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Thread: Revival attempt of C64C mobo rev 250469

  1. #1

    Default Revival attempt of C64C mobo rev 250469

    I got this black screen C64 motherboard recently and yesterday begun my attempt
    to bring it back to life.

    I first connected the pcb to power cable and to scart. Switched the power on.
    There was nothing, absolutely nothing. I would expect some sort of flash or blink
    from TV black screen change into a C64 black screen but there was no any kind of reaction on the television
    to turning power on.

    I then measured with a multimeter that there were at least some voltages coming to the pcb. Noticed that
    small green resistor near the tv modulator was getting awfully hot. There was also strange / bad odor coming
    from the tv modulator area. And below the pcb I noticed that someone had at some point removed (or tried to remove)
    that tv modulator before me. I had another same revision pcb that I have used as spares before and it was helpful again
    to donate tv modulator to this one. After replacing the tv-modulator, I have now clear flash/blink that I would expect to see
    when power is switched on to the pcb - yet the screen remains black. the resistor is no longer getting hot after power on.
    I'm hoping that this part of the issue is solved anyway.

    I let the pcb be on for a few minutes and ram chip at U10 was getting very hot. So I pulled that and it did not work on
    other pcb either - that was one issue, but still black screen. I noticed physical damage on one of the caps (pic 3) and cut one leg from it
    just to make sure it is not shorted and causing issue, but it was not. VIC chip and 8701 were on sockets and are tested good on
    working pcb.

    I think my next steps to check are the second ram at U11, both 6526 chips and the cpu. Hopefully I dont have to pull
    everything out before I reach the fried chip (assuming it is a chip issue in the first place). I need to find jupiter lander also,
    as it bypasses kernal and might reveal something.


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  2. #2

    Default

    Oh man, this is one unforgiving pcb !

    Tested so far by unsoldering and checking with working pcb:
    - Tv modulator BAD
    - U10 ram BAD
    - U11 ram BAD
    - U22 7406 logic BAD
    - U2 6526 GOOD !
    - U1 6526 - boots to blue screen but cursor missing - BAD
    - CPU - BAD

    This is one of those motherboards that make you wonder what on earth has happened to
    it in the past. Of course there is the possibility, that I have damaged some (or more) of the
    pulled chips while desoldering, but I usually get good chips out of the board without damaging them.
    At least I was successful in pulling the chips to avoid damaging traces on the motherboard.

    Still, I socketed all the slots that I had removed chips and tried the motherboard with tested and working chips but the
    screen remains black. There is still something wrong with it. And I'm soon running out of good candidates to check.

    Let's see... I guess not every unit is such that I can fix.




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  3. #3
    Join Date
    Sep 2012
    Location
    Indiana, USA
    Posts
    514

    Default

    With so many bad components, it might lighting damage.

  4. #4

    Default

    Bad power supply ...

  5. #5

    Default

    Yes, probably overvoltage from power supply, with or without lightning accident.

    I continued troubleshooting last night. The board was powered on for a while on my worktable
    and I noticed U4 kernal/basic rom getting a bit hot. Heating chips are always suspects. They normally
    get warm/hot too, but there is the fine line of detecting what may be caused by shorted chip.

    I often check back on Ray Carlsen's work and comments on chips, quote from Ray's site for U4 here:
    "U4 251913-01 BASIC and KERNAL ROM (same IC as U32 in C128 )
    Blank screen, no border. Cartridges don't work except the few that bypass the Kernal ROM such as CBM Kickman and Jupiter Lander. Those can be used as a diagnostic for a bad Kernal."

    Now, I have been trying the 'bypass' trick here, with jupiter lander and I also have tried
    the pcb with deat test cart and it remained black. I tried to be smart here and checked
    the schematics. U3 (7408 ) logic chip seems to control CS of U4 chip - so maybe that logic
    chip is the cause then for example keeping the chip selected at all times. Makes sense yes ?

    Well, I pulled 7408 out and replaced with a new, but pcb remained black. Have not tested that
    logic chip if it was faulty or not, but I will.

    Followed by that I decided that I'm anyway going to pull kernal rom U4 from the pcb. And I managed
    to finally break a trace there :/. Well, it is bound to happen some times even though you try to be careful.

    It was late and I had no time or energy to fix the trace yet, but I was still going to chect the board
    without U4 connected. As Jupiter lander or dead test cart might still bring it live. And you know what - it did !
    Yeah, finally light at the end of the tunnel =) .

    Now, I have some fixing and tidying up to do with this. I also want to test it comprehensively, before I can
    assume it fixed.

    Just a comment on Ray's advices, I appreciate and value his work and willingness to share information very, very much.
    He also gives a disclamer, that this is not an exact science to repair old commodores. Eventhough at this case the symptom
    or failure did not match Ray's suggestion, does not mean that in most cases his comment may be correct. Ray has probably
    helped many to bring old commodores back to life and I give all my thumbs up for him.

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    Last edited by VintageVic; March 5th, 2020 at 12:37 AM.

  6. #6
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
    Posts
    7,342

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    Congratulations!!!

    And I second your comment about Ray!

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