17 Nov
2025
17 Nov
'25
12:41 a.m.
(sending again from a different email address as I don't think my first
email got through, apologies if this is a duplicate)
I have a VT100 that I was working on a while back and then set aside for a
bit. I have started to look at it again. I have noticed that the Video Shift
Register, a 74S299, gets very hot, I can smell the heat and the chip gets
almost too hot to touch, reaching almost 40 celsius. I have a working VT102
for comparison and the same chip there does not get so hot (it reaches about
30 celsius). I have already tried replacing the chip, but the new one gets
equally hot.
I looked at the signals the chip is receiving and the one that stands out as
different is the CLK input (pin 12). It looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2024/04/vt100-74s299-clk-s
ignal.png. On the VT102 it looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-clk-s
ignal.png. It is much spikier on the VT100 and I was told at one point that
this could be the cause of the hot running for the chip. Is that a
reasonable assumption?
Assuming the spikes are the cause of the hot running. I am trying to see why
there is a difference. I have noticed that on the VT102 there is a 68R
resistor between the DC011 which produces the signal and the 74S299. You can
see this as R86 in the VT101 printset
https://bitsavers.org/pdf/dec/terminal/vt101/MP-01066-00C_VT101_Family_Field
_Maintenance_Print_Set_Apr82.pdf (p47 of the PDF). The VT100 printset dated
Feb 82
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.p
df shows an inductor L8 being used (PDF p17), but my VT100 does not have
this and so must be described by the March 80 printset
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Mar80.pdf (PDF
p17).
Could the absence of L8 explain the spikier DOT CLK signal and the hotter
74S299?
Incidentally, I suspect that the flyback transformer on my VT100 has failed.
If anyone has a flyback transformer going spare, especially in the UK, then
I would love to hear from you.
Thanks
Rob
17 Nov
17 Nov
12:19 p.m.
If the part is getting hot, that can mean 1 of 2 different problems.
1.
The part has failed internally.
2.
An output has external short to one of the voltage rails
3.
The VCC input has gone above 5V and this part has fail and is now loading it down and
hopefully protecting the other parts.
For any of these, you will need to remove the part. The worst one is that the regulator
has failed as just removing the part and powering the system up can cause more additional
damage.
First check the VCC voltage. If it is not 5V you may have a regulator problem. If it is
not 5V you'll need remove the part and use a variac ( or similar ) to bring the power
up slowly, watching the rail voltage.
Next since you've removed the part, you can now continue. With the terminal powered
up, we want to look for another possible short causing the part to run hot. Use a 50 (
about 47 is a 50 ) ohm resistor. Connect one end to circuit ground. Attach it to each
output pin and measure the voltage If another input has shorted to 5V, you would see more
than a 0.5 volts on the resistor it is likely a shorted input.
If all these don't seem to be the problem replace the part. Do it with a socket. If
you unsolder is make sure to note which pin locations top and bottom so that you can check
them with a meter after installing the socket.
Dwight
________________________________
From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Sent: Sunday, November 16, 2025 10:41 PM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Cc: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Subject: [cctalk] Hot Video Shift Register on VT100
(sending again from a different email address as I don't think my first
email got through, apologies if this is a duplicate)
I have a VT100 that I was working on a while back and then set aside for a
bit. I have started to look at it again. I have noticed that the Video Shift
Register, a 74S299, gets very hot, I can smell the heat and the chip gets
almost too hot to touch, reaching almost 40 celsius. I have a working VT102
for comparison and the same chip there does not get so hot (it reaches about
30 celsius). I have already tried replacing the chip, but the new one gets
equally hot.
I looked at the signals the chip is receiving and the one that stands out as
different is the CLK input (pin 12). It looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2024/04/vt100-74s299-clk-s
ignal.png. On the VT102 it looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-clk-s
ignal.png. It is much spikier on the VT100 and I was told at one point that
this could be the cause of the hot running for the chip. Is that a
reasonable assumption?
Assuming the spikes are the cause of the hot running. I am trying to see why
there is a difference. I have noticed that on the VT102 there is a 68R
resistor between the DC011 which produces the signal and the 74S299. You can
see this as R86 in the VT101 printset
https://bitsavers.org/pdf/dec/terminal/vt101/MP-01066-00C_VT101_Family_Field
_Maintenance_Print_Set_Apr82.pdf (p47 of the PDF). The VT100 printset dated
Feb 82
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.p
df shows an inductor L8 being used (PDF p17), but my VT100 does not have
this and so must be described by the March 80 printset
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Mar80.pdf (PDF
p17).
Could the absence of L8 explain the spikier DOT CLK signal and the hotter
74S299?
Incidentally, I suspect that the flyback transformer on my VT100 has failed.
If anyone has a flyback transformer going spare, especially in the UK, then
I would love to hear from you.
Thanks
Rob
12:35 p.m.
<quote> If it is not 5V you'll need remove the part and use a variac ( or
similar ) to bring the power up slowly, watching the rail voltage. </quote>
I would incline to using bench power supplies with current limits, as the voltage source,
when investigating failing boards / components. Perhaps not possible or perhaps it
requires a lot of strip down, but then you can wind on the DC "gently", observe
currents and respond apropriately. Switched mode PSUs, esp wide range ones, can be
insensitive to AC line voltage.
Martin
-----Original Message-----
From: dwight via cctalk [mailto:cctalk@classiccmp.org]
Sent: 17 November 2025 18:19
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Cc: dwight <dkelvey(a)hotmail.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
If the part is getting hot, that can mean 1 of 2 different problems.
1.
The part has failed internally.
2.
An output has external short to one of the voltage rails
3.
The VCC input has gone above 5V and this part has fail and is now loading it down and
hopefully protecting the other parts.
For any of these, you will need to remove the part. The worst one is that the regulator
has failed as just removing the part and powering the system up can cause more additional
damage.
First check the VCC voltage. If it is not 5V you may have a regulator problem. If it is
not 5V you'll need remove the part and use a variac ( or similar ) to bring the power
up slowly, watching the rail voltage.
Next since you've removed the part, you can now continue. With the terminal powered
up, we want to look for another possible short causing the part to run hot. Use a 50 (
about 47 is a 50 ) ohm resistor. Connect one end to circuit ground. Attach it to each
output pin and measure the voltage If another input has shorted to 5V, you would see more
than a 0.5 volts on the resistor it is likely a shorted input.
If all these don't seem to be the problem replace the part. Do it with a socket. If
you unsolder is make sure to note which pin locations top and bottom so that you can check
them with a meter after installing the socket.
Dwight
________________________________
From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Sent: Sunday, November 16, 2025 10:41 PM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Cc: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Subject: [cctalk] Hot Video Shift Register on VT100
(sending again from a different email address as I don't think my first email got
through, apologies if this is a duplicate)
I have a VT100 that I was working on a while back and then set aside for a bit. I have
started to look at it again. I have noticed that the Video Shift Register, a 74S299, gets
very hot, I can smell the heat and the chip gets almost too hot to touch, reaching almost
40 celsius. I have a working VT102 for comparison and the same chip there does not get so
hot (it reaches about
30 celsius). I have already tried replacing the chip, but the new one gets equally hot.
I looked at the signals the chip is receiving and the one that stands out as different is
the CLK input (pin 12). It looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2024/04/vt100-74s299-clk-s
ignal.png. On the VT102 it looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-clk-s
ignal.png. It is much spikier on the VT100 and I was told at one point that this could be
the cause of the hot running for the chip. Is that a reasonable assumption?
Assuming the spikes are the cause of the hot running. I am trying to see why there is a
difference. I have noticed that on the VT102 there is a 68R resistor between the DC011
which produces the signal and the 74S299. You can see this as R86 in the VT101 printset
https://bitsavers.org/pdf/dec/terminal/vt101/MP-01066-00C_VT101_Family_Field
_Maintenance_Print_Set_Apr82.pdf (p47 of the PDF). The VT100 printset dated Feb 82
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.p
df shows an inductor L8 being used (PDF p17), but my VT100 does not have this and so must
be described by the March 80 printset
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Mar80.pdf (PDF p17).
Could the absence of L8 explain the spikier DOT CLK signal and the hotter 74S299?
Incidentally, I suspect that the flyback transformer on my VT100 has failed.
If anyone has a flyback transformer going spare, especially in the UK, then I would love
to hear from you.
Thanks
Rob
1:23 p.m.
Just an fyi, vt100 flybacks fail frequently and are largely unobtainable. Many list
members have asked for spares with no luck. So if your flyback is truly bad there’s no use
trying to fix any other part of it until you get the flyback working.
Sent from my iPhone
On Nov 17, 2025, at 10:35, Martin Bishop via cctalk
<cctalk(a)classiccmp.org> wrote:
<quote> If it is not 5V you'll need remove the part and use a variac ( or
similar ) to bring the power up slowly, watching the rail voltage. </quote>
I would incline to using bench power supplies with current limits, as the voltage source,
when investigating failing boards / components. Perhaps not possible or perhaps it
requires a lot of strip down, but then you can wind on the DC "gently", observe
currents and respond apropriately. Switched mode PSUs, esp wide range ones, can be
insensitive to AC line voltage.
Martin
-----Original Message-----
From: dwight via cctalk [mailto:cctalk@classiccmp.org]
Sent: 17 November 2025 18:19
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Cc: dwight <dkelvey(a)hotmail.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
If the part is getting hot, that can mean 1 of 2 different problems.
1.
The part has failed internally.
2.
An output has external short to one of the voltage rails
3.
The VCC input has gone above 5V and this part has fail and is now loading it down and
hopefully protecting the other parts.
For any of these, you will need to remove the part. The worst one is that the regulator
has failed as just removing the part and powering the system up can cause more additional
damage.
First check the VCC voltage. If it is not 5V you may have a regulator problem. If it is
not 5V you'll need remove the part and use a variac ( or similar ) to bring the power
up slowly, watching the rail voltage.
Next since you've removed the part, you can now continue. With the terminal powered
up, we want to look for another possible short causing the part to run hot. Use a 50 (
about 47 is a 50 ) ohm resistor. Connect one end to circuit ground. Attach it to each
output pin and measure the voltage If another input has shorted to 5V, you would see more
than a 0.5 volts on the resistor it is likely a shorted input.
If all these don't seem to be the problem replace the part. Do it with a socket. If
you unsolder is make sure to note which pin locations top and bottom so that you can check
them with a meter after installing the socket.
Dwight
________________________________
From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Sent: Sunday, November 16, 2025 10:41 PM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Cc: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Subject: [cctalk] Hot Video Shift Register on VT100
(sending again from a different email address as I don't think my first email got
through, apologies if this is a duplicate)
I have a VT100 that I was working on a while back and then set aside for a bit. I have
started to look at it again. I have noticed that the Video Shift Register, a 74S299, gets
very hot, I can smell the heat and the chip gets almost too hot to touch, reaching almost
40 celsius. I have a working VT102 for comparison and the same chip there does not get so
hot (it reaches about
30 celsius). I have already tried replacing the chip, but the new one gets equally hot.
I looked at the signals the chip is receiving and the one that stands out as different is
the CLK input (pin 12). It looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2024/04/vt100-74s299-clk-s
ignal.png. On the VT102 it looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-clk-s
ignal.png. It is much spikier on the VT100 and I was told at one point that this could be
the cause of the hot running for the chip. Is that a reasonable assumption?
Assuming the spikes are the cause of the hot running. I am trying to see why there is a
difference. I have noticed that on the VT102 there is a 68R resistor between the DC011
which produces the signal and the 74S299. You can see this as R86 in the VT101 printset
https://bitsavers.org/pdf/dec/terminal/vt101/MP-01066-00C_VT101_Family_Field
_Maintenance_Print_Set_Apr82.pdf (p47 of the PDF). The VT100 printset dated Feb 82
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.p
df shows an inductor L8 being used (PDF p17), but my VT100 does not have this and so must
be described by the March 80 printset
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Mar80.pdf (PDF p17).
Could the absence of L8 explain the spikier DOT CLK signal and the hotter 74S299?
Incidentally, I suspect that the flyback transformer on my VT100 has failed.
If anyone has a flyback transformer going spare, especially in the UK, then I would love
to hear from you.
Thanks
Rob
2:10 p.m.
On Mon, Nov 17, 2025 at 2:24 PM Wayne S via cctalk
<cctalk(a)classiccmp.org> wrote:
Just an fyi, vt100 flybacks fail frequently and are
largely unobtainable. Many list members have asked for spares with no luck.
Yep. I'm one of those. I watched a puff of smoke come out of mine
when I was testing it right after I repaired the PSU (PSU is fine now,
and I've been using this terminal with an external monitor - I need an
actual VT100 because I'm really trying to get a Retrographics board
working and it doesn't fit in a VT102 - totally different packaging).
Ultimately, it's just a 12V mono monitor but it's likely I would pull
the entire CRT assembly back to the analog board with the edge
connector. That takes +12V, H. sync. V. sync, and video.
-ethan
22 Nov
22 Nov
11:53 a.m.
Thanks for the reply. I had time today to go through some of these items.
-----Original Message-----
From: dwight via cctalk <cctalk(a)classiccmp.org>
Sent: 17 November 2025 18:19
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: dwight <dkelvey(a)hotmail.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
If the part is getting hot, that can mean 1 of 2 different problems.
1.
The part has failed internally.
2.
An output has external short to one of the voltage rails
3.
The VCC input has gone above 5V and this part has fail and is now loading
it
down and hopefully protecting the other parts.
For any of these, you will need to remove the part. The worst one is that
the
regulator has failed as just removing the part and
powering the system up
can
cause more additional damage.
First check the VCC voltage. If it is not 5V you may have a regulator
problem. If
it is not 5V you'll need remove the part and use a
variac ( or similar )
to bring
the power up slowly, watching the rail voltage.
I don't think Vcc is the problem. With the part installed Vcc was 5.1V. With
the part removed it was still 5.1V. Pretty sure that is within the tolerance
of the PSU, and according to the datasheet I have this is within the limits
for the part.
Next since you've removed the part, you can now continue. With the
terminal
powered up, we want to look for another possible short
causing the part to
run hot. Use a 50 ( about 47 is a 50 ) ohm resistor. Connect one end to
circuit
ground. Attach it to each output pin and measure the
voltage If another
input
has shorted to 5V, you would see more than a 0.5
volts on the resistor it
is
likely a shorted input.
With the part removed and the terminal powered off, I used my DMM to measure
the resistance of the two 74S299 output pins (8 and 17) compared to Vcc and
to GND. Pin 17 was 1.2M to GND, pin 8 was open circuit to GND. Pin 17 was
open circuit to Vcc, pin 8 was open circuit to Vcc.
I then did what you suggested and, with the part removed, connected the
output, pin 17, to GND with a 47 ohm resistor. I switched it on and the
voltage across the resistor was 0.03V. I also tried connecting the output
pin to 5V and the voltage across the resistor was 0V. So I don't think there
are any shorts.
I should explain that I think the 74S299 is working fine because, although I
have a problem with the monitor board/flyback, the external video output
works fine.
Given that it works OK but is just hot, is it possible that this is just
normal? Or is it the spikiness of the clock input that is the problem? I
checked the clock input with the chip still removed and it is still spikier
than on my VT102.
If all these don't seem to be the problem replace the part. Do it with a
socket.
If you unsolder is make sure to note which pin
locations top and bottom so
that you can check them with a meter after installing the socket.
I need to order some sockets.
Dwight
________________________________
From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Sent: Sunday, November 16, 2025 10:41 PM
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Subject: [cctalk] Hot Video Shift Register on VT100
(sending again from a different email address as I don't think my first
email
got through, apologies if this is a duplicate)
I have a VT100 that I was working on a while back and then set aside for a
bit.
I have started to look at it again. I have noticed
that the Video Shift
Register, a
74S299, gets very hot, I can smell the heat and the
chip gets almost too
hot to
touch, reaching almost 40 celsius. I have a working
VT102 for comparison
and
the same chip there does not get so hot (it reaches
about
30 celsius). I have already tried replacing the chip, but the new one gets
equally hot.
I looked at the signals the chip is receiving and the one that stands out
as
different is the CLK input (pin 12). It looks like
this:
https://rjarratt.wordpress.com/wp-content/uploads/2024/04/vt100-74s299-
clk-s
ignal.png. On the VT102 it looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-
clk-s
ignal.png. It is much spikier on the VT100 and I was told at one point
that this
could be the cause of the hot running for the chip. Is
that a reasonable
assumption?
Assuming the spikes are the cause of the hot running. I am trying to see
why
there is a difference. I have noticed that on the
VT102 there is a 68R
resistor
between the DC011 which produces the signal and the
74S299. You can see
this as R86 in the VT101 printset
https://bitsavers.org/pdf/dec/terminal/vt101/MP-01066-
00C_VT101_Family_Field
_Maintenance_Print_Set_Apr82.pdf (p47 of the PDF). The VT100 printset
dated Feb 82
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Fe
b82.p
df shows an inductor L8 being used (PDF p17), but my VT100 does not have
this and so must be described by the March 80 printset
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Mar80.pdf
(PDF p17).
Could the absence of L8 explain the spikier DOT CLK signal and the hotter
74S299?
Incidentally, I suspect that the flyback transformer on my VT100 has
failed.
If anyone has a flyback transformer going spare,
especially in the UK,
then I
would love to hear from you.
Thanks
Rob
24 Nov
24 Nov
4:28 p.m.
Sorry if I wasn't clear in my earlier reply, but there were some answers
inline. The summary is that Vcc looks fine, there does not seem to be any
short on the outputs of the shift register and the chip seems to be working
correctly. It just gets hot and I wonder if this could be due to the CLK
input being a bit spikier on the VT100 than on my VT102? Could this be why
later revisions of the VT100 introduced an inductor on the DOT CLK output
from the DC011?
Thanks
Rob
socket.
-----Original Message-----
From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Sent: 22 November 2025 17:53
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Thanks for the reply. I had time today to go through some of these items.
With
the
-----Original Message-----
From: dwight via cctalk <cctalk(a)classiccmp.org>
Sent: 17 November 2025 18:19
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: dwight <dkelvey(a)hotmail.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
If the part is getting hot, that can mean 1 of 2 different problems.
1.
The part has failed internally.
2.
An output has external short to one of the voltage rails
3.
The VCC input has gone above 5V and this part has fail and is now
loading
it
down and hopefully protecting the other parts.
For any of these, you will need to remove the part. The worst one is
that
the
regulator has failed as just removing the part
and powering the system
up
can
cause more additional damage.
First check the VCC voltage. If it is not 5V you may have a regulator
problem. If
it is not 5V you'll need remove the part and
use a variac ( or similar
)
to bring
the power up slowly, watching the rail voltage.
I don't think Vcc is the problem. With the part installed Vcc was 5.1V. part removed it was still 5.1V. Pretty sure that is
within the tolerance
of the
PSU, and according to the datasheet I have this is
within the limits for
the
part.
voltage
Next since you've removed the part, you can now continue. With the
terminal
powered up, we want to look for another possible
short causing the
part to run hot. Use a 50 ( about 47 is a 50 ) ohm resistor. Connect
one end to
circuit
ground. Attach it to each output pin and measure
the voltage If
another
input
has shorted to 5V, you would see more than a 0.5
volts on the
resistor it
is
likely a shorted input.
With the part removed and the terminal powered off, I used my DMM to
measure the resistance of the two 74S299 output pins (8 and 17) compared
to Vcc and to GND. Pin 17 was 1.2M to GND, pin 8 was open circuit to GND.
Pin 17 was open circuit to Vcc, pin 8 was open circuit to Vcc.
I then did what you suggested and, with the part removed, connected the
output, pin 17, to GND with a 47 ohm resistor. I switched it on and the across the resistor was 0.03V. I also tried connecting
the output pin to
5V and
the voltage across the resistor was 0V. So I don't
think there are any
shorts.
I should explain that I think the 74S299 is working fine because, although
I
have a problem with the monitor board/flyback, the
external video output
works fine.
Given that it works OK but is just hot, is it possible that this is just
normal? Or
is it the spikiness of the clock input that is the
problem? I checked the
clock
input with the chip still removed and it is still
spikier than on my
VT102.
If all these don't seem to be the problem replace the part. Do it with
a
socket.
> If you unsolder is make sure to note which pin locations top and
> bottom so that you can check them with a meter after installing the
I need to order some sockets.
Dwight
________________________________
From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Sent: Sunday, November 16, 2025 10:41 PM
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Rob Jarratt
<robert.jarratt(a)ntlworld.com>
Subject: [cctalk] Hot Video Shift Register on VT100
(sending again from a different email address as I don't think my
first
email
got through, apologies if this is a duplicate)
I have a VT100 that I was working on a while back and then set aside
for a
bit.
I have started to look at it again. I have
noticed that the Video
Shift
Register, a
74S299, gets very hot, I can smell the heat and
the chip gets almost
too
hot to
touch, reaching almost 40 celsius. I have a
working VT102 for
comparison
and
the same chip there does not get so hot (it
reaches about
30 celsius). I have already tried replacing the chip, but the new one
gets equally hot.
I looked at the signals the chip is receiving and the one that stands
out
as
different is the CLK input (pin 12). It looks
like this:
https://rjarratt.wordpress.com/wp-content/uploads/2024/04/vt100-74s299
-
clk-s
ignal.png. On the VT102 it looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299
-
clk-s
ignal.png. It is much spikier on the VT100 and I was told at one point
that this
could be the cause of the hot running for the
chip. Is that a
reasonable assumption?
Assuming the spikes are the cause of the hot running. I am trying to
see
why
there is a difference. I have noticed that on the
VT102 there is a 68R
resistor
between the DC011 which produces the signal and
the 74S299. You can
see this as R86 in the VT101 printset
https://bitsavers.org/pdf/dec/terminal/vt101/MP-01066-
00C_VT101_Family_Field
_Maintenance_Print_Set_Apr82.pdf (p47 of the PDF). The VT100 printset
dated Feb 82
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_F
e
b82.p
df shows an inductor L8 being used (PDF p17), but my VT100 does not
have this and so must be described by the March 80 printset
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Mar80.pdf
(PDF p17).
Could the absence of L8 explain the spikier DOT CLK signal and the
hotter 74S299?
Incidentally, I suspect that the flyback transformer on my VT100 has
failed.
If anyone has a flyback transformer going spare,
especially in the UK,
then I
> would love to hear from you.
>
>
>
> Thanks
>
>
>
> Rob
> 
4:40 p.m.
Sorry if I wasn't clear in my earlier reply, but there were some answers
inline. The summary is that Vcc looks fine, there does not seem to be any
short on the outputs of the shift register and the chip seems to be working
correctly. It just gets hot and I wonder if this could be due to the CLK
input being a bit spikier on the VT100 than on my VT102? Could this be why
later revisions of the VT100 introduced an inductor on the DOT CLK output
from the DC011?
Perhaps one way to test the hypothesis would be to insert an inductor like
those used on the later revisions in the DOT CLK output and check whether
a) everything still works and b) the shift register runs cooler?
Regards,
Peter Coghlan.
5:04 p.m.
Another test would be to disconnect the output and see if that runs the
device cooler. Maybe the load, while not a short, is simply too great for
the device.
But I am leaning towards the inputs containing spikes that need to be
dissipated as heat in clamping diodes. A decent CRO on the device input
pins while it is out of circuit will show that as well.
Kindest regards,
Doug Jackson
em: doug(a)doughq.com
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
cctalk(a)classiccmp.org> wrote:
Sorry if I wasn't clear in my earlier reply, but there were some answers
inline. The summary is that Vcc looks fine, there does not seem to be any
short on the outputs of the shift register and the chip seems to be
working
correctly. It just gets hot and I wonder if this
could be due to the CLK
input being a bit spikier on the VT100 than on my VT102? Could this be
why
later revisions of the VT100 introduced an
inductor on the DOT CLK output
from the DC011?
Perhaps one way to test the hypothesis would be to insert an inductor like
those used on the later revisions in the DOT CLK output and check whether
a) everything still works and b) the shift register runs cooler?
Regards,
Peter Coghlan.
5:48 p.m.
-----Original Message-----
From: Doug Jackson via cctalk <cctalk(a)classiccmp.org>
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Cc: Doug Jackson <doug(a)doughq.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that runs the
device cooler. Maybe the load, while not a short, is simply too great for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to be
dissipated as heat in clamping diodes. A decent CRO on the device input pins
while it is out of circuit will show that as well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are suggesting I try here?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
cctalk(a)classiccmp.org> wrote:
> >
> > Sorry if I wasn't clear in my earlier reply, but there were some
> > answers inline. The summary is that Vcc looks fine, there does not
> > seem to be any short on the outputs of the shift register and the
> > chip seems to be
> working
> > correctly. It just gets hot and I wonder if this could be due to the
> > CLK input being a bit spikier on the VT100 than on my VT102? Could
> > this be
> why
> > later revisions of the VT100 introduced an inductor on the DOT CLK
> > output from the DC011?
> >
>
> Perhaps one way to test the hypothesis would be to insert an inductor
> like those used on the later revisions in the DOT CLK output and check
> whether
> a) everything still works and b) the shift register runs cooler?
>
> Regards,
> Peter Coghlan.
>
7:14 p.m.
The oscilloscope will show you the actual driving waveform.
You can then see the level of the spikes
Kindest regards,
Doug Jackson
em: doug(a)doughq.com
ph: 0414 986878
On Tue, 25 Nov 2025 at 10:48, Rob Jarratt <robert.jarratt(a)ntlworld.com>
wrote:
-----Original Message-----
From: Doug Jackson via cctalk <cctalk(a)classiccmp.org>
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <
cctalk(a)classiccmp.org>
Cc: Doug Jackson <doug(a)doughq.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that runs the
device cooler. Maybe the load, while not a short, is simply too great
for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to be
dissipated as heat in clamping diodes. A decent CRO on the device input
pins
while it is out of circuit will show that as
well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
suggesting I try here?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
cctalk(a)classiccmp.org> wrote:
> >
> > Sorry if I wasn't clear in my earlier reply, but there were some
> > answers inline. The summary is that Vcc looks fine, there does not
> > seem to be any short on the outputs of the shift register and the
> > chip seems to be
> working
> > correctly. It just gets hot and I wonder if this could be due to the
> > CLK input being a bit spikier on the VT100 than on my VT102? Could
> > this be
> why
> > later revisions of the VT100 introduced an inductor on the DOT CLK
> > output from the DC011?
> >
>
> Perhaps one way to test the hypothesis would be to insert an inductor
> like those used on the later revisions in the DOT CLK output and check
> whether
> a) everything still works and b) the shift register runs cooler?
>
> Regards,
> Peter Coghlan.
>
28 Nov
28 Nov
3:52 p.m.
I checked the CLK input for the 74S299 with the 74S299 removed and the signal is still
spiky and looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-clk-…
I removed the other load on this signal, the DC012, and the CLK signal still looked spiky.
I also put back the 74S299 (now socketed) with DC012 removed and it still gets hot. I
tried another new 74S299 just in case I had a bad one and that also got hot without a load
on the output.
I am thinking that it is either the spiky input or it is just expected to get this hot.
Regards
Rob
From: Doug Jackson <doug(a)doughq.com>
Sent: 25 November 2025 01:15
To: rob(a)jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
The oscilloscope will show you the actual driving waveform.
You can then see the level of the spikes
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Tue, 25 Nov 2025 at 10:48, Rob Jarratt <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> > wrote:
-----Original Message-----
From: Doug Jackson via cctalk <cctalk(a)classiccmp.org
<mailto:cctalk@classiccmp.org> >
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org
<mailto:cctalk@classiccmp.org> >
Cc: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that runs the
device cooler. Maybe the load, while not a short, is simply too great for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to be
dissipated as heat in clamping diodes. A decent CRO on the device input pins
while it is out of circuit will show that as well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are suggesting I try here?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> > wrote:
> >
> > Sorry if I wasn't clear in my earlier reply, but there were some
> > answers inline. The summary is that Vcc looks fine, there does not
> > seem to be any short on the outputs of the shift register and the
> > chip seems to be
> working
> > correctly. It just gets hot and I wonder if this could be due to the
> > CLK input being a bit spikier on the VT100 than on my VT102? Could
> > this be
> why
> > later revisions of the VT100 introduced an inductor on the DOT CLK
> > output from the DC011?
> >
>
> Perhaps one way to test the hypothesis would be to insert an inductor
> like those used on the later revisions in the DOT CLK output and check
> whether
> a) everything still works and b) the shift register runs cooler?
>
> Regards,
> Peter Coghlan.
>
4:58 p.m.
Is that a negative excursion at the bottom of the waveform? now low -
looks like -0.3v to me - With '"logic, that will likely turn on the
protection diodes on the inputs, making them warm.
Double check where the ground actually is on the CRO display.
It's also interesting that the input doesn't even reach 4v, let alone 5v.
Whats driving that?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com
ph: 0414 986878
On Sat, 29 Nov 2025 at 08:52, Rob Jarratt <robert.jarratt(a)ntlworld.com>
wrote:
I checked the CLK input for the 74S299 with the 74S299
removed and the
signal is still spiky and looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-clk-…
I removed the other load on this signal, the DC012, and the CLK signal
still looked spiky. I also put back the 74S299 (now socketed) with DC012
removed and it still gets hot. I tried another new 74S299 just in case I
had a bad one and that also got hot without a load on the output.
I am thinking that it is either the spiky input or it is just expected to
get this hot.
Regards
Rob
*From:* Doug Jackson <doug(a)doughq.com>
*Sent:* 25 November 2025 01:15
*To:* rob(a)jarratt.me.uk
*Cc:* General Discussion: On-Topic and Off-Topic Posts <
cctalk(a)classiccmp.org>
*Subject:* Re: [cctalk] Re: Hot Video Shift Register on VT100
The oscilloscope will show you the actual driving waveform.
You can then see the level of the spikes
Kindest regards,
Doug Jackson
em: doug(a)doughq.com
ph: 0414 986878
On Tue, 25 Nov 2025 at 10:48, Rob Jarratt <robert.jarratt(a)ntlworld.com>
wrote:
-----Original Message-----
From: Doug Jackson via cctalk <cctalk(a)classiccmp.org>
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <
cctalk(a)classiccmp.org>
Cc: Doug Jackson <doug(a)doughq.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that runs the
device cooler. Maybe the load, while not a short, is simply too great
for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to be
dissipated as heat in clamping diodes. A decent CRO on the device input
pins
while it is out of circuit will show that as
well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
suggesting I try here?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
cctalk(a)classiccmp.org> wrote:
> >
> > Sorry if I wasn't clear in my earlier reply, but there were some
> > answers inline. The summary is that Vcc looks fine, there does not
> > seem to be any short on the outputs of the shift register and the
> > chip seems to be
> working
> > correctly. It just gets hot and I wonder if this could be due to the
> > CLK input being a bit spikier on the VT100 than on my VT102? Could
> > this be
> why
> > later revisions of the VT100 introduced an inductor on the DOT CLK
> > output from the DC011?
> >
>
> Perhaps one way to test the hypothesis would be to insert an inductor
> like those used on the later revisions in the DOT CLK output and check
> whether
> a) everything still works and b) the shift register runs cooler?
>
> Regards,
> Peter Coghlan.
>
29 Nov
29 Nov
1:10 a.m.
Yes, it is a negative voltage. I measured this by attaching the ground lead to the ground
connection on the 74S299.
The DC011 that is producing the CLK signal is socketed, but the one on my VT102 isn’t. I
would try swapping them to see what happens, but I don’t really want to touch a fully
working board.
Thanks
Rob
From: Doug Jackson <doug(a)doughq.com>
Sent: 28 November 2025 22:59
To: rob(a)jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
Is that a negative excursion at the bottom of the waveform? now low - looks like -0.3v to
me - With '"logic, that will likely turn on the protection diodes on the inputs,
making them warm.
Double check where the ground actually is on the CRO display.
It's also interesting that the input doesn't even reach 4v, let alone 5v. Whats
driving that?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Sat, 29 Nov 2025 at 08:52, Rob Jarratt <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> > wrote:
I checked the CLK input for the 74S299 with the 74S299 removed and the signal is still
spiky and looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-clk-…
I removed the other load on this signal, the DC012, and the CLK signal still looked spiky.
I also put back the 74S299 (now socketed) with DC012 removed and it still gets hot. I
tried another new 74S299 just in case I had a bad one and that also got hot without a load
on the output.
I am thinking that it is either the spiky input or it is just expected to get this hot.
Regards
Rob
From: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
Sent: 25 November 2025 01:15
To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk>
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org
<mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
The oscilloscope will show you the actual driving waveform.
You can then see the level of the spikes
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Tue, 25 Nov 2025 at 10:48, Rob Jarratt <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> > wrote:
-----Original Message-----
From: Doug Jackson via cctalk <cctalk(a)classiccmp.org
<mailto:cctalk@classiccmp.org> >
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org
<mailto:cctalk@classiccmp.org> >
Cc: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that runs the
device cooler. Maybe the load, while not a short, is simply too great for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to be
dissipated as heat in clamping diodes. A decent CRO on the device input pins
while it is out of circuit will show that as well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are suggesting I try here?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> > wrote:
> >
> > Sorry if I wasn't clear in my earlier reply, but there were some
> > answers inline. The summary is that Vcc looks fine, there does not
> > seem to be any short on the outputs of the shift register and the
> > chip seems to be
> working
> > correctly. It just gets hot and I wonder if this could be due to the
> > CLK input being a bit spikier on the VT100 than on my VT102? Could
> > this be
> why
> > later revisions of the VT100 introduced an inductor on the DOT CLK
> > output from the DC011?
> >
>
> Perhaps one way to test the hypothesis would be to insert an inductor
> like those used on the later revisions in the DOT CLK output and check
> whether
> a) everything still works and b) the shift register runs cooler?
>
> Regards,
> Peter Coghlan.
>
2:36 a.m.
Rob,
I have a few silly thoughts, but maybe they can be useful.
The negative going signal is something I would not be happy with, as it may damage the IC
(over time).
TTL does not like negative voltages on their pins.
Is there between the DC011 (signal generating) and the LS299 (signal "user")
wiring, or are they on the same PCB?
In case of wiring, capacitive coupling may cause negative going voltages.
Could a clamping diode with an as low as possible Vf (< 0.3 V) help to reduce the
negative voltage? It is not a permanent solution, because the DC011 might be a bit unhappy
with that diode, but for a test it might help to draw a conclusion.
If all does not help, in getting the LS299 running cooler, consider some temperature
conductive paste and a small "IC heatsink". Tie the heatsink onto the IC with a
piece of waxed lacing. The IC can radiate more efficiently the generated heat, so that it
can run cooler. This could drop temperature with some 10 degrees.
Success hunting down this issue,
Henk
-----Oorspronkelijk bericht-----
Van: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Verzonden: zaterdag 29 november 2025 08:11
Aan: 'Doug Jackson' <doug(a)doughq.com>om>; rob(a)jarratt.me.uk
CC: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>rg>; Rob Jarratt <robert.jarratt(a)ntlworld.com>
Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
Yes, it is a negative voltage. I measured this by attaching the ground lead to the ground
connection on the 74S299.
The DC011 that is producing the CLK signal is socketed, but the one on my VT102 isn’t. I
would try swapping them to see what happens, but I don’t really want to touch a fully
working board.
Thanks
Rob
From: Doug Jackson <doug(a)doughq.com>
Sent: 28 November 2025 22:59
To: rob(a)jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
Is that a negative excursion at the bottom of the waveform? now low - looks like -0.3v to
me - With '"logic, that will likely turn on the protection diodes on the inputs,
making them warm.
Double check where the ground actually is on the CRO display.
It's also interesting that the input doesn't even reach 4v, let alone 5v. Whats
driving that?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Sat, 29 Nov 2025 at 08:52, Rob Jarratt <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> > wrote:
I checked the CLK input for the 74S299 with the 74S299 removed and the signal is still
spiky and looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-clk-…
I removed the other load on this signal, the DC012, and the CLK signal still looked spiky.
I also put back the 74S299 (now socketed) with DC012 removed and it still gets hot. I
tried another new 74S299 just in case I had a bad one and that also got hot without a load
on the output.
I am thinking that it is either the spiky input or it is just expected to get this hot.
Regards
Rob
From: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
Sent: 25 November 2025 01:15
To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk>
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org
<mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
The oscilloscope will show you the actual driving waveform.
You can then see the level of the spikes
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Tue, 25 Nov 2025 at 10:48, Rob Jarratt <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> > wrote:
-----Original Message-----
From: Doug Jackson via cctalk <cctalk(a)classiccmp.org
<mailto:cctalk@classiccmp.org> >
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org
<mailto:cctalk@classiccmp.org> >
Cc: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that runs the
device cooler. Maybe the load, while not a short, is simply too great for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to be
dissipated as heat in clamping diodes. A decent CRO on the device input pins
while it is out of circuit will show that as well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are suggesting I try here?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> > wrote:
> >
> > Sorry if I wasn't clear in my earlier reply, but there were some
> > answers inline. The summary is that Vcc looks fine, there does not
> > seem to be any short on the outputs of the shift register and the
> > chip seems to be
> working
> > correctly. It just gets hot and I wonder if this could be due to the
> > CLK input being a bit spikier on the VT100 than on my VT102? Could
> > this be
> why
> > later revisions of the VT100 introduced an inductor on the DOT CLK
> > output from the DC011?
> >
>
> Perhaps one way to test the hypothesis would be to insert an inductor
> like those used on the later revisions in the DOT CLK output and check
> whether
> a) everything still works and b) the shift register runs cooler?
>
> Regards,
> Peter Coghlan.
>
3:17 a.m.
Hi Rob,
this is not intended to indicate the cause or solution, but merely to help
analyze how critical your IC temperature is: Did you actually measure the
temperature of the IC? I recently bought an inexpensive infrared
thermometer (like:https://www.ebay.co.uk/itm/156810933790) that can measure
the temperature of small areas relatively accurately. I made the experience
that I cannot judge temperatures very well with my fingers.
According to Wikipedia, TTL ICs are specified for temperatures up to
70 °C. This
temperature should obviously be avoided, but my fingers already perceive
anything above about 50 °C as "hot".
I hope this can help a litte!
Ulli
Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk <
cctalk(a)classiccmp.org>gt;:
Rob,
I have a few silly thoughts, but maybe they can be useful.
The negative going signal is something I would not be happy with, as it
may damage the IC (over time).
TTL does not like negative voltages on their pins.
Is there between the DC011 (signal generating) and the LS299 (signal
"user") wiring, or are they on the same PCB?
In case of wiring, capacitive coupling may cause negative going voltages.
Could a clamping diode with an as low as possible Vf (< 0.3 V) help to
reduce the negative voltage? It is not a permanent solution, because the
DC011 might be a bit unhappy with that diode, but for a test it might help
to draw a conclusion.
If all does not help, in getting the LS299 running cooler, consider some
temperature conductive paste and a small "IC heatsink". Tie the heatsink
onto the IC with a piece of waxed lacing. The IC can radiate more
efficiently the generated heat, so that it can run cooler. This could drop
temperature with some 10 degrees.
Success hunting down this issue,
Henk
-----Oorspronkelijk bericht-----
Van: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Verzonden: zaterdag 29 november 2025 08:11
Aan: 'Doug Jackson' <doug(a)doughq.com>om>; rob(a)jarratt.me.uk
CC: 'General Discussion: On-Topic and Off-Topic Posts' <
cctalk(a)classiccmp.org>gt;; Rob Jarratt <robert.jarratt(a)ntlworld.com>
Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
Yes, it is a negative voltage. I measured this by attaching the ground
lead to the ground connection on the 74S299.
The DC011 that is producing the CLK signal is socketed, but the one on my
VT102 isn’t. I would try swapping them to see what happens, but I don’t
really want to touch a fully working board.
Thanks
Rob
From: Doug Jackson <doug(a)doughq.com>
Sent: 28 November 2025 22:59
To: rob(a)jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts <
cctalk(a)classiccmp.org>
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
Is that a negative excursion at the bottom of the waveform? now low -
looks like -0.3v to me - With '"logic, that will likely turn on the
protection diodes on the inputs, making them warm.
Double check where the ground actually is on the CRO display.
It's also interesting that the input doesn't even reach 4v, let alone 5v.
Whats driving that?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Sat, 29 Nov 2025 at 08:52, Rob Jarratt <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> > wrote:
I checked the CLK input for the 74S299 with the 74S299 removed and the
signal is still spiky and looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299-clk-…
I removed the other load on this signal, the DC012, and the CLK signal
still looked spiky. I also put back the 74S299 (now socketed) with DC012
removed and it still gets hot. I tried another new 74S299 just in case I
had a bad one and that also got hot without a load on the output.
I am thinking that it is either the spiky input or it is just expected to
get this hot.
Regards
Rob
From: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
Sent: 25 November 2025 01:15
To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk>
Cc: General Discussion: On-Topic and Off-Topic Posts <
cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
The oscilloscope will show you the actual driving waveform.
You can then see the level of the spikes
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Tue, 25 Nov 2025 at 10:48, Rob Jarratt <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> > wrote:
-----Original Message-----
From: Doug Jackson via cctalk <cctalk(a)classiccmp.org <mailto:
cctalk(a)classiccmp.org> >
Sent: 24 November 2025 23:04
To: General Discussion: On-Topic and Off-Topic Posts <
cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
Cc: Doug Jackson <doug(a)doughq.com
<mailto:doug@doughq.com> >
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Another test would be to disconnect the output and see if that runs the
device cooler. Maybe the load, while not a short, is simply too great
for the
device.
I think I can try this, the DC012 it drives is socketed.
But I am leaning towards the inputs containing spikes that need to be
dissipated as heat in clamping diodes. A decent CRO on the device input
pins
while it is out of circuit will show that as
well.
I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
suggesting I try here?
Kindest regards,
Doug Jackson
em: doug(a)doughq.com <mailto:doug@doughq.com>
ph: 0414 986878
On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> > wrote:
> >
> > Sorry if I wasn't clear in my earlier reply, but there were some
> > answers inline. The summary is that Vcc looks fine, there does not
> > seem to be any short on the outputs of the shift register and the
> > chip seems to be
> working
> > correctly. It just gets hot and I wonder if this could be due to the
> > CLK input being a bit spikier on the VT100 than on my VT102? Could
> > this be
> why
> > later revisions of the VT100 introduced an inductor on the DOT CLK
> > output from the DC011?
> >
>
> Perhaps one way to test the hypothesis would be to insert an inductor
> like those used on the later revisions in the DOT CLK output and check
> whether
> a) everything still works and b) the shift register runs cooler?
>
> Regards,
> Peter Coghlan.
>
30 Nov
30 Nov
6:56 a.m.
-----Original Message-----
From: Hans-Ulrich Hölscher via cctalk <cctalk(a)classiccmp.org>
Sent: 29 November 2025 09:17
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Hans-Ulrich Hölscher
<vaxorcist(a)googlemail.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Hi Rob,
this is not intended to indicate the cause or solution, but merely to help
analyze how critical your IC temperature is: Did you actually measure the
temperature of the IC? I recently bought an inexpensive infrared
thermometer (like:https://www.ebay.co.uk/itm/156810933790) that can
measure the temperature of small areas relatively accurately. I made the
experience that I cannot judge temperatures very well with my fingers.
According to Wikipedia, TTL ICs are specified for temperatures up to
70 °C. This
temperature should obviously be avoided, but my fingers already perceive
anything above about 50 °C as "hot".
I hope this can help a litte!
Yes, I have an IR Thermometer and the chip gets to about 40 celsius. I
realise that this is well below the maximum rating, but I can smell the heat
and it worries me a bit. If I knew this was expected then I would be OK with
it, but it feels anomalous and I have felt the chip to be hotter than this
sometimes. The observation about the negative voltages also concerns me a
bit, although I do have spares for the 74S299 should I need them.
It looks now like the DC011 is just producing a DOT CLK output with negative
spikes. Maybe this is why later revisions introduced an inductor or maybe it
is just a bad DC011 or something is wrong with what is around the DC011.
I am going to look at the DC011 inputs to see if anything looks amiss. The
other thing I would like to do is to see what happens if I can add an
inductor to the output of the DC011. But I don't know how to read the spec
on the printset to know what I might need. The printset says "BEAD, FERRITE
.1380DX.380LG", can anyone tell me what this might mean in terms of an
inductor that I could try?
Thanks
Rob
Ulli
Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk <
cctalk(a)classiccmp.org>gt;:
> Rob,
>
> I have a few silly thoughts, but maybe they can be useful.
> The negative going signal is something I would not be happy with, as
> it may damage the IC (over time).
> TTL does not like negative voltages on their pins.
> Is there between the DC011 (signal generating) and the LS299 (signal
> "user") wiring, or are they on the same PCB?
> In case of wiring, capacitive coupling may cause negative going
voltages.
> Could a clamping diode with an as low as possible
Vf (< 0.3 V) help to
> reduce the negative voltage? It is not a permanent solution, because
> the
> DC011 might be a bit unhappy with that diode, but for a test it might
> help to draw a conclusion.
>
> If all does not help, in getting the LS299 running cooler, consider
> some temperature conductive paste and a small "IC heatsink". Tie the
> heatsink onto the IC with a piece of waxed lacing. The IC can radiate
> more efficiently the generated heat, so that it can run cooler. This
> could drop temperature with some 10 degrees.
>
> Success hunting down this issue,
> Henk
>
> -----Oorspronkelijk bericht-----
> Van: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
> Verzonden: zaterdag 29 november 2025 08:11
> Aan: 'Doug Jackson' <doug(a)doughq.com>om>; rob(a)jarratt.me.uk
> CC: 'General Discussion: On-Topic and Off-Topic Posts' <
> cctalk(a)classiccmp.org>gt;; Rob Jarratt <robert.jarratt(a)ntlworld.com>
> Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
>
> Yes, it is a negative voltage. I measured this by attaching the ground
> lead to the ground connection on the 74S299.
>
>
>
> The DC011 that is producing the CLK signal is socketed, but the one on
> my
> VT102 isnt. I would try swapping them to see what happens, but I
> dont really want to touch a fully working board.
>
>
>
> Thanks
>
>
>
> Rob
>
>
>
> From: Doug Jackson <doug(a)doughq.com>
> Sent: 28 November 2025 22:59
> To: rob(a)jarratt.me.uk
> Cc: General Discussion: On-Topic and Off-Topic Posts <
> cctalk(a)classiccmp.org>
> Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
>
>
>
> Is that a negative excursion at the bottom of the waveform? now low -
> looks like -0.3v to me - With '"logic, that will likely turn on the
> protection diodes on the inputs, making them warm.
>
>
>
> Double check where the ground actually is on the CRO display.
>
>
>
> It's also interesting that the input doesn't even reach 4v, let alone
5v.
> Whats driving that?
>
>
>
>
>
> Kindest regards,
>
>
>
> Doug Jackson
>
>
>
> em: doug(a)doughq.com <mailto:doug@doughq.com>
>
> ph: 0414 986878
>
>
>
>
>
>
>
>
>
> On Sat, 29 Nov 2025 at 08:52, Rob Jarratt <robert.jarratt(a)ntlworld.com
> <mailto:robert.jarratt@ntlworld.com> > wrote:
>
> I checked the CLK input for the 74S299 with the 74S299 removed and the
> signal is still spiky and looks like this:
> https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s299
> -clk-signal.png
>
>
>
> I removed the other load on this signal, the DC012, and the CLK signal
> still looked spiky. I also put back the 74S299 (now socketed) with
> DC012 removed and it still gets hot. I tried another new 74S299 just
> in case I had a bad one and that also got hot without a load on the
output.
>
>
>
> I am thinking that it is either the spiky input or it is just expected
> to get this hot.
>
>
>
> Regards
>
>
>
> Rob
>
>
>
> From: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
> Sent: 25 November 2025 01:15
> To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk>
> Cc: General Discussion: On-Topic and Off-Topic Posts <
> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
> Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
>
>
>
> The oscilloscope will show you the actual driving waveform.
>
>
>
> You can then see the level of the spikes
>
>
>
> Kindest regards,
>
>
>
> Doug Jackson
>
>
>
> em: doug(a)doughq.com <mailto:doug@doughq.com>
>
> ph: 0414 986878
>
>
>
>
>
>
>
>
>
> On Tue, 25 Nov 2025 at 10:48, Rob Jarratt <robert.jarratt(a)ntlworld.com
> <mailto:robert.jarratt@ntlworld.com> > wrote:
>
>
>
> > -----Original Message-----
> > From: Doug Jackson via cctalk <cctalk(a)classiccmp.org <mailto:
> cctalk(a)classiccmp.org> >
> > Sent: 24 November 2025 23:04
> > To: General Discussion: On-Topic and Off-Topic Posts <
> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
> > Cc: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
> > Subject: [cctalk] Re: Hot Video Shift Register on VT100
> >
> > Another test would be to disconnect the output and see if that runs
> > the device cooler. Maybe the load, while not a short, is simply too
> > great
> for the
> > device.
>
> I think I can try this, the DC012 it drives is socketed.
>
> >
> > But I am leaning towards the inputs containing spikes that need to
> > be dissipated as heat in clamping diodes. A decent CRO on the
> > device input
> pins
> > while it is out of circuit will show that as well.
>
> I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
> suggesting I try here?
>
>
> >
> > Kindest regards,
> >
> > Doug Jackson
> >
> > em: doug(a)doughq.com <mailto:doug@doughq.com>
> > ph: 0414 986878
> >
> >
> >
> >
> > On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
> > cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> > wrote:
> >
> > > >
> > > > Sorry if I wasn't clear in my earlier reply, but there were some
> > > > answers inline. The summary is that Vcc looks fine, there does
> > > > not seem to be any short on the outputs of the shift register
> > > > and the chip seems to be
> > > working
> > > > correctly. It just gets hot and I wonder if this could be due to
> > > > the CLK input being a bit spikier on the VT100 than on my VT102?
> > > > Could this be
> > > why
> > > > later revisions of the VT100 introduced an inductor on the DOT
> > > > CLK output from the DC011?
> > > >
> > >
> > > Perhaps one way to test the hypothesis would be to insert an
> > > inductor like those used on the later revisions in the DOT CLK
> > > output and check whether
> > > a) everything still works and b) the shift register runs cooler?
> > >
> > > Regards,
> > > Peter Coghlan.
> > >
>
>
8:30 a.m.
I have traced the clock signal back. The MAS CLK input on the DC011 also has
the negative spikes. This is coming from a 74S157. The input to that is an
oscillator and that too has the negative spikes.
So, it seems the 74S157 is just passing through the inputs.
There is a capacitor across pins 4 and 2 of the oscillator (Vcc and Gnd),
would a failure there cause this kind of negative spike? It seems unlikely
to me, but would be happy to be corrected.
The oscillator is rated at 24.0734MHz. I can find lots rated at exactly
24MHz and I guess the difference in frequency is small enough not to matter,
right?
I will order the part and also a new capacitor for it in case that is where
the fault lies and see.
Thanks
Rob
-----Original Message-----
From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Sent: 30 November 2025 12:57
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: 'Hans-Ulrich Hölscher' <vaxorcist(a)googlemail.com>om>; Rob Jarratt
<robert.jarratt(a)ntlworld.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
realise
-----Original Message-----
From: Hans-Ulrich Hölscher via cctalk <cctalk(a)classiccmp.org>
Sent: 29 November 2025 09:17
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Hans-Ulrich Hölscher
<vaxorcist(a)googlemail.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Hi Rob,
this is not intended to indicate the cause or solution, but merely to
help analyze how critical your IC temperature is: Did you actually
measure the temperature of the IC? I recently bought an inexpensive
infrared thermometer (like:https://www.ebay.co.uk/itm/156810933790)
that can measure the temperature of small areas relatively accurately.
I made the experience that I cannot judge temperatures very well with my
fingers.
According to Wikipedia, TTL ICs are specified for temperatures up to
70 °C. This
temperature should obviously be avoided, but my fingers already
perceive anything above about 50 °C as "hot".
I hope this can help a litte!
Yes, I have an IR Thermometer and the chip gets to about 40 celsius. I that this is well below the maximum rating, but I can
smell the heat and
it
worries me a bit. If I knew this was expected then I
would be OK with it,
but it
feels anomalous and I have felt the chip to be hotter
than this sometimes.
The observation about the negative voltages also concerns me a bit,
although
I do have spares for the 74S299 should I need them.
It looks now like the DC011 is just producing a DOT CLK output with
negative
spikes. Maybe this is why later revisions introduced
an inductor or maybe
it is
just a bad DC011 or something is wrong with what is
around the DC011.
I am going to look at the DC011 inputs to see if anything looks amiss. The
other thing I would like to do is to see what happens if I can add an
inductor
to the output of the DC011. But I don't know how
to read the spec on the
printset to know what I might need. The printset says "BEAD, FERRITE
.1380DX.380LG", can anyone tell me what this might mean in terms of an
inductor that I could try?
Thanks
Rob
VT102?
Ulli
Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk <
cctalk(a)classiccmp.org>gt;:
> Rob,
>
> I have a few silly thoughts, but maybe they can be useful.
> The negative going signal is something I would not be happy with, as
> it may damage the IC (over time).
> TTL does not like negative voltages on their pins.
> Is there between the DC011 (signal generating) and the LS299 (signal
> "user") wiring, or are they on the same PCB?
> In case of wiring, capacitive coupling may cause negative going
voltages.
> Could a clamping diode with an as low as
possible Vf (< 0.3 V) help
> to reduce the negative voltage? It is not a permanent solution,
> because the
> DC011 might be a bit unhappy with that diode, but for a test it
> might help to draw a conclusion.
>
> If all does not help, in getting the LS299 running cooler, consider
> some temperature conductive paste and a small "IC heatsink". Tie the
> heatsink onto the IC with a piece of waxed lacing. The IC can
> radiate more efficiently the generated heat, so that it can run
> cooler. This could drop temperature with some 10 degrees.
>
> Success hunting down this issue,
> Henk
>
> -----Oorspronkelijk bericht-----
> Van: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
> Verzonden: zaterdag 29 november 2025 08:11
> Aan: 'Doug Jackson' <doug(a)doughq.com>om>; rob(a)jarratt.me.uk
> CC: 'General Discussion: On-Topic and Off-Topic Posts' <
> cctalk(a)classiccmp.org>gt;; Rob Jarratt <robert.jarratt(a)ntlworld.com>
> Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
>
> Yes, it is a negative voltage. I measured this by attaching the
> ground lead to the ground connection on the 74S299.
>
>
>
> The DC011 that is producing the CLK signal is socketed, but the one
> on my
> VT102 isnt. I would try swapping them to see what happens, but I
> dont really want to touch a fully working board.
>
>
>
> Thanks
>
>
>
> Rob
>
>
>
> From: Doug Jackson <doug(a)doughq.com>
> Sent: 28 November 2025 22:59
> To: rob(a)jarratt.me.uk
> Cc: General Discussion: On-Topic and Off-Topic Posts <
> cctalk(a)classiccmp.org>
> Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
>
>
>
> Is that a negative excursion at the bottom of the waveform? now low
> - looks like -0.3v to me - With '"logic, that will likely turn on
> the protection diodes on the inputs, making them warm.
>
>
>
> Double check where the ground actually is on the CRO display.
>
>
>
> It's also interesting that the input doesn't even reach 4v, let
> alone
5v.
> Whats driving that?
>
>
>
>
>
> Kindest regards,
>
>
>
> Doug Jackson
>
>
>
> em: doug(a)doughq.com <mailto:doug@doughq.com>
>
> ph: 0414 986878
>
>
>
>
>
>
>
>
>
> On Sat, 29 Nov 2025 at 08:52, Rob Jarratt
> <robert.jarratt(a)ntlworld.com <mailto:robert.jarratt@ntlworld.com> >
wrote:
>
> I checked the CLK input for the 74S299 with the 74S299 removed and
> the signal is still spiky and looks like this:
> https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s2
> 99
> -clk-signal.png
>
>
>
> I removed the other load on this signal, the DC012, and the CLK
> signal still looked spiky. I also put back the 74S299 (now socketed)
> with
> DC012 removed and it still gets hot. I tried another new 74S299 just
> in case I had a bad one and that also got hot without a load on the
output.
>
>
>
> I am thinking that it is either the spiky input or it is just
> expected to get this hot.
>
>
>
> Regards
>
>
>
> Rob
>
>
>
> From: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
> Sent: 25 November 2025 01:15
> To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk>
> Cc: General Discussion: On-Topic and Off-Topic Posts <
> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
> Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
>
>
>
> The oscilloscope will show you the actual driving waveform.
>
>
>
> You can then see the level of the spikes
>
>
>
> Kindest regards,
>
>
>
> Doug Jackson
>
>
>
> em: doug(a)doughq.com <mailto:doug@doughq.com>
>
> ph: 0414 986878
>
>
>
>
>
>
>
>
>
> On Tue, 25 Nov 2025 at 10:48, Rob Jarratt
> <robert.jarratt(a)ntlworld.com <mailto:robert.jarratt@ntlworld.com> >
wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: Doug Jackson via cctalk <cctalk(a)classiccmp.org <mailto:
> > cctalk(a)classiccmp.org> >
> > > Sent: 24 November 2025 23:04
> > > To: General Discussion: On-Topic and Off-Topic Posts <
> > cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
> > > Cc: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
> > > Subject: [cctalk] Re: Hot Video Shift Register on VT100
> > >
> > > Another test would be to disconnect the output and see if that
> > > runs the device cooler. Maybe the load, while not a short, is
> > > simply too great
> > for the
> > > device.
> >
> > I think I can try this, the DC012 it drives is socketed.
> >
> > >
> > > But I am leaning towards the inputs containing spikes that need to
> > > be dissipated as heat in clamping diodes. A decent CRO on the
> > > device input
> > pins
> > > while it is out of circuit will show that as well.
> >
> > I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
> > suggesting I try here?
> >
> >
> > >
> > > Kindest regards,
> > >
> > > Doug Jackson
> > >
> > > em: doug(a)doughq.com <mailto:doug@doughq.com>
> > > ph: 0414 986878
> > >
> > >
> > >
> > >
> > > On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
> > > cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> > wrote:
> > >
> > > > >
> > > > > Sorry if I wasn't clear in my earlier reply, but there were
> > > > > some answers inline. The summary is that Vcc looks fine, there
> > > > > does not seem to be any short on the outputs of the shift
> > > > > register and the chip seems to be
> > > > working
> > > > > correctly. It just gets hot and I wonder if this could be due
> > > > > to the CLK input being a bit spikier on the VT100 than on my
> > > > > Could this be
> > > > why
> > > > > later revisions of the VT100 introduced an inductor on the DOT
> > > > > CLK output from the DC011?
> > > > >
> > > >
> > > > Perhaps one way to test the hypothesis would be to insert an
> > > > inductor like those used on the later revisions in the DOT CLK
> > > > output and check whether
> > > > a) everything still works and b) the shift register runs cooler?
> > > >
> > > > Regards,
> > > > Peter Coghlan.
> > > >
> >
> >
9:24 a.m.
Keep in mind that ringing (artifact caused by the inductance of the
scope probe ground lead) can result in a false display of overshoots...
How short is your ground lead?
-Charles
On 11/30/25 08:30, Rob Jarratt via cctalk wrote:
I have traced the clock signal back. The MAS CLK input
on the DC011 also has
the negative spikes. This is coming from a 74S157. The input to that is an
oscillator and that too has the negative spikes.
So, it seems the 74S157 is just passing through the inputs.
There is a capacitor across pins 4 and 2 of the oscillator (Vcc and Gnd),
would a failure there cause this kind of negative spike? It seems unlikely
to me, but would be happy to be corrected.
The oscillator is rated at 24.0734MHz. I can find lots rated at exactly
24MHz and I guess the difference in frequency is small enough not to matter,
right?
I will order the part and also a new capacitor for it in case that is where
the fault lies and see.
Thanks
Rob
-----Original Message-----
From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Sent: 30 November 2025 12:57
To: 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org>
Cc: 'Hans-Ulrich Hölscher' <vaxorcist(a)googlemail.com>om>; Rob Jarratt
<robert.jarratt(a)ntlworld.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
realise
-----Original Message-----
From: Hans-Ulrich Hölscher via cctalk <cctalk(a)classiccmp.org>
Sent: 29 November 2025 09:17
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Hans-Ulrich Hölscher
<vaxorcist(a)googlemail.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Hi Rob,
this is not intended to indicate the cause or solution, but merely to
help analyze how critical your IC temperature is: Did you actually
measure the temperature of the IC? I recently bought an inexpensive
infrared thermometer (like:https://www.ebay.co.uk/itm/156810933790)
that can measure the temperature of small areas relatively accurately.
I made the experience that I cannot judge temperatures very well with my
fingers.
According to Wikipedia, TTL ICs are specified for
temperatures up to
70 °C. This
temperature should obviously be avoided, but my fingers already
perceive anything above about 50 °C as "hot".
I hope this can help a litte!
Yes, I have an IR Thermometer and the chip gets to
about 40 celsius. I that this is well below the maximum rating, but I
can smell the heat and
it
worries me a bit. If I knew this was expected
then I would be OK with it,
but it
feels anomalous and I have felt the chip to be
hotter than this sometimes.
The observation about the negative voltages also concerns me a bit,
although
I do have spares for the 74S299 should I need
them.
It looks now like the DC011 is just producing a DOT CLK output with
negative
spikes. Maybe this is why later revisions
introduced an inductor or maybe
it is
just a bad DC011 or something is wrong with what
is around the DC011.
I am going to look at the DC011 inputs to see if anything looks amiss. The
other thing I would like to do is to see what happens if I can add an
inductor
to the output of the DC011. But I don't know
how to read the spec on the
printset to know what I might need. The printset says "BEAD, FERRITE
.1380DX.380LG", can anyone tell me what this might mean in terms of an
inductor that I could try?
Thanks
Rob
VT102?
>>>>>> Could this be
>>>>> why
>>>>>> later revisions of the VT100 introduced an inductor on the DOT
>>>>>> CLK output from the DC011?
>>>>>>
>>>>> Perhaps one way to test the hypothesis would be to insert an
>>>>> inductor like those used on the later revisions in the DOT CLK
>>>>> output and check whether
>>>>> a) everything still works and b) the shift register runs cooler?
>>>>>
>>>>> Regards,
>>>>> Peter Coghlan.
>>>>>
>>> Ulli
Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk <
cctalk(a)classiccmp.org>gt;:
> Rob,
>
> I have a few silly thoughts, but maybe they can be useful.
> The negative going signal is something I would not be happy with, as
> it may damage the IC (over time).
> TTL does not like negative voltages on their pins.
> Is there between the DC011 (signal generating) and the LS299 (signal
> "user") wiring, or are they on the same PCB?
> In case of wiring, capacitive coupling may cause negative going
voltages.
> Could a clamping diode with an as low as
possible Vf (< 0.3 V) help
> to reduce the negative voltage? It is not a permanent solution,
> because the
> DC011 might be a bit unhappy with that diode, but for a test it
> might help to draw a conclusion.
>
> If all does not help, in getting the LS299 running cooler, consider
> some temperature conductive paste and a small "IC heatsink". Tie the
> heatsink onto the IC with a piece of waxed lacing. The IC can
> radiate more efficiently the generated heat, so that it can run
> cooler. This could drop temperature with some 10 degrees.
>
> Success hunting down this issue,
> Henk
>
> -----Oorspronkelijk bericht-----
> Van: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
> Verzonden: zaterdag 29 november 2025 08:11
> Aan: 'Doug Jackson' <doug(a)doughq.com>om>; rob(a)jarratt.me.uk
> CC: 'General Discussion: On-Topic and Off-Topic Posts' <
> cctalk(a)classiccmp.org>gt;; Rob Jarratt <robert.jarratt(a)ntlworld.com>
> Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
>
> Yes, it is a negative voltage. I measured this by attaching the
> ground lead to the ground connection on the 74S299.
>
>
>
> The DC011 that is producing the CLK signal is socketed, but the one
> on my
> VT102 isnt. I would try swapping them to see what happens, but I
> dont really want to touch a fully working board.
>
>
>
> Thanks
>
>
>
> Rob
>
>
>
> From: Doug Jackson <doug(a)doughq.com>
> Sent: 28 November 2025 22:59
> To: rob(a)jarratt.me.uk
> Cc: General Discussion: On-Topic and Off-Topic Posts <
> cctalk(a)classiccmp.org>
> Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
>
>
>
> Is that a negative excursion at the bottom of the waveform? now low
> - looks like -0.3v to me - With '"logic, that will likely turn on
> the protection diodes on the inputs, making them warm.
>
>
>
> Double check where the ground actually is on the CRO display.
>
>
>
> It's also interesting that the input doesn't even reach 4v, let
> alone
5v.
> Whats driving that?
>
>
>
>
>
> Kindest regards,
>
>
>
> Doug Jackson
>
>
>
> em: doug(a)doughq.com <mailto:doug@doughq.com>
>
> ph: 0414 986878
>
>
>
>
>
>
>
>
>
> On Sat, 29 Nov 2025 at 08:52, Rob Jarratt
> <robert.jarratt(a)ntlworld.com <mailto:robert.jarratt@ntlworld.com> >
wrote:
> I checked the CLK input for the 74S299 with
the 74S299 removed and
> the signal is still spiky and looks like this:
> https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s2
> 99
> -clk-signal.png
>
>
>
> I removed the other load on this signal, the DC012, and the CLK
> signal still looked spiky. I also put back the 74S299 (now socketed)
> with
> DC012 removed and it still gets hot. I tried another new 74S299 just
> in case I had a bad one and that also got hot without a load on the
output.
>
>
> I am thinking that it is either the spiky input or it is just
> expected to get this hot.
>
>
>
> Regards
>
>
>
> Rob
>
>
>
> From: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
> Sent: 25 November 2025 01:15
> To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk>
> Cc: General Discussion: On-Topic and Off-Topic Posts <
> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
> Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
>
>
>
> The oscilloscope will show you the actual driving waveform.
>
>
>
> You can then see the level of the spikes
>
>
>
> Kindest regards,
>
>
>
> Doug Jackson
>
>
>
> em: doug(a)doughq.com <mailto:doug@doughq.com>
>
> ph: 0414 986878
>
>
>
>
>
>
>
>
>
> On Tue, 25 Nov 2025 at 10:48, Rob Jarratt
> <robert.jarratt(a)ntlworld.com <mailto:robert.jarratt@ntlworld.com> >
wrote:
>>
>>
>>> -----Original Message-----
>>> From: Doug Jackson via cctalk <cctalk(a)classiccmp.org <mailto:
>> cctalk(a)classiccmp.org> >
>>> Sent: 24 November 2025 23:04
>>> To: General Discussion: On-Topic and Off-Topic Posts <
>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>> Cc: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com> >
>>> Subject: [cctalk] Re: Hot Video Shift Register on VT100
>>>
>>> Another test would be to disconnect the output and see if that
>>> runs the device cooler. Maybe the load, while not a short, is
>>> simply too great
>> for the
>>> device.
>> I think I can try this, the DC012 it drives is socketed.
>>
>>> But I am leaning towards the inputs containing spikes that need to
>>> be dissipated as heat in clamping diodes. A decent CRO on the
>>> device input
>> pins
>>> while it is out of circuit will show that as well.
>> I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
>> suggesting I try here?
>>
>>
>>> Kindest regards,
>>>
>>> Doug Jackson
>>>
>>> em: doug(a)doughq.com <mailto:doug@doughq.com>
>>> ph: 0414 986878
>>>
>>>
>>>
>>>
>>> On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> > wrote:
>>>
>>>>> Sorry if I wasn't clear in my earlier reply, but there were
>>>>> some answers inline. The summary is that Vcc looks fine, there
>>>>> does not seem to be any short on the outputs of the shift
>>>>> register and the chip seems to be
>>>> working
>>>>> correctly. It just gets hot and I wonder if this could be due
>>>>> to the CLK input being a bit spikier on the VT100 than on my
10:23 a.m.
-----Original Message-----
From: Charles Morris via cctalk <cctalk(a)classiccmp.org>
Sent: 30 November 2025 15:24
To: cctalk(a)classiccmp.org
Cc: Charles Morris <charlesmorris800(a)protonmail.com>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Keep in mind that ringing (artifact caused by the inductance of the scope
probe ground lead) can result in a false display of overshoots...
How short is your ground lead?
Quite short. But what I saw is the exact same shape as the input to the chip that gets hot
and measuring the same chip on a VT102 I don't see this. So I am fairly sure this is
real.
-Charles
On 11/30/25 08:30, Rob Jarratt via cctalk wrote:
> I have traced the clock signal back. The MAS CLK input on the DC011
> also has the negative spikes. This is coming from a 74S157. The input
> to that is an oscillator and that too has the negative spikes.
>
> So, it seems the 74S157 is just passing through the inputs.
>
> There is a capacitor across pins 4 and 2 of the oscillator (Vcc and
> Gnd), would a failure there cause this kind of negative spike? It
> seems unlikely to me, but would be happy to be corrected.
>
> The oscillator is rated at 24.0734MHz. I can find lots rated at
> exactly 24MHz and I guess the difference in frequency is small enough
> not to matter, right?
>
> I will order the part and also a new capacitor for it in case that is
> where the fault lies and see.
>
> Thanks
>
> Rob
>
>
>> -----Original Message-----
>> From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
>> Sent: 30 November 2025 12:57
>> To: 'General Discussion: On-Topic and Off-Topic Posts'
>> <cctalk(a)classiccmp.org>
>> Cc: 'Hans-Ulrich Hölscher' <vaxorcist(a)googlemail.com>om>; Rob Jarratt
>> <robert.jarratt(a)ntlworld.com>
>> Subject: [cctalk] Re: Hot Video Shift Register on VT100
>>
>>> -----Original Message-----
>>> From: Hans-Ulrich Hölscher via cctalk <cctalk(a)classiccmp.org>
>>> Sent: 29 November 2025 09:17
>>> To: General Discussion: On-Topic and Off-Topic Posts
>> <cctalk(a)classiccmp.org>
>>> Cc: Hans-Ulrich Hölscher <vaxorcist(a)googlemail.com>
>>> Subject: [cctalk] Re: Hot Video Shift Register on VT100
>>>
>>> Hi Rob,
>>> this is not intended to indicate the cause or solution, but merely
>>> to help analyze how critical your IC temperature is: Did you
>>> actually measure the temperature of the IC? I recently bought an
>>> inexpensive infrared thermometer
>>> (like:https://www.ebay.co.uk/itm/156810933790)
>>> that can measure the temperature of small areas relatively accurately.
>>> I made the experience that I cannot judge temperatures very well
>>> with my
>> fingers.
>>> According to Wikipedia, TTL ICs are specified for temperatures up to
>>> 70 °C. This
>>> temperature should obviously be avoided, but my fingers already
>>> perceive anything above about 50 °C as "hot".
>>>
>>> I hope this can help a litte!
>> Yes, I have an IR Thermometer and the chip gets to about 40 celsius.
>> I
> realise
>> that this is well below the maximum rating, but I can smell the heat
>> and
> it
>> worries me a bit. If I knew this was expected then I would be OK with
>> it,
> but it
>> feels anomalous and I have felt the chip to be hotter than this sometimes.
>> The observation about the negative voltages also concerns me a bit,
> although
>> I do have spares for the 74S299 should I need them.
>>
>> It looks now like the DC011 is just producing a DOT CLK output with
> negative
>> spikes. Maybe this is why later revisions introduced an inductor or
>> maybe
> it is
>> just a bad DC011 or something is wrong with what is around the DC011.
>>
>> I am going to look at the DC011 inputs to see if anything looks
>> amiss. The other thing I would like to do is to see what happens if I
>> can add an
> inductor
>> to the output of the DC011. But I don't know how to read the spec on
>> the printset to know what I might need. The printset says "BEAD,
>> FERRITE .1380DX.380LG", can anyone tell me what this might mean in
>> terms of an inductor that I could try?
>>
>> Thanks
>>
>> Rob
>>
>>
>>> Ulli
>>>
>>> Am Sa., 29. Nov. 2025 um 09:44 Uhr schrieb Henk Gooijen via cctalk <
>>> cctalk(a)classiccmp.org>gt;:
>>>
>>>> Rob,
>>>>
>>>> I have a few silly thoughts, but maybe they can be useful.
>>>> The negative going signal is something I would not be happy with,
>>>> as it may damage the IC (over time).
>>>> TTL does not like negative voltages on their pins.
>>>> Is there between the DC011 (signal generating) and the LS299
>>>> (signal
>>>> "user") wiring, or are they on the same PCB?
>>>> In case of wiring, capacitive coupling may cause negative going
>> voltages.
>>>> Could a clamping diode with an as low as possible Vf (< 0.3 V) help
>>>> to reduce the negative voltage? It is not a permanent solution,
>>>> because the
>>>> DC011 might be a bit unhappy with that diode, but for a test it
>>>> might help to draw a conclusion.
>>>>
>>>> If all does not help, in getting the LS299 running cooler, consider
>>>> some temperature conductive paste and a small "IC heatsink".
Tie
>>>> the heatsink onto the IC with a piece of waxed lacing. The IC can
>>>> radiate more efficiently the generated heat, so that it can run
>>>> cooler. This could drop temperature with some 10 degrees.
>>>>
>>>> Success hunting down this issue,
>>>> Henk
>>>>
>>>> -----Oorspronkelijk bericht-----
>>>> Van: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
>>>> Verzonden: zaterdag 29 november 2025 08:11
>>>> Aan: 'Doug Jackson' <doug(a)doughq.com>om>; rob(a)jarratt.me.uk
>>>> CC: 'General Discussion: On-Topic and Off-Topic Posts' <
>>>> cctalk(a)classiccmp.org>gt;; Rob Jarratt
<robert.jarratt(a)ntlworld.com>
>>>> Onderwerp: [cctalk] Re: Hot Video Shift Register on VT100
>>>>
>>>> Yes, it is a negative voltage. I measured this by attaching the
>>>> ground lead to the ground connection on the 74S299.
>>>>
>>>>
>>>>
>>>> The DC011 that is producing the CLK signal is socketed, but the one
>>>> on my
>>>> VT102 isn’t. I would try swapping them to see what happens, but I
>>>> don’t really want to touch a fully working board.
>>>>
>>>>
>>>>
>>>> Thanks
>>>>
>>>>
>>>>
>>>> Rob
>>>>
>>>>
>>>>
>>>> From: Doug Jackson <doug(a)doughq.com>
>>>> Sent: 28 November 2025 22:59
>>>> To: rob(a)jarratt.me.uk
>>>> Cc: General Discussion: On-Topic and Off-Topic Posts <
>>>> cctalk(a)classiccmp.org>
>>>> Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
>>>>
>>>>
>>>>
>>>> Is that a negative excursion at the bottom of the waveform? now
>>>> low
>>>> - looks like -0.3v to me - With '"logic, that will likely turn
on
>>>> the protection diodes on the inputs, making them warm.
>>>>
>>>>
>>>>
>>>> Double check where the ground actually is on the CRO display.
>>>>
>>>>
>>>>
>>>> It's also interesting that the input doesn't even reach 4v, let
>>>> alone
>> 5v.
>>>> Whats driving that?
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Kindest regards,
>>>>
>>>>
>>>>
>>>> Doug Jackson
>>>>
>>>>
>>>>
>>>> em: doug(a)doughq.com <mailto:doug@doughq.com>
>>>>
>>>> ph: 0414 986878
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Sat, 29 Nov 2025 at 08:52, Rob Jarratt
>>>> <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> >
>> wrote:
>>>> I checked the CLK input for the 74S299 with the 74S299 removed and
>>>> the signal is still spiky and looks like this:
>>>> https://rjarratt.wordpress.com/wp-content/uploads/2025/11/vt102-74s
>>>> 2
>>>> 99
>>>> -clk-signal.png
>>>>
>>>>
>>>>
>>>> I removed the other load on this signal, the DC012, and the CLK
>>>> signal still looked spiky. I also put back the 74S299 (now
>>>> socketed) with
>>>> DC012 removed and it still gets hot. I tried another new 74S299
>>>> just in case I had a bad one and that also got hot without a load
>>>> on the
>> output.
>>>>
>>>>
>>>> I am thinking that it is either the spiky input or it is just
>>>> expected to get this hot.
>>>>
>>>>
>>>>
>>>> Regards
>>>>
>>>>
>>>>
>>>> Rob
>>>>
>>>>
>>>>
>>>> From: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com>
>
>>>> Sent: 25 November 2025 01:15
>>>> To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk>
>>>> Cc: General Discussion: On-Topic and Off-Topic Posts <
>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>>> Subject: Re: [cctalk] Re: Hot Video Shift Register on VT100
>>>>
>>>>
>>>>
>>>> The oscilloscope will show you the actual driving waveform.
>>>>
>>>>
>>>>
>>>> You can then see the level of the spikes
>>>>
>>>>
>>>>
>>>> Kindest regards,
>>>>
>>>>
>>>>
>>>> Doug Jackson
>>>>
>>>>
>>>>
>>>> em: doug(a)doughq.com <mailto:doug@doughq.com>
>>>>
>>>> ph: 0414 986878
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Tue, 25 Nov 2025 at 10:48, Rob Jarratt
>>>> <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> >
>> wrote:
>>>>
>>>>
>>>>> -----Original Message-----
>>>>> From: Doug Jackson via cctalk <cctalk(a)classiccmp.org <mailto:
>>>> cctalk(a)classiccmp.org> >
>>>>> Sent: 24 November 2025 23:04
>>>>> To: General Discussion: On-Topic and Off-Topic Posts <
>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
>>>>> Cc: Doug Jackson <doug(a)doughq.com <mailto:doug@doughq.com>
>
>>>>> Subject: [cctalk] Re: Hot Video Shift Register on VT100
>>>>>
>>>>> Another test would be to disconnect the output and see if that
>>>>> runs the device cooler. Maybe the load, while not a short, is
>>>>> simply too great
>>>> for the
>>>>> device.
>>>> I think I can try this, the DC012 it drives is socketed.
>>>>
>>>>> But I am leaning towards the inputs containing spikes that need to
>>>>> be dissipated as heat in clamping diodes. A decent CRO on the
>>>>> device input
>>>> pins
>>>>> while it is out of circuit will show that as well.
>>>> I guess CRO means Cathode Ray Oscilloscope? Not sure what you are
>>>> suggesting I try here?
>>>>
>>>>
>>>>> Kindest regards,
>>>>>
>>>>> Doug Jackson
>>>>>
>>>>> em: doug(a)doughq.com <mailto:doug@doughq.com>
>>>>> ph: 0414 986878
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Tue, 25 Nov 2025 at 09:52, Peter Coghlan via cctalk <
>>>>> cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
wrote:
>>>>>
>>>>>>> Sorry if I wasn't clear in my earlier reply, but there
were some
>>>>>>> answers inline. The summary is that Vcc looks fine, there
does
>>>>>>> not seem to be any short on the outputs of the shift
register
>>>>>>> and the chip seems to be
>>>>>> working
>>>>>>> correctly. It just gets hot and I wonder if this could be
due to
>>>>>>> the CLK input being a bit spikier on the VT100 than on my
> VT102?
>>>>>>> Could this be
>>>>>> why
>>>>>>> later revisions of the VT100 introduced an inductor on the
DOT
>>>>>>> CLK output from the DC011?
>>>>>>>
>>>>>> Perhaps one way to test the hypothesis would be to insert an
>>>>>> inductor like those used on the later revisions in the DOT CLK
>>>>>> output and check whether
>>>>>> a) everything still works and b) the shift register runs cooler?
>>>>>>
>>>>>> Regards,
>>>>>> Peter Coghlan.
>>>>>>
>>>>
24 Nov
24 Nov
5:05 p.m.
I would scope it , Then pull vcc and check current, what is hot? check
resistance across the chip, maybe internal shorts?
Sorry if seggested and answered, TL/DR all of it.
On Mon, Nov 24, 2025 at 2:52 PM Peter Coghlan via cctalk <
cctalk(a)classiccmp.org> wrote:
Sorry if I wasn't clear in my earlier reply, but there were some answers
inline. The summary is that Vcc looks fine, there does not seem to be any
short on the outputs of the shift register and the chip seems to be
working
correctly. It just gets hot and I wonder if this
could be due to the CLK
input being a bit spikier on the VT100 than on my VT102? Could this be
why
later revisions of the VT100 introduced an
inductor on the DOT CLK output
from the DC011?
Perhaps one way to test the hypothesis would be to insert an inductor like
those used on the later revisions in the DOT CLK output and check whether
a) everything still works and b) the shift register runs cooler?
Regards,
Peter Coghlan.
5:42 p.m.
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 24 November 2025 22:41
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Hot Video Shift Register on VT100
Yes, I did think of doing this, but I don't like the idea of breaking a
track for this purpose if the outcome is uncertain. I guess maybe even if
this does work the circuit was presumably designed to work fairly hot,
although perhaps the revision added the inductor to reduce the heat?
I will consider it though, but I am not too sure what kind of part to use.
If I have read it correctly the revised print set says "BEAD, FERRITE
.1380DX.380LG", I am not quite sure that this means, although I am sure I
can get ferrite cores. Does that description mean anything to anyone?
Sorry if I wasn't clear in my earlier reply, but there were some
answers inline. The summary is that Vcc looks fine, there does not
seem to be any short on the outputs of the shift register and the chip
seems to be working correctly. It just gets hot and I wonder if this
could be due to the CLK input being a bit spikier on the VT100 than on
my VT102? Could this be why later revisions of the VT100 introduced an
inductor on the DOT CLK output from the DC011?
Perhaps one way to test the hypothesis would be to insert an inductor like
those used on the later revisions in the DOT CLK output and check whether
a) everything still works and b) the shift register runs cooler?
Regards,
Peter Coghlan.
30 Nov
30 Nov
11:53 a.m.
On Mon, Nov 17, 2025 at 12:51 AM Rob Jarratt via cctalk
<cctalk(a)classiccmp.org> wrote:
I have a VT100 that I was working on a while back and
then set aside for a
bit. I have started to look at it again. I have noticed that the Video Shift
Register, a 74S299, gets very hot, I can smell the heat and the chip gets
almost too hot to touch, reaching almost 40 celsius.
What if you tried replacing it with a 74F299?
Ought to be like half the power dissipation and at least as fast.
Unless the sharper edges cause issues.
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