Great, my VT52 is shot.

Here's a video of it with sound on. https://i.imgur.com/X1qVYGP.mp4

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 at classiccmp.org <mailto:cctalk at 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 > >