22 Nov
2025
22 Nov
'25
9:53 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