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Thread: 32k PET 2001-N - Video and running issues

  1. #11

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    Quote Originally Posted by Hutch View Post
    What I meant to say was that IF the processor was not running, I'd expect more random characters on the screen, not large blocks of @.
    That's why I think the vRam may be faulty and the processor is probably fine.

    Yeah, I've been fooled by a bad CPU toggling address lines before. No reason to doubt it's working though.
    You can try powering on with the CPU removed and see if the screen looks any different. Might confirm it's getting to clearing the screen.

    Hah... well, progress of a sort...

    I do have spare 6502s.. I swapped out the 6502 and after some (very little) random stuff, the screen clears... and never comes back.

    I'm going to look through my tubes for some more 2114s just in case I have them. At this point I have the 6502, character ROM, Kernal, edit, basic 1&2, and the... 6522? I forget.. I pulled the PIAs out. Without VRAM I do get a nice checkerboard (when the screen wants to cooperate). I didn't feel like shocking myself, but I'm going to have to pull that video board out.

    I took a shot at breadboarding the video mixer with a hodgepodge of what I have on hand.. No joy. Not even a flicker. I'm not assuming it's the video circuit; it's more likely the components that I used and the rat's nest of wires.

  2. #12
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    When you say the screen is dark, with the brightness up, are there signs of a a raster scan?

    You could monitor the vertical drive, horizontal drive and video data on the main board. Do they go away when the screen goes dark. If so there may be a problem with the main board video timing logic. If the signals stay, but the screen goes dark, then there may be a problem with the video board.

  3. #13
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    Dave_m thinks in exactly the same way that I do!

    Before poking around with the monitor, make sure you have sensible H and V DRIVE signals from the main logic board (and something on the video signal as well).

    Since you don't have the CRTC controller, you should be able to remove the CPU (so the random power-up pattern should never go away), scope the H, V and video signals and leave the PET run like that for a while. If the picture stays like that, it is more likely not to be the monitor to blame. If the monitor goes blank, scope the drive signals again. If they are still there, the monitor is to blame. If the signals are not correct, the logic board is to blame.

    Dave

  4. #14

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    Quote Originally Posted by daver2 View Post
    Dave_m thinks in exactly the same way that I do!

    Before poking around with the monitor, make sure you have sensible H and V DRIVE signals from the main logic board (and something on the video signal as well).

    Since you don't have the CRTC controller, you should be able to remove the CPU (so the random power-up pattern should never go away), scope the H, V and video signals and leave the PET run like that for a while. If the picture stays like that, it is more likely not to be the monitor to blame. If the monitor goes blank, scope the drive signals again. If they are still there, the monitor is to blame. If the signals are not correct, the logic board is to blame.

    Dave
    Yep. I always have h-drive and v-drive on the board. It sort of acts like the connector/wire is loose somewhere. Often it will go blank, sometimes it will drop down to a single narrow band.

  5. #15

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    After swapping out the 6502 and reseating everybody, I get a mostly clean screen with a free RAM report.. Even a flashing cursor.

    It will not start reliably, however, which makes it seem as though the CPU isn't always starting. The last five times, for instance, it was just the same RAM report leftover in the chips with no screen clear. There are still some brokenish characters ($ and @ in a few places), so there might still be SRAM problems. The keyboard also seems to be non-responsive, which is a new symptom (it did work some time ago). Maybe a bad PIA? Not sure.

    At this point, the monitor dropping it is the biggest hinderance to progress. I'm going to both wire up a reset switch so that I don't have to keep flipping the power and take out the board for the 9" monitor and give it a good going over. Before I picked this thing up back about 8 years ago, mice had made quite a nest for themselves. The greatest concentration of them was in the 9" CRT housing. It looks like they sharpened their teeth on the heat sink for *years*. In many ways, I'm surprised it works at all.

  6. #16
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    Monitor wise then we will have to follow the H and V drive through the chain. The fact that you say you get a narrow band sometimes is also a good indicator of vertical circuitry problems.

    For the main logic board - you could try my diagnostic ROM that fits in the 2K EDIT ROM and see if that can diagnose a consistent issue.

    Dave

  7. #17

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    Horizontal or vertical band?

    I'd put the old 6502 back in.

  8. #18

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    Quote Originally Posted by KC9UDX View Post
    Horizontal or vertical band?

    I'd put the old 6502 back in.
    Horizontal. It's not constant. When the image is up, it's up and rock solid; bright, clear. Then it will go out, sometimes completely, sometimes like something is loose, sometimes to just a horizontal band that seems to lack sync. More often, completely nothing for long stretches. It does seem happiest when it's cool.

  9. #19

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    Quote Originally Posted by daver2 View Post
    Monitor wise then we will have to follow the H and V drive through the chain. The fact that you say you get a narrow band sometimes is also a good indicator of vertical circuitry problems.

    For the main logic board - you could try my diagnostic ROM that fits in the 2K EDIT ROM and see if that can diagnose a consistent issue.

    Dave
    Hey Dave-

    I thought I had posted asking for a link to the 2k Edit Rom diagnostic. I can't seem to locate it with Google.

    Thanks

  10. #20
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