On Sat, Apr 18, 2020, 1:01 PM Mattis Lind <[email protected]> wrote:
> > > > >> 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. > Told you my memory sucked... Warner /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 >>> > <[email protected] <mailto:[email protected]>>: >>> > >>> > 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 >>> > >>> >>
