24 Nov
2025
24 Nov
'25
2:28 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
>