[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