> We had a VT52 do something akin to that once back in the day... The tech > we had that worked on this stuff said he had to fix something in the > vertical refresh circuit to achieve vertical hold... My vague memories > likely won't help you much, but this is quite distinctive and others of the > group will be able to suggest things :) >
Vertical hold implies that there is an oscillator that need to get into hold or lock with an incoming signal. There is none in a VT52 so that is not possible. There is simply no way to adjust vertical hold in a VT52. /Mattis > Warner > > >> >> 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 >> > >> >
