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I have to say I don't know or don't remember as it was quite a long time ago.

I have a DECstation 5000/240 which had a PSU problem. I don't know if the 200 uses the same PSU, but take a look at these blog posts of mine to see if they help at all: https://robs-old-computers.com/category/retro-computing/decstation-5000-240/ Regards Rob

Hello again. I haven't been able to look at the VT100 again until today. I decided to try a regular PC PSU supplying the VT100 video board, just to exclude any possibility that the VT100 PSU might be causing any issues with the over/undershoots. I know I was told the PSU would not be the problem...

I think Henk is right, the box is just terrible. It would be good to take a closer look and see what is salvageable. Cirencester is quite a way from me though, not sure I would make the journey specially given the condition of the machine.

Yes I calibrated the probe by connecting it to the two contacts on the scope designed for this purpose. I used a plastic tool to turn the dial until the square wave was perfectly square. Only then did I try to measure the signal on the chip. I tried to keep the length of the ground as short as I...

I checked my probe against the scope's calibration output and used a test clip so that I could connect the probe ground wire directly to the chip's ground pin. This is what I got: So I think the under/overshoot is genuine. How do I go about finding the cause?

That might be a good idea, but I would prefer to get to the cause of the excessive heat really (if I can!)

Thanks Alon. I am using a 1:10 probe. I am pretty confident that the probe I was using was calibrated using the scope output, and I was using the same probe on the good VT102 as the bad VT100. However, I do know that I would not have connected the probe ground wire to the chip's ground pin. So...

I just measured 5.1V on Vcc to the chip. This chip is a recent replacement for the original one because I thought it might be faulty, so it shouldn't be a bad part if both of them get hot. A few posts back there was discussion about the under/overshoot because there is some of that going on...

There is certainly the space to add a heatsink, but I think there must be an underlying problem causing it to get so hot in the first place. As noted in my last reply, the same chip in a VT102 does not get anything like as hot. Maybe I could add a heatsink anyway, just to try to help it out.

I did wonder if it might be normal, but I checked on a working VT102 and the same chip does not get anything like as hot. I can smell the heat and it is too hot to touch. I have been trying to use my multimeter to measure the temperature as it has a temperature function. I don't know if I am...

Well @bigfix sent me two DC012s. I took the old one out, soldered in a socket and installed one of the "new" chips. As you can see below, the external video now works! :) I now need to reassemble the monitor board and see if the flyback is OK, but I have a strong suspicion that it has failed...

I have been trying to find these parts in the UK but I have not been able to find them. Maybe other similar fasteners would work. Maybe once I get the VT100 working again I will try to find them.

I have been looking at this further and comparing my faulty VT100 with a working VT102. I suspect that the DC012 chip may be faulty in that the scan count outputs (SC0-SC3) seem to be only ever all zeroes or all ones. On the working VT102 I can see these outputs count up from 0-8 then F then...

I hadn't considered that it would be the first row of pixels, only the first row of characters. When I use characters that have pixels at the top then I do see them repeated all the way down the screen. What is curious though is that it isn't just characters like the vertical bar, which has a...

Thanks for the suggestions, I will go through them in the coming days. I did think that it could be repeating the first line over and over, but I don't think that is the case, because when I type letters in local mode the full-height cursor moves across the screen, but no letters appear. It...

Looking at the schematic, the Feb82 printset has an inductor (L8) on the DOT CLK H output from the DC011, it is the output of L8 which drives the CLK input of the shift register (the blue trace in the scope screenshots above). However, my board does not have L8, and I see that the Mar80 version...

Finding any of those sounds difficult, but I guess I will need to check them. There is a cap across Vcc-Gnd on the shift register which I can check.

Thanks Alon, but I used the same probes and same grounds in both cases. I think I have noticed generally spikier signals on the faulty machine than on the good one. I checked the PSU output capacitors and they seemed OK, but maybe I should check again?

I replaced the 74S299 with a new one but the fault did not go away and the new one also gets very hot. I have checked the inputs to the shift register. I get the following signals: The blue signal is BV4 DOT CLK and the yellow signal is BV4 VSR LOAD H. By comparison the same signals on a...