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

  1. #1

    Default 32k PET 2001-N - Video and running issues

    Hey-

    My life is still in turmoil, but I decided to have a go at my PET before moving it on to a new home (yes, some of you know who I'm looking at).

    At this point, it is semi-conscious again... but...

    I appear to have a strong clock, phi1, phi2, and there appears to be data appearing on the data lines of the 6502 (see the logic analyzer output). The display, however, is not happy looking. Additionally, while the display in the photo looks awesome (compared to complete darkness), it will randomly flicker to darkness or perhaps even go dark for long periods. Nothing is particularly hot on the board, I just burned a complete set of EPROMs (and verified them), and I've been over the board three times looking for solder joints, etc.

    When the screen goes dark, the tube is still glowing, so I'm guessing that it's a video circuitry problem, but I'm stumped as how to tell if it's on the board or on the CRT board. (This is a 2001-N 32k..)

    Any suggestions? I know we have some PET pros around here. Photos attached.
    IMG_3086.jpgIMG_3087.jpgIMG_3088.jpg

  2. #2

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    Based on that photo of the screen, my first guess would be the video RAM.
    It's mostly @ which is screen code zero which tells me the video circuits are reading mostly zeros from video ram.

    There is some data on screen though which could just mean some addresses have some other bits stuck on.
    Are the characters on screen the same every time you power on?
    Does anything change if you type blindly ?

    The screen flickering and going dark is likely something in the CRT analog board. I have one that does the same thing but have not gotten around to pulling it and re-flowing the solder joints.
    If that doesn't work, I'll need to roll up my sleeves and start testing components.
    Last edited by Hutch; June 28th, 2019 at 02:38 PM.

  3. #3

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    Interesting. I replaced the VRAM about four years ago, so I feel fairly confident that they are good. The content is generally the same, though a few characters might change. The @ had me concerned that the processor wasn't running, which is why I had the logic analyzer hooked up. The good news is the character generator is definitely working.

    Regarding the analog CRT board, I think I have the parts to make a composite adapter to attach to the board. I had been thinking about making that and hooking to a composite monitor for testing; asking here first was just me being lazy.

    If you (or anyone else) have any other thoughts, please let me know.

  4. #4

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    In my experience, if the video ram is OK and the processor is not running, the video ram would display a more random selection of characters based on the random bit patterns in the ram at power on.
    What's being displayed is much less random. If the video ram is socketed, it's worth a quick swap if you have spares.

    If you feel certain that the video ram is good, then of course there could be some problem in the video logic causing it to read incorrect data from the ram, but in that case I'd expect even less randomness, like a single bit stuck on or off and every character shifted by the same value.
    The character ROM and the shift register appear to be working fine.

    I really need one of those composite adapters myself but have been too lazy to make one. Hoping I could find one on Ebay or one of the retro computing sellers online.
    Last edited by Hutch; June 28th, 2019 at 03:00 PM.

  5. #5

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    Ok. Well, more tomorrow. I spent three hours on it today, which got it back to where it is now from completely non-functional. I may start with the composite adapter so that I have a better view of the screen.

    In your quote you say, "...and the processor is not running...", which is why I included the logic analyzer output. There is definitely data appearing on DA0-DA7 on the processor, along with good clock coming out of it. It *looks* like it's running.

    I may have some spare VRAM, and I think I did socket those chips. If I did, I'll swap in some spares. If not, I think I'll start by wiring the analyzer up to the address bus and see if the CPU is fetching instructions.

  6. #6

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    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.
    Last edited by Hutch; June 28th, 2019 at 03:20 PM.

  7. #7

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    Oh, super idea!

  8. #8

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    Which logic board does this PET have? If it has a CRTC, I expect none of your problems to be with the monitor.

    Either way, the ROM may be bad or not being read correctly; of course zero page and stack RAM are suspect, too.

  9. #9

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    Quote Originally Posted by KC9UDX View Post
    Which logic board does this PET have? If it has a CRTC, I expect none of your problems to be with the monitor.

    Either way, the ROM may be bad or not being read correctly; of course zero page and stack RAM are suspect, too.
    This is not a CRTC PET. Also, as I said, I just burned fresh EPROMs for it; they are fine.

  10. #10

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    I'd try an NOP generator.

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