lördag 18 april 2020 skrev Chris Zach <c...@alembic.crystel.com>: > Here's a video of it with sound on. > > https://i.imgur.com/X1qVYGP.mp4
You have video even in the horisontal retrace. I still think you have a problem somewhere around the video flip flop. Check that part of the circuit. /Mattis > > C > > On 4/18/2020 4:45 AM, Mattis Lind wrote: > >> Hello Chris! >> >> Sorry that your trusty VT52 failed. But it shouldn't be too hard to >> repair it! >> >> I have successfully restored a VT52 (actually a VT50 with a brain >> transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is >> that the capacitors were in good shape. Especially the bigger filter >> capacitor. I think that DEC used great quality capacitors. >> >> It is always a good idea to check the voltages so that they are within >> the spec. But it should not be necessary to replace all the capacitors in >> my opinion. >> >> The VT52 is a quite interesting design with a very simplistic CPU built >> from TTL components and a microprogram that has 1024 by 8 bits. Most of the >> operations inside it is controlled from this program. In mine the sockets >> for the mikroprogram were bad which meant that it failed to do anything. >> Then I had a couple of gates that were bad. >> >> Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk < >> cctalk@classiccmp.org <mailto:cctalk@classiccmp.org>>: >> >> Wonderful: A few weeks ago I forgot to turn off my VT52 and left it >> running for a day or two. Now the screen is filled with snow and it >> looks like the text is all over the place horozontally. >> >> >> It would be great to have a picture since it could give some hints on >> what is wrong with it. My understanding is that the scan is working both >> horizontally and vertically. Otherwise you would only have a straight line. >> Unlike many common CRTs this CRT circuitry has no oscillator in it that >> creates a picture without input. And since the horisontal >> deflection circuit also creates the high voltage there would be no picture >> at all unless the horisontal signal is there. >> >> This means that the two control boards in the bottom creates some kind of >> horisontal and vertical signals. These signals comes from a simple divider >> chain. There could be some kind of problem in the divider chain. But then >> it would be more likely to not work at all if one of the counters are bad. >> But it is a good idea to check the H and V signal som they are inline with >> the spec. Around 15kHz and 60 or 50 Hz respectively. >> >> Since I haven't seen how the output looks like I speculating a bit. It >> could be the case that the divider chain is correct but the video signal is >> not generated in sync with the divider chain. Then the characters would end >> up all over the place on screen. >> >> There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is >> the video flip-flop. It controls when the screen is rendered. It might be >> the case it could cause your problem. I think it is a good idea to check >> the signals around this flip-flop. >> >> I am always a bit skeptical towards 74Hxx. My impression is that they >> fail more often than standard 74xx. >> >> When I repaired my VT52 I made a quick circuit that created a composite >> video signal from the control board. The signal was fed into a standard CRT >> monitor which I had around. I could then run the control boards flat on >> the bench to take measurements rather than assembling prior to each test. I >> thought that it was quite handy. >> >> >> Any tips or thoughts on where to start looking to fix? The keyboard >> seems to be working as does the RS232 input (the snow on the screen >> changes when the pdp11 talks to it) >> >> >> This indicate that since you are using RS232 you have at least some kind >> of voltage level on +12 V and -12 V since otherwise there would be no >> communication. And +5V is probably not that bad either. Since not much >> would operate in that case. Likewise the -5V is probably ok since the char >> rom would not work very well without the bias. >> >> Good luck with your repair! >> >> /Mattis >> >> >> Thanks! >> Chris >> >>