Great, my VT52 is shot.

Chris Zach cz at
Sat Apr 18 11:46:55 CDT 2020

Here's a video of it with sound on.


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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