Great, my VT52 is shot.

[Warner Losh]

On Sat, Apr 18, 2020, 1:01 PM Mattis Lind <mattislind at gmail.com> 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
>>> > <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
>>> >
>>>
>>