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