18 Apr
2020
18 Apr
'20
8:35 p.m.
On Sat, Apr 18, 2020, 1:01 PM Mattis Lind <mattislind at gmail.com> wrote:
Told you my memory sucked...
Warner
/Mattis
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.
> 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
>> >
>>
>