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Thread: Commodore Pet 8032SK with monitor not working

  1. #11

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    Once the power supply rails are known to be good and assuming the horizontal drive signal is present at point 13 on the schematic, the first thing to check would be if the H output stage is working. If it is working, then 400V will be present on the output (cathode) of D752. A less risky place to test, if your meter was not up to a 400V value, would be the E30 test point.

    If the H scan output stage is not working there will be no EHT and the screen will be black.

    In these monitors the most vulnerable two transistors are the H output transistor Q721 and the driver transistor Q711. The reason being is that if they are presented with an abnormal drive signal from the computer, then more energy is stored in the line opt transformer, Q721's collector voltage gets exceeded and when it fails it takes out Q711 in the process. It was a risky design relying on the timing of the HOR.DRV signal from the computer.

    It is interesting they tacked in an over-voltage detect circuit, but that won't always save Q721. It is also worth checking the energy recovery diode D721 for shorts and the other diodes around the line opt transformer area.
    Last edited by Hugo Holden; October 21st, 2019 at 03:59 PM.

  2. #12

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    This is video signal at point 9 after d201
    20191022_180640.jpg

  3. #13

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    Quote Originally Posted by Hugo Holden View Post
    Once the power supply rails are known to be good and assuming the horizontal drive signal is present at point 13 on the schematic, the first thing to check would be if the H output stage is working. If it is working, then 400V will be present on the output (cathode) of D752. A less risky place to test, if your meter was not up to a 400V value, would be the E30 test point.

    If the H scan output stage is not working there will be no EHT and the screen will be black.

    In these monitors the most vulnerable two transistors are the H output transistor Q721 and the driver transistor Q711. The reason being is that if they are presented with an abnormal drive signal from the computer, then more energy is stored in the line opt transformer, Q721's collector voltage gets exceeded and when it fails it takes out Q711 in the process. It was a risky design relying on the timing of the HOR.DRV signal from the computer.

    It is interesting they tacked in an over-voltage detect circuit, but that won't always save Q721. It is also worth checking the energy recovery diode D721 for shorts and the other diodes around the line opt transformer area.
    Hi Hugo, at the end of D752 i have 0v....i set my multimeter to 1000V scale

  4. #14
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    There is no video signal there...

    The $64,000 question is "do you have a video signal and the horizontal and vertical drive signals at the monitor connector of the PET main board"? If not, there is no point in going further with the monitor unless the main board is working...

    Dave

  5. #15

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    Quote Originally Posted by daver2 View Post
    There is no video signal there...

    The $64,000 question is "do you have a video signal and the horizontal and vertical drive signals at the monitor connector of the PET main board"? If not, there is no point in going further with the monitor unless the main board is working...

    Dave
    Sure, Pet's board is working 100%

  6. #16
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    >>> This is video signal at point 9 after d201

    Ah, but you didn't do what I asked you to do...

    "You will have to speed the timebase up to get to see the video signal detail."

    Your photograph of the video signal shows a timebase of 10 milliseconds/division which is far too slow to resolve the video signal.

    Dave

  7. #17

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    Quote Originally Posted by daver2 View Post
    >>> This is video signal at point 9 after d201

    Ah, but you didn't do what I asked you to do...

    "You will have to speed the timebase up to get to see the video signal detail."

    Your photograph of the video signal shows a timebase of 10 milliseconds/division which is far too slow to resolve the video signal.

    Dave

    If i set up timebase i can't see nothing....

    The problem is that i don't have 400v

  8. #18
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    Well, you don't have a video signal either by the look of it. So even if you did have 400 Volts, you would still have a picture of a black cat in a coal cellar at midnight...

    Dave

  9. #19

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    ehehe what can i do now?

  10. #20

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    As noted before, for the monitor's H(horizontal) output stage to be working (and get the 400V and the other outputs from the horizontal output transformer) the HOR.DRIVE signal must be present and normal and of course the power supply must be good and the monitor H circuitry good.

    So with no 400V , first check the HOR.DRIVE signal and if that is not there, the problem likely is in the computer, not the monitor. If it is there & normal, then the problem is in the video monitor , likely in the H driver or H output transistor stages.

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