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Thread: PET 2001 with the wrong plug

  1. #11
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    Quote Originally Posted by JamesGame View Post

    Where does this leave me, not sure if to consider a professional restoration or reusing the case for a NUC or Pi.
    From the display indications at turn-off, it seems like the main board is almost working. Check all the DC voltages. All the DC voltages can be found on the 4116 RAM chips. Also check the AC going to the CRT board. There are two wires going from the transformer secondary up to the CRT board. It should be around 16 VAC as it feeds a DC regulator chip which outputs 12VDC. The nine schematics for the 2001-16 can be found here:

    http://zimmers.net/anonftp/pub/cbm/s...01N/index.html

  2. #12

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    I sure wouldn't scrap that board. In my opinion, what you are seeing is evidence of a stuck /IRQ or /NMI. But do what Dave says first.

    After that, remove any and all 6520, 6521, 6522, 6820, and 6821s. Make sure you note where you got them from; xx20 and xx21 are interchangeable, but 6522 is not. If it will boot without those, put them back in one at a time until it stops working again. Then you'll know which chip is the problem.

    If pulling all those out doesn't cause a change, there are other things we can help you check.

    Fixing PETs is not difficult, but fun!

  3. #13
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    Quote Originally Posted by KC9UDX View Post
    Fixing PETs is not difficult, but fun!
    KC,
    I wonder what problem could get good ASCII characters on the screen as the voltage slowly decays away? Out of spec power supply voltage? With a good raster on the display, it should not be a video board problem I hope. Do you think we should have the OP check the video data line looking for a DC voltage that is not stuck at zero or +5 V? If it measures a DC average of 1 V or so, there must be pulses indicating a good video signal going to the video board. Although come to think of it, there will be little text on the screen so it may be hard to tell the voltage difference between a white screen and a screen with few characters. Maybe we could get reverse video by grounding a signal.

    Always an interesting problem with the PETs!

  4. #14

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    Quote Originally Posted by dave_m View Post
    KC,
    I wonder what problem could get good ASCII characters on the screen as the voltage slowly decays away? Out of spec power supply voltage? With a good raster on the display, it should not be a video board problem I hope. Do you think we should have the OP check the video data line looking for a DC voltage that is not stuck at zero or +5 V? If it measures a DC average of 1 V or so, there must be pulses indicating a good video signal going to the video board. Although come to think of it, there will be little text on the screen so it may be hard to tell the voltage difference between a white screen and a screen with few characters. Maybe we could get reverse video by grounding a signal.

    Always an interesting problem with the PETs!
    I've seen this in other systems that work similar to PETs, but not specifically in a PET. If the 6502 is prevented from running the boot code whilst the power supply voltages are all good, but free to run when the voltages fall, it could be any number of things. The easiest thing to check, in my opinion, is if a 6520 or 6522 is stuck pulling down /IRQ. I find, with my little experience with PETs, that this is actually a common problem, usually showing up as a stuck or missing cursor. But maybe this other problem could happen.

    I've thought for a long time that we could really benefit from a flow chart for PET repair. We have seen a large number of cases here and at the very least we always first check the power supply, next probably the clock, then the 6502 control lines.

    Would you agree that it's a safe first step to remove all the I/O chips? I think that most if not all PET models will at least fully boot without them. Is this true? If so, other than the flaky socket problem, this might be a good thing to eliminate first.

    I guess to make my initial assumption possible, we need to know how long after power-down the text appears on the screen. After all, the boot process is not very fast. If there's not a significant delay, then I think you're right, we should go down the video chain. There's a simple way to generate video by pulling the character generator ROM and shorting one of its data lines. I wonder if it's safe to do that with the ROM installed? Otherwise, I think you're on to something with the reverse video. I don't have schematics handy, and my memory is lousy, so you're probably much better equipped at this point.

  5. #15
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    Quote Originally Posted by KC9UDX View Post

    Would you agree that it's a safe first step to remove all the I/O chips? I think that most if not all PET models will at least fully boot without them. Is this true? If so, other than the flaky socket problem, this might be a good thing to eliminate first.
    I agree we should do the easy stuff first that will give us some data points. Yes, with the 6520 PIA's and the 6522 VIA removed, if all else is working, we should get the Commodore welcome screen with nothing else working.

  6. #16
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    The European 3016 = 2001-16N in the USA.

    Don't assume whomever made the power supply switch-a-roo knew what they were doing back then but I would suggest you find a working 3000 PET that has not been altered and compare your 2001-16N with that. Either a 3016 or 3032 will do.

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