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Thread: PET 9" Monitor, losing vertical deflection randomly

  1. #1
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    Default PET 9" Monitor, losing vertical deflection randomly

    Seems to be losing vertical deflection. Been getting steadily worse over time and seems better when it's cold vs when it's warmed up.
    Was thinking about re-capping but I know there are some caps that require very low ESR and may be hard to find.
    Not sure what I should be looking for at Mouser or DigiKey to be sure I get the right specs.

    Do you see anything in the video that points to a particular component or area?
    Thanks.

    Video Link: https://youtu.be/gECCG7t_qdU

  2. #2

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    Yes I have had something similar. It turned out to be a combination of IC’s in my case not capacitors.
    I replaced D8 (vertical drive) and that temporarily fixed it but then I think E5 failed (screen collapsed to half the size) after that..... from what I recall.

    Andy

  3. #3
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    I'll break out the scope and see what's happening with H & V sync when the picture jumps.
    I didn't consider a problem with the digital side since I'd expect it to work or just not work at all.

  4. #4
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    Are you losing the vertical drive signal (60 Hz) from the mainboard? If so, it is generated by some complicated sequencer logic (see middle of sheet 3) composed of flip flops and counter chips and a failure of any of those chips can cause those symptoms. If the vertical drive is steady then the problem will be in the video board. The horizontal signal circuit is much simpler and is usually OK in the main board.

  5. #5

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    In a similar case I had, it was a marginal CR8 on the monitor's PCB. It might also be some intermittent solder joint on either (or both) the main PET board or the monitor board.
    In any case, it can't be easily solved without checking where the fault lies. A logic probe could help understanding if the fault is caused by loss of the vertical drive from the mainboard. An oscilloscope would help finding the fault wherever it lies.

    Frank

    PS: I think also a cracked R20 did a similar effect on another of those monitors.
    Last edited by iz8dwf; September 27th, 2020 at 12:44 PM. Reason: typo

  6. #6
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    I hooked the scope up to the vertical sync pin on the logic board.
    Normally the vertical pin is 5v with negative pulses to ground at 60Hz.
    I watched it until it warmed up enough to start doing it and when the picture collapses vertically, the pulses stop and it's a steady 5v
    but sometimes it drops to a steady ground level and when that happens there's no picture at all.
    So I guess the problem is on the logic board in the area around D8 or B6.

    I assumed it was something in the CRT so I was looking there.
    Thanks for the re-direction.

    p.s. it does it even with the monitor unplugged, so it's not being dragged down by something in the CRT.

    I guess I'll start tracing signals back from v-sync.

  7. #7
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    Quote Originally Posted by Hutch View Post
    So I guess the problem is on the logic board in the area around D8 or B6.
    Exactly right. Unfortunately when a state sequencer stops, it is hard to tell cause from effect.

    You will have to go down the counter outputs from chips D5, D6 and D7 looking for a struck output. Try to ignore chip E6 (74100) for the moment. It holds the starting address of character rows (i.e, decimal 40, 80 120) so they can be repeated for eight scan lines to form a character row. Lets hope right now that the 74100 is not the problem as it is confusing. It is probably a bad counter or logic gate that is causing the problem. The 74177 counters are high failure items.

  8. #8
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    Strangeness. My scope kinds sucks. It's a very old 20MHz analog scope so it may not be fast enough for me to see what's happening but ....

    I can't see anything on the Q output (pin3) of C7. Nada. But I do see toggling on Q-not. That output drives the flip-flops at B6 so I know there has to be a signal there.
    It's tricky waiting for something to happen.
    I was looking at the Q output (pin3) of the flip-flop at B6 and I was seeing the normal 60(ish) Hz pattern there while the picture was stable and when it finally collapsed, I was getting something MUCH faster. Like 250KHz.
    I don't know what frequency would normally be clocking that flip-flop but could it fail in a way that passes it's clock through or is something else down the line running faster?
    I can't figure out why I can't see any signal on pin 3 of C7 or pin 12 of B6. I probably just need a new scope.

    Also note: When the picture collapses, I don't lose H-Sync or video.

    schematic.JPG

  9. #9
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    The C7 pin 3 flip flop output is a key signal to clock the two B6 state sequencer flip flops. When it stops, it usually means the sequencer is struck due to a problem somewhere in conditions leading to the next step. However if the Q/ is toggling and the Q is not, let's hope that you found the problem and the C7 is bad. Change it!

  10. #10
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    The four states of the B6 flip flops are listed on the top of the page next to B6. When things are working it takes 16.66 mS to go through the four states which gives the Vertical Drive 60 Hz signal. The counters I mentioned previously are used to determine the timing of the states and also are used as addresses for the video memory refresh circuitry.

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