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Thread: Faulty 8032-SK with no V-Sync - Gary

  1. #21

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    Previous tests seems to indicate ROM selection is working properly, so maybe the inconsistent CRC is really just faulty ROMs (still not 100% sold), I turned my attention to the fact that the defaults ROM set won't generate Video and that it seems the IRQ from the VIA and PIAs are stuck high.

    This can only be caused by the VIA or one of the PIAs and I've already eliminated the IEEE PIA. So, based on symptoms, I took the decision to first replace the keyboard PIA and that turned out to be the correct decision.

    I did not have a spare 6520 so I used the IEEE chip in the place of the keyboard PIA.

    The machine now boots to the BASIC prompt!

    Test Bench - small.jpg
    Success - small.jpg

  2. #22

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    So far, I had to replace:

    1. 6502 - legs rusted off
    2. Character ROM - legs rusted off
    3. 555 - Reset was stuck high
    4. 6545 - No V-Sync output
    5. 6520 Keyboard PIA - IRQ was stuck low

    Still need to fix the VRAM, likely a 4114 that's faulty.

    Decided to try and not replace all the ROMs but just leave a ROM replacement card in place.

    Replaced chips - 1 - small.jpg

  3. #23
    Join Date
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    I suspect your two problems (/IRQ = high and no video) are caused by separate things - but the same IC.

    Normally, /IRQ = high would only mean that the keyboard wouldn't work and the cursor not flash. I would still expect to see some sort of video (especially if you removed the keyboard PIA UB12).

    No video would imply that the keyboard PIA is (in some way) upsetting the video and/or synch lines to the monitor.

    Ah, just re-read the two posts again. Post #21 states "IRQ ... stuck high" and post #22 states "IRQ ... stuck low". Yes, /IRQ stuck low would not work at all !

    Dave

  4. #24
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    Feb 2009
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    Good work so far. Are you getting a blinking cursor proper keyboard response?

  5. #25

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    Oops, sorry about the stuck high/low typo

    It was definitely the keyboard PIA that stopped the video being generated with the normal ROM set. Both H and V-sync were being generated properly, it was just the Video signal that was missing. But your PETTESTER was quite happy to output video with the faulty PIA in circuit, so I assume you disable /IRO?

    Iím not getting a blinking cursor yet. I donít have a keyboard plugged in at this point. Is a keyboard needed for the cursor?

    Could one jumper some of the Row/Column pins on the keyboard connector on the PCB to emulate a rudimentary keyboard?

  6. #26

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    BTW, any idea where to find a 6520 PIA? Are there other popular vintage kit where I might be able to salvage one?

    I pulled the replacement CRTC from an old PC CGA card.

  7. #27
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    The keyboard is not needed for the cursor. You may not yet be getting the 60 Hz interrupt. A Motorola 6820 or 6821 may be easier to find than a 6520.

  8. #28

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    Quote Originally Posted by dave_m View Post
    The keyboard is not needed for the cursor. You may not yet be getting the 60 Hz interrupt. A Motorola 6820 or 6821 may be easier to find than a 6520.
    Ah, OK. Which chip generates the 60Hz (guess 50Hz, in my case) interrupt? I currently have the IEEE PIA pulled as I used it in the keyboard PIA slot.

  9. #29
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    Quote Originally Posted by Jannie View Post
    Ah, OK. Which chip generates the 60Hz (guess 50Hz, in my case) interrupt? I currently have the IEEE PIA pulled as I used it in the keyboard PIA slot.
    The keyboard PIA UB12 is the only chip that issues an interrupt. The Vertical Drive signal goes to a UB12 input at pin 18. The PIA is initialized to issue an /IRQ when it sees a pulse on pin 18. Can you remove the VIA if it is on a socket to make sure it is not causing some problem with the interrupt? It is connected to /IRQ also but is not programmed to issue interrupts.

  10. #30

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    Quote Originally Posted by dave_m View Post
    The keyboard PIA UB12 is the only chip that issues an interrupt. The Vertical Drive signal goes to a UB12 input at pin 18. The PIA is initialized to issue an /IRQ when it sees a pulse on pin 18. Can you remove the VIA if it is on a socket to make sure it is not causing some problem with the interrupt? It is connected to /IRQ also but is not programmed to issue interrupts.
    Was hoping you're not going to suggest that.

    The VIA is not socketed....but, it's becoming more clear I'll likely have to socket it. Not a big issue, but I prefer to not desolder chips on these old boards where I can avoid it. Must say though, the PET board seems to be relatively easy to desolder chips and socket.

    According to the PET diagnostic board, the /IEQ line is not being held anymore, after I replaced the keyboard PIA, so I was hoping the VIA is OK. But I'll socket it and check.

    BTW, I saw Vert Drive also goes to the VIA on pin-15. Not sure if it's used there.
    Last edited by Jannie; October 4th, 2020 at 11:31 AM.

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