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Thread: dead 8032 with strange behaviour

  1. #1
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    Default dead 8032 with strange behaviour

    Hello,

    While I was trying to fix another 8032 with a suspected CRT board (http://www.vcfed.org/forum/showthrea...8032-CRT-issue), I picked up a supposedly working 8032 board I have, with the idea to compare signals, etc.
    But nothing is easy in this life and the board doesn't work anymore. So I decided to fix this board first.

    It doesn't do the familiar "CHIRP" and of course no video.

    I already checked all the ROMs (fixed a couple: char and one of the three basic ICs). Then I tried to see with the probe if some lines were stuck high or low, and the surprising result is this one:

    scope results.pdf

    As you can see in this pdf, every pin of the big ICs (cpu, pia, via and crtc) is high or low, but there's no pulsing signal (Note: NC means no signal from the probe).

    The very strange pin is the #27 (line D6): it is NC (no signal, neither low, nor high or pulse), but if you remove the 6502 it goes LOW.

    I have a RAM/ROM adapter, but currently it works only as RAM replacement: I tried it, but nothing happens, so at least I know the SRAM ICs aren't the problem (at least, not only).

    Any idea? What should I check?

    Forgot to mention +5v and +12v seem to be ok.

  2. #2

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    Is the clock running? Does the reset circuit work?

  3. #3
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    Quote Originally Posted by KC9UDX View Post
    Is the clock running? Does the reset circuit work?
    Good question...
    The crystal return a pulse on probe, but the 555 pin 3 (I suppose it should pulse) start with a high state, and then goes to low.

  4. #4

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    I assume you mean the reset circuit. If so, that's correct, it's working.

    What do you see on pin 37 of the 6502?

  5. #5
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    Quote Originally Posted by KC9UDX View Post
    I assume you mean the reset circuit. If so, that's correct, it's working.
    Please, explain me exactly what I should to check... unfortunately I don't know much about hardware, so you should not assume I'm understanding, LOL

    Quote Originally Posted by KC9UDX View Post
    What do you see on pin 37 of the 6502?
    Well, I resumed all the 65xx ICs pin status in the pdf I linked at the beggining of this thread (http://www.vcfed.org/forum/attachmen...7&d=1561936364), however at 6502 pin 37 I have a LOW state.

    What I don't understand is why I have no signal at all (like a not connected pin) at 65xx ICs pin #27 if the 6502 is inserted, or Low if 6502 isn't in the socket.

  6. #6

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    I assume you're using a logic probe: these things give strange results when definite signals are not applied. So a high impedance state cannot be accurately determined with one. Basically all the probe is telling you is that it's not measuring a high or low.

    It seems that your clock oscillator is not oscillating. I'm not sure what an MMOS 6502 does without a clock. What you see on pin 27 might be perfectly normal under this condition, but that's moot. You'll need to fix the clock to get sane results.

  7. #7
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    Quote Originally Posted by KC9UDX View Post
    I assume you're using a logic probe: these things give strange results when definite signals are not applied. So a high impedance state cannot be accurately determined with one. Basically all the probe is telling you is that it's not measuring a high or low.

    It seems that your clock oscillator is not oscillating. I'm not sure what an MMOS 6502 does without a clock. What you see on pin 27 might be perfectly normal under this condition, but that's moot. You'll need to fix the clock to get sane results.
    Thank you! I'm trying to fix the clock. I started from the 8032029-06.gif schematic, master timing. I've seen the pull up line (connected to pin 1 of the three 74177: UD3, UE6 and UE7) is HIGH. Is it right?

    I replaced some ICs suspected to be bad (expecially UD3) and now I have a pulse state at 6502 pin 37 and pin 3; Low at pin 4 (irq, shouldn't be a pulse state?), Low at pins 9..20 and 22..25; pulse at pins 26..30 (pin 27 had no signal at all, now it pulses), High at pin 31, Pulse at 32 and 33, high at 34.

    Time to go to sleep; next task, tomorrow (maybe)...

    Every help or suggestion will be very appreciated!

    bye for now.

  8. #8

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    Your clock sounds normal now. Nice work!

    /IRQ should only go low when there's an interrupt; and it shouldn't do that long enough to appear to be low with a probe. When it's stuck low, the 6502 executes the ISR over and over and has no time to do anything else; making the it appear to be doing nothing.

    There are only a few sources of /IRQ in a PET. Assuming the PIAs and VIAs are all socketed, you can pull those out of the sockets for now.

  9. #9
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    The 6520s and the 6522 were ok, I tested them in a working 8032. And -surprise- I inserted them back in place and the IRQ was ok (high). I can't say what happened and why I didn't work.


    Strange, this board worked fine some years ago, then suddenly stopped to work. I left it in on a shelf, believing just one chip was faulty, but until now I detected at least 4 bad ICs (not to mention the suspected ones):

    74154 I replaced yesterday, after I discovered a no signal at the ROMs pin 20; now the ROM ICs are running just warm (they were hot);
    char ROM
    Basic ROM (the one in the middle)
    74LS177

    And it doesn't work yet. No video output, no chirp...


    As I previously told, I have a ROM/RAM adapter that works fine at least as RAM replacement; I tested with no result, so I'm assuming that probably it's not a RAM problem.

    Any cue about should I check now? Any chance that UE14 and/or UE5 could prevent the computer from boot?

  10. #10

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    I'd be concerned about hot ROMs. Can you swap them with the other 8032?

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