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Thread: Dead Commodore 8032 CBM

  1. #11
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    Quote Originally Posted by Juleshsmith View Post

    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
    UA20 is not a RAM chip so that is OK. I'll explain address lines with follow up message. In mean time look at sheet one of schematic and find the 16 address lines. These signals are named BA15 through BA0 (for buffered Address). This address 'bus' of 16 signals is routed to many chips including the RAM, ROM and Input/Output chips like the 6520 and 6522.
    -Dave

  2. #12
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    There are 16 address lines out of the 6502 CPU. This is how the CPU identifies which particular memory or I/O location it wants to either read from or write to. Since each address line can either be in a 1 state(+5V) or 0 state (0 V), there are a total of two to the power of 16 combinations of addresses which equals over 64,000 locations (64K).

    Anyway, the state of the upper four address lines (BA15, BA14, BA13 and BA12) can give us a clue as to which of the five ROMs the CPU is trying to 'read'. So check the voltage of each signal starting with BA15 and write down the pattern, i.e., +5C, +5V, 0V and +5V. This tells us what ROM was being addressed. This example pattern means the upper bits were in a 1101 state which in computer shorthand is called hexadecimal 'D'. This means the UD8 ROM is being addressed and may be bad as the computer is hung up there. Measure those 4 signals and report back.
    -Dave

  3. #13
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    Excellent Dave.

    Not sure what this means...

    AB15 +62mV
    AB14 +52mV
    AB13 +51mV
    AB12 + 41mV

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

  4. #14
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    Quote Originally Posted by Juleshsmith View Post
    Excellent Dave.

    Not sure what this means...

    AB15 +62mV
    AB14 +52mV
    AB13 +51mV
    AB12 + 41mV
    I'll have to think about this. Apparently the machine got stuck while doing something in low RAM. It should not have been fetching instructions there. It should have been reading or writing data into perhaps zero page locations which should not have hung it up, but bad data in zero page RAM could have certainly caused the machine not to boot. So it is possible that you have bad RAM. Time for a piggyback test on the lower 16K RAM. Do you happen to have any spare 4116 dynamic RAM chips?

  5. #15
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    Excellent thx Dave.

    I think my DOA 8032 has a spare 4116 or 2. I'll get onto now!
    "DAVID: Is this a game... or is it real? JOSHUA: What's the difference?" 'Wargames' 1983

  6. #16
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    Ok. I've piggy-backed all the lower 16k RAMs (4116s) (UA5, UA7... UA9) but alas no change.

    Most of the DOA 8032 chips are socketed which makes it's easy. About 20% of the good 8032s are socketed.

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

  7. #17
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    curiously I tested ABs again
    AB12 + 0.9v
    AB13 +3.4v
    AB14 +1.1v
    AB15 +3.9v

    Fluctuating... does that still go along with your bad RAM theory Dave?
    "DAVID: Is this a game... or is it real? JOSHUA: What's the difference?" 'Wargames' 1983

  8. #18
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    Quote Originally Posted by Juleshsmith View Post

    Fluctuating... does that still go along with your bad RAM theory Dave?
    Not really, as now the addresses seem to fluctuate between $Exxx and $Axxx. 'E' is OK as that is the EDIT ROM where the screen is handled, but what does not make sense is how the machine gets to $Axxx. That is the spare ROM location. You may need to look with the scope to tell how much pulsing is going on. If we get lucky and the Edit ROM is bad, no soldering is necessary as that part is always on a socket. Make sure UD7 (EDIT ROM) is properly seated in socket. How tarnished does the UD7 socket look? Commodore used cheap sockets.
    -Dave

  9. #19
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    Reseated UD7 Dave. Socket looks good. No dry joints underneath Alas no change
    "DAVID: Is this a game... or is it real? JOSHUA: What's the difference?" 'Wargames' 1983

  10. #20
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    Quote Originally Posted by Juleshsmith View Post
    Reseated UD7 Dave. Socket looks good. No dry joints underneath Alas no change
    At this stage, you have some options.

    If your 8032 happens to have the two PIA and VIA chips on sockets (normally they are soldered), you could remove them and if one of them is the only problem, the welcome screen will come up. Also replace the 6502 CPU, but that chip rarely fails. Use proper 24/40 pin chip removal and insertion tools to avoid bending pins. See photos.

    If you feel like a lot of troubleshooting, build a simple NO OP Generator, then you can fire up your scope, and start checking address lines and chip selects hoping to spot a problem.

    If you want a quick chance to fix everything without troubleshooting, you can buy a RAM/ROM replacement module as there is a good likelihood that it's one of those chips. At least two good vendors sell a small board that fits into the CPU socket. The cost is about $70 USD maybe a little more with shipping so far away.

    I could send you some replacement EPROMs if you want to try unsoldering 24 pin chips. I've always been terrified when I have to remove a big chip as there is a possibility of damaging traces and plated thru holes. But using good technique which we can show you, it is doable.

    One question: What technique did you use with the piggyback test. With power off, did you place one RAM at a time and then try a power-on to see any flicker of change on the screen?


    40J743_AS01.jpg 70176423_large.jpg

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