Great, my VT52 is shot.

Mattis Lind mattislind at
Sat Apr 18 13:52:17 CDT 2020

lördag 18 april 2020 skrev Chris Zach <cz at>:

> Here's a video of it with sound on.

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


> 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): 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 <mailto:cctalk at>>:
>>     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

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