Great, my VT52 is shot.

[Mattis Lind]

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>:

> 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
>