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Thread: PET 9" Monitor, losing vertical deflection randomly

  1. #21

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    Update: Previous theory was wrong.
    Replaced D8, B6 & C7 and no joy. Must be further up the line.
    Not many other parts I can see that will have an effect on vSync without also affecting HSync and/or Video. C6, E8, E5, maybe D6 & D7.
    It seems my scope is not up to the task of seeing glitches in these signals.

  2. #22
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    Quote Originally Posted by Hutch View Post
    Update:
    Not many other parts I can see that will have an effect on vSync without also affecting HSync and/or Video
    No, there is little circuitry in common between HSync and VSync. VSync is the output of a complicated sequencer and HSync is an output a simple countdown circuit. They only have part of the clock countdown circuit in common.

    It seems my scope is not up to the task of seeing glitches in these signals.
    Why did you replace D8 (pin 6) without looking at its two inputs? I said 'something is forcing the D8-pin 6 NAND gate output to be stuck high'. You should have checked to see what input was LOW and followed that chain. Sorry I was not clearer. Your scope should be able to see a 0.5 uS pulse unless its bandwidth is very low. A 20 MHz scope should be fine. You may be having problems triggering the scope properly as I suggested earlier. Select negative or positive trigger depending on the active edge of the signal and tweak the trigger adjust knob.

    The sequencer is struck and a lot of test points will look bad. We simply need to start looking at the inputs of D8-pin 6 and work back from there. Take scope photos with your phone if possible.
    -Dave
    Last edited by dave_m; September 30th, 2020 at 04:25 PM.

  3. #23

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    A couple of general points might help:

    For very narrow pulses that are difficult to see on some scopes due to the beam intensity being very low during the pulse, it can be deduced that the pulse is in fact there if the scope's time-base has a trigger light. Because the light only comes on with repetitive triggering. (In really good scopes like the 2465B the trigger level is shown on the screen and you can adjust it, until the triggering just stops and therefore work out the amplitude of an invisible super narrow pulse).

    The issue or question often crops up in logic circuits, when a pulse appears to be missing on the output of some IC, whether is might be loaded by the inputs of IC's that the output is feeding. In most cases this is a very unlikely scenario because of the failure modes of the input circuitry on the logic gates. They more likely go open circuit than develop an extremely low resistance to +5V or ground (not impossible but rare). So if a signal is "missing" on a chip's output, much more likely it is the chip's logic conditions at its inputs & resets pins, etc that are incorrect, or in the case of failure (when the chip's inputs are correct), that the output stage of the chip has failed to sink or source current.

    One somewhat diabolical fault I have seen twice recently with failed TTL's is the inputs going open. When this happens , internally in the chip it can behave as though the input is logic high. If its one of the inputs on a gate for example, the gate, say a NAND can then output pulses at times from its other working input, when its not supposed to, and that causes havoc, say if it is in some loop in a counter reset system.

    Intermittent IC behavior (assuming its not the socket connections) can be temperature related. So with an intermittent fault, its worth attempting to cool each suspect chip with freeze spray, in case you can locate it that way. Though having said that I recently had an counter IC that just randomly dropped out one output and it was not affected by temperature at all.
    Last edited by Hugo Holden; September 30th, 2020 at 04:45 PM. Reason: typo

  4. #24

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    Yeah, my focus was on D8 pin 11 and the B6 flip-flops.
    Working back from there. Like I said, my scope wasn't showing me anything on the B6 inputs/C7 output.
    I had the desoldering tool hot and I was in the neighborhood.

    It's not easy to trace the signals since my scope is 100% analog 30 year old scrap, and the problem is intermittent. Holding the scope probe on any pin long enough to see a glitch is problematic, especially since I can't capture it.
    I don't have any way to clip on a chip pin without the risk of shorting out. I should have better tools but I do this sort of thing very rarely.

  5. #25

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    Quote Originally Posted by Hutch View Post
    Yeah, my focus was on D8 pin 11 and the B6 flip-flops.
    Working back from there. Like I said, my scope wasn't showing me anything on the B6 inputs/C7 output.
    I had the desoldering tool hot and I was in the neighborhood.

    It's not easy to trace the signals since my scope is 100% analog 30 year old scrap, and the problem is intermittent. Holding the scope probe on any pin long enough to see a glitch is problematic, especially since I can't capture it.
    I don't have any way to clip on a chip pin without the risk of shorting out. I should have better tools but I do this sort of thing very rarely.
    I sometimes use small single clips or multi-clips like these:

    https://www.ebay.com/itm/IC-TEST-CLI...QAAOSwULJfIEW~

    TTL IC's are fairly tolerant with brief shorts from outputs to inputs or outputs to ground, as the outputs are poor at sourcing current but good at sinking it to ground. But they don't do well if the output is low and shorted to +5V as it can destroy the lower transistor in the totem pole output stage. Some years ago I made TTL IC test clips with LED's on them to show the output logic state.

  6. #26

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    OK, progress.
    I started in the area of D8 pin 3 and D8 pin 6 (bottom of page, sheet 3)
    Checking the inputs to D8, I see the input to D8 pin1 disappear when the picture jumps.
    That comes from the 74100 pin 4, but I'm also losing the input to C2 pin 10 which drives the latch.
    That comes from B1 pin 8. Looking at B1 pin 13, which is also the clock that drives the A1 counter, the signal there is rock steady.
    It's actually the first time I'm seeing any signal that's not changing when the glitch happens.

    So, correct me if I'm wrong here but ....
    The clock to A1 on Pin 1 is stable all the time.
    The counter reset comes from C7 which I already replaced.
    The A1 outputs on 8, 9 &11 which go to B9, 10 & 12 are dropping to zero volts completely when they disappear, not just stopping the count wherever it is. The pulses never stop with the lines high.

    If A1 stops counting I would lose the low address bits to the character ROM but the shift register would still be loading whatever byte it's pointing to and still sending out a video signal, even if it's the wrong row of pixels? No?
    If B1 is not being triggered because it's inputs are stuck low, C2 doesn't drive the latch at E6?
    E6 is latching a video ram address? And the logic below that is detecting when it's at the end of a row (column 40)?

    In any case, it does look like I have a bad counter at A1. Or am I missing another possibility that could cause the outputs to drop when the input clock is stable?
    Thanks

    p.s. FWIW, this is the scope I'm struggling with. (Just for laughs)IMG_4407.JPG

  7. #27
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    Quote Originally Posted by Hutch View Post
    In any case, it does look like I have a bad counter at A1. Or am I missing another possibility that could cause the outputs to drop when the input clock is stable?
    A1-pin2 (Reset) must be LOW to allow the counter to run. If it is pulsing high a lot then the counter will stay reset. If pin 2 is LOW most of the time as it should be, then I agree the counter may be bad.


    this is the scope I'm struggling with. (Just for laughs)IMG_4407.JPG
    20 MHz bandwidth is a little low, but should be good enough to see most of the signals in a PET. The Reject sticker is a little worrisome. It may just mean the scope is out of calibration, but could mean something important is broken.

  8. #28

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    Dave :would a bad counter A1 linked to the character ROM cause screen bounce and collapse .... in my case a bad A1 was bad characters on the screen (horizontal lines over characters).

    Hutch:- Just a thought, are the 5V regulators OK and are the 74ls177's pulsing correctly ?

    Andy



    Quote Originally Posted by dave_m View Post
    A1-pin2 (Reset) must be LOW to allow the counter to run. If it is pulsing high a lot then the counter will stay reset. If pin 2 is LOW most of the time as it should be, then I agree the counter may be bad.




    20 MHz bandwidth is a little low, but should be good enough to see most of the signals in a PET. The Reject sticker is a little worrisome. It may just mean the scope is out of calibration, but could mean something important is broken.
    Last edited by AndyG; September 30th, 2020 at 10:31 PM.

  9. #29
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    Quote Originally Posted by AndyG View Post
    would a bad counter A1 linked to the character ROM cause screen bounce and collapse ....
    ...and are the 74ls177's pulsing correctly ?
    Andy,
    Yes it could as it might keep the main counters D6 and D7 from reaching the terminal count (detected by D8-pin3) to increment the sequencer.

    The counters D6 and D7 should have been the first things checked as I suggested in message #4 and #7, but our OP is having fun shotgunning. He'll find the problem soon enough.

  10. #30

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    Quote Originally Posted by dave_m View Post
    ... The counters D6 and D7 should have been the first things checked as I suggested in message #4 and #7, but our OP is having fun shotgunning. ...
    Yeah ..... Sometimes it's just easier to swap parts if you have them on hand, Especially with an intermittent problem that refuses to glitch while you're watching.
    I was mistaken about the signal around C7 and B6 because my scope refuses to show me any signal from C7 which sidetracked me and got me focused on the wrong area.

    Other than the one input to the AND gate at E8 pin8, I'm not sure how D6&D7 would have an effect on the vSync signal.
    I have to admit I don't have a complete understanding of the whole circuit.
    Since the output of E6 is dropping, I'm not sure I could trust anything I might see on D6&D7.
    I was working the circuit backwards and ran into the signal drop from A1 before I got to D6&D7.

    I checked the output of E8, pin 8 and, it does drop but one of the inputs (12) comes from the counter at A1 and that is dropping as I said earlier.
    So if E8 is not triggered because one of the inputs is stuck low, it didn't make any sense to keep working back to D6&D7
    Also the pin 1 input to E8 pin 3 is dropping because the output of E6 is dropping. All of this I ran into working backwards before I got to D6&D7.

    Keep in mind also, the vsync signal doesn't drop for long and my scope isn't capable of capturing transients. It rarely drops for more than a few frames.
    When I'm probing pins I never know for sure if it just glitched for a millisecond or if my scope just missed a trigger. I can't look at the screen and the scope at the same time but the CRT usually makes a popping or clicking noise when it happens so I'm relying on that noise which I'm sure is not good for the monitor.
    It's a poor tech who blames his tools but I do really need to talk my wife into letting me buy a new digital scope.

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