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Thread: Commodore Pet 8032SK with monitor not working

  1. #211
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    Quote Originally Posted by ilpaninaro View Post
    Ok, i resetted two 4116 chip and now pet turn on!
    I inserted your pettester and i can see this !
    Attachment 57060
    OK your Monitor is working OK. And the tests indicate that the RAM is good. But as the EDIT ROM does not bring up the system, then perhaps a ROM is bad. I can tell that the FD16 sum check on the kernal ROM is good, but do not know the numbers for the other ROMs. Dave2 do you know the expected sums for BASIC4 ROMs?

  2. #212
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    Just edited my previous post with that information... It is in the manual as well !

    Dave

  3. #213
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    Quote Originally Posted by daver2 View Post
    Just edited my previous post with that information... It is in the manual as well !

    Dave
    OK all ROMs checked good! Of course test could not check the EDIT ROM. We should be close.

  4. #214

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    News:

    I changed edit rom socket because
    it seemed defective...
    now when i turn on sometimes i have regular screen, sometimes i ve a mid screen full of garbage chars and sometimes black screen
    When i have regular screen i noticed that the words of the basic are oblique and not perfectly horizontal
    at screen....
    20191103_155311.jpg,20191103_180103.jpg

  5. #215
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    The first picture (that will drive Dwight insane...) is most likely a faulty monitor circuit somewhere.

    The second picture is most likely a faulty PET logic board.

    Dave

  6. #216

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    The tilt indicates the Yoke requires rotating. To do this the clamp screw must be loosened. Sometimes the white plastic tongues under it can be stuck to the neck glass, so if that is the case loosen the clamp right off, slip it away and free those one by one using a plastic tool or fingernail. Don't put excessive force on the glass neck, don't scratch it either with metal tools, only use gentle forces and when you re tighten the clamp, only enough to stop the Yoke rotating with gentle pressure. Don't over-tighten the clamp.

    Interesting that the computer comes up ok sometimes and not others. I'm not familiar with the power on reset system, if its an R-C based circuit, perhaps the reset pulse is not long enough compared to the speed that the supply voltage comes up. Both time constants of course get shortened by aged electrolytics.

    Notice that I have put your monitor picture up the right way. This is what Dwight was asking for. Its hard to look at sideways pictures. You can rotate images in any photo edit software.
    Attached Images Attached Images

  7. #217

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    Quote Originally Posted by Hugo Holden View Post
    The tilt indicates the Yoke requires rotating. To do this the clamp screw must be loosened. Sometimes the white plastic tongues under it can be stuck to the neck glass, so if that is the case loosen the clamp right off, slip it away and free those one by one using a plastic tool or fingernail. Don't put excessive force on the glass neck, don't scratch it either with metal tools, only use gentle forces and when you re tighten the clamp, only enough to stop the Yoke rotating with gentle pressure. Don't over-tighten the clamp.

    Interesting that the computer comes up ok sometimes and not others. I'm not familiar with the power on reset system, if its an R-C based circuit, perhaps the reset pulse is not long enough compared to the speed that the supply voltage comes up. Both time constants of course get shortened by aged electrolytics.
    Ok thanks Hugo, i try to rotate

  8. #218
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    Yes, you can see it on the PETTESTER diagnostic screen as well. I thought that one was OK until I went back and had a look.

    This is what Dwight has been saying - the photograph needs to be framed such that we can see what is actually being displayed on the screen rather than us guessing.

    The RESET circuit is a C/R network driving a 555 timer. These are notoriously poor. But I am not getting feedback from an earlier post I made regarding how long the PET is left OFF for before trying it again. Too short a time period and the reset circuitry may not work.

    Just had another thought... If this PET has a CRTC, if the 6502 CPU fails to start properly it may not initialise the CRTC and then we will get a black screen. All these faults may now be interrelated.

    Dave

  9. #219

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    Finally a straight on picture. I was't sure if it was an issue with bypass capacitors or that the yoke was rotated. As Hugo says, be careful around the neck. If after you rotate the yoke, you don't like the centering, there are two tabs sticking out that you can use to center the display. If the two tabs are together, they are the strongest pulling things around the screen. If they are 180 out, they have no effect.
    Move them around a little and you'll get the feel. They are slightly magnetized.
    As for erratic reset, it is often that the crystal doesn't start up very fast and the reset circuit isn't slow enough. Sometime it is the IC the makes the crystal oscillator. Some times it is the crystal. Some times it is the electrolytic/tantalum capacitor in the reset. Some times you can get the crystal oscillator to start a little faster by adding a 5 pf cap on one end or the other of the crystal to ground.
    What works for you is up to you.
    I'm curious, why do you generally ignore my request? Is it that you don't like me?
    Dwight

  10. #220

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    All are generally happy with the screen. I'm not as happy. I'm still a little concerned about the 400V not measuring 400v. It should be relatively stable. I don't buy the fact that it could have significant noise. If it did, it would show distortion in the line size. It is possible that the value was in error and should be higher.
    A higher voltage than needed will do two things. It will burn the phosphor on the screen sooner and make a smaller height and width on the screen. I'd be a lot happier if someone with one of these would measure theirs and report the value.
    Of course it could be the cheap meter has a bad scale.
    Dwight

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