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Thread: Dead Commodore 8032 CBM

  1. #1
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    Default Dead Commodore 8032 CBM

    Hi VCFED world.

    My beloved CBM 8032 died last week As is often the case, I proudly showed her off to dinner guests last Saturday night, hit the power button... NOTHING!!!, not even the cheerful welcoming 'chirp'. I was guttered, as were my guests. She was last turned on and working approx 2 weeks ago. No signs of illness then. Hasn't been moved.

    She was last sick in 2012 and the wonderful people on VCFED guided me through her recovery.

    I was thinking this time I'd take it to a pro, but alas we don't really have anyone near where I live who repairs vintage computers.

    I have an elementary knowledge of electronics, so need a lot of hand holding (as you would have noticed from my last post)

    Here's what I know:
    1) No chirp on start up, no video display, but there is a faint glow coming from the back of the CRT.

    2) The PSU shows the following; Pins 9 and 11= 22.4VAC (S/B 21VAC), Pins 4 and 6 = 18.6VAC (S/B 22VAC), Pins 7 and 8 + 16.8VAC (S/B 18VAC)- Are these acceptable voltages?

    3) Power is getting to the data cassette (runs when you hit play, fast fwd etc)

    4) No obvious 'hot' chips. No obvious damage or dry joints although I only had a quick scan but could look more closely if I knew which chips to look at.

    5) There are previous blogs I've read about dead 8032s, but a bit above my skill set... http://www.vcfed.org/forum/archive/i...p/t-40896.html

    I have a second non working 8032 which I can use for parts

    In simple terms, where do I start? I don't have a suitable meter to test caps but I could purchase one if that would be a good start?

    Which chips are possibly the suspects? Could you also give me their location (E.G. UA9 etc)

    Many many thx guys. And so the journey begins...
    "DAVID: Is this a game... or is it real? JOSHUA: What's the difference?" 'Wargames' 1983

  2. #2
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    Quote Originally Posted by Juleshsmith View Post
    And so the journey begins...
    OK, you will need a DVM or at least a good VOM. Make sure the battery in it is good. You first need to find out if the problem is in the main board or in the CRT section. It could be for example that a bad ROM or something is keeping the 6545 CRT Controller from being initialized and without a good Horizontal Drive signal, there will be no high voltage generated and therefore a dark screen.

    First thing, Power off and press on all the socketed chips and jiggle/reseat the connectors including the J7 video connector to remove any oxide on the connections. Power up and see if any better. But before you remove any connectors/cables, take photos to make sure you can hook things back up correctly.

    Then you should check if you have good DC voltages. With the black (-) probe attached to the PET chassis for a ground, Check a 4116 RAM chip like UA4 and verify the voltage on the following pins:
    pin 1 -5V
    pin 8 +12V
    pin 9 +5V

    Then look for an active Horizontal Drive on UC2- pin 3. It is a pulsing signal but should have a DC average reading of over 1.5 VDC. If it is zero there is probably a problem on the main board.

    That should get you started. With a little good luck, you will get your beloved PET back on the air.

  3. #3
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    Excellent, thx dave_m!

    Sound like a really good start. I have a VOM but a bottom of the range, and I don't think it's able to test caps. I've jiggled and reset the connectors but alas no change.

    Couple of questions before I start... if the problem was in the CRT section, would you still expect to hear the "chirp" on start up (mine has no "chirp")? If that was the case then you could rule out that the problem was in the CRT section?

    Thx
    "DAVID: Is this a game... or is it real? JOSHUA: What's the difference?" 'Wargames' 1983

  4. #4
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    Quote Originally Posted by Juleshsmith View Post
    Couple of questions before I start... if the problem was in the CRT section, would you still expect to hear the "chirp" on start up (mine has no "chirp")? If that was the case then you could rule out that the problem was in the CRT section?
    Yes, you are right. No chirp probably means the main board has the problem. There may be no pulsing on the Horizontal Drive signal as the 6545 may not be initialized. Check voltages then let us know what chips are on sockets for easy replacement. Can you solder if necessary?

  5. #5
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    Excellent!

    Checked UA4
    Pin 1 -4.89
    Pin 8 +11.71
    Pin 9 +5.07

    UC2

    Pin 3 +5v

    J
    "DAVID: Is this a game... or is it real? JOSHUA: What's the difference?" 'Wargames' 1983

  6. #6
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    PS- UC 2 pin 3 was a solid + 5v (not pulsating)
    "DAVID: Is this a game... or is it real? JOSHUA: What's the difference?" 'Wargames' 1983

  7. #7
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    Quote Originally Posted by Juleshsmith View Post
    PS- UC 2 pin 3 was a solid + 5v (not pulsating)
    OK, good measurements. Power looks OK and no activity from the 6545 CRTC so most likely the computer is not running in program. Check the SYNC signal to see if the CPU is fetching instructions or is hung up. UB14-pin 7 on the 6502 chip. If it reads zero, there are no instruction fetches going on. It should read over 1 volt. Also check Phase 2 Output Clock on pin 39 of the 6502 (UB14). It should be running at 1 MHz square wave and so read about 2.5V on the DC Voltmeter. Can you borrow a scope if needed?
    -Dave

  8. #8
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    6502 (UB14) pin 7 = +29.5v
    Pin 39 = +2.3v

    I've got a scope Dave but have never used it (time to learn!)

    RE socketed chips. Most chips are hard wired. A few years ago I socketed a few of the RAM chips. 6502 is socketed.

    Yep, I can solder! And as I mention, I have a DOA 8032 I can use for spares....

    Really appreciate your step by step help on this
    "DAVID: Is this a game... or is it real? JOSHUA: What's the difference?" 'Wargames' 1983

  9. #9
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    Quote Originally Posted by Juleshsmith View Post
    6502 (UB14) pin 7 = +29.5v
    Pin 39 = +2.3v

    I've got a scope Dave but have never used it (time to learn!)
    If you meant that CPU pin 7 (Sync) is about 30 mV or a definite LOW, then the PET may have executed the dreaded KILL instruction. It is an illegal OP Code that causes the machine to wait forever. This may be because the CPU read from a bad ROM. Take a look at the most significant bits of the address bus (A15, A14, A13 and A12) to see if it might point to the bad ROM. For example A15 A14 A13 A12 = 1110 then the E000 address ROM may be bad (UD7). See sheet 4 of the schematic for ROM addresses. If a ROM is bad you will have options to proceed. You can attempt to unsolder and replace a 24 pin ROM or just replace all RAM and ROM at once with a small RAM/ROM replacement board that fits easily in the CPU socket. If you decide to replace the ROM, I will send you an EPROM replacement. Anyway, one step at a time.

  10. #10
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    Ah, yep 30 mV (didn't see the "m" on my new multimeter)

    "Take a look at the most significant bits of the address bus (A15, A14, A13 and A12) to see if it might point to the bad ROM. For example A15 A14 A13 A12 = 1110 then the E000 address ROM may be bad (UD7)"


    Ok, just getting a little lost now (pls forgive my ignorance)... What do you mean by "significant bits" ? When you say A15 is that UA15 (etc)? And not sure what you mean with "A15 A14... =1110" What's "1110"

    I tested all the RAMs (pins 1, 8, 9) and the only one that was off was UA20 Pin 9 = 16mV

    Thx again Dave
    "DAVID: Is this a game... or is it real? JOSHUA: What's the difference?" 'Wargames' 1983

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