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Thread: C64 Breadbin blackscreen need help

  1. #1
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    Default C64 Breadbin blackscreen need help

    so I was given two C64 breadbin units in rough shape with smashed keyboards. Be it that I feel the need to fix EVERYTHING I have spent some hours on these models. Got the first unit working by just cleaning contacts.

    The second one is proving tricky. I have swapped and socketed all the major IC's and TTL logic chips and RAM. All the TTL checks ok, all the major IC's work fine on another unit. I swap all the RAM around but just a black screen. Swapped Fuse (for sanity sake) and electrolytic Caps even though they are within tolerances.

    Could use some pointers on some points to check.

    * Just a note, the Vic II chip when plugged into a working machine has vertical lines but still works.

  2. #2

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    Quote Originally Posted by VERAULT View Post
    so I was given two C64 breadbin units in rough shape with smashed keyboards. Be it that I feel the need to fix EVERYTHING I have spent some hours on these models. Got the first unit working by just cleaning contacts.

    The second one is proving tricky. I have swapped and socketed all the major IC's and TTL logic chips and RAM. All the TTL checks ok, all the major IC's work fine on another unit. I swap all the RAM around but just a black screen. Swapped Fuse (for sanity sake) and electrolytic Caps even though they are within tolerances.

    Could use some pointers on some points to check.

    * Just a note, the Vic II chip when plugged into a working machine has vertical lines but still works.
    Hi,
    wow... you socketed and swapped everything it seems.
    Are the supplies ok? Did you check with an oscilloscope how the vital signals look like at the CPU pins?
    Did you check how the video signal looks like right at the VIC-II output pin?
    Assuming all that desolder/solder work didn't introduce any other problem, it could be a broken track somewhere, a shorted capacitor, an open resistor...
    I've repaired many stubborn C64 black screens, but I wouldn't work blind (i.e. without oscilloscope support) on a repair like this.

    Frank IZ8DWF

  3. #3
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    Well unfortunately I need to make the repair without a working scope. Have a logic probe and good multimeter. I'm guessing I cant check for good video signal with either however.

  4. #4

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    Quote Originally Posted by VERAULT View Post
    Well unfortunately I need to make the repair without a working scope. Have a logic probe and good multimeter. I'm guessing I cant check for good video signal with either however.
    I think at this stage it will be difficult then.
    Might be a single address signal or data signal not going where it should go.
    With a logic probe, you can go on all data pins on all the relevant chips and see if something is stuck/floating. If not the data bus, then same check on the address bus reaching all the way down to the chips.
    Then the select and R/W signals and so on...
    Of course assuming that you checked at least the clocks, reset and interrupts (both).
    You must have lots of time and patience to attempt this imho

    Frank

  5. #5
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    Quote Originally Posted by iz8dwf View Post
    I think at this stage it will be difficult then.
    Might be a single address signal or data signal not going where it should go.
    With a logic probe, you can go on all data pins on all the relevant chips and see if something is stuck/floating. If not the data bus, then same check on the address bus reaching all the way down to the chips.
    Then the select and R/W signals and so on...
    Of course assuming that you checked at least the clocks, reset and interrupts (both).
    You must have lots of time and patience to attempt this imho

    Frank
    Thanks Frank, When it comes to this stuff I have plenty of patience.. Just no so much time.

  6. #6
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    Does anyone have a link to a schematic for a 1984 C64 board Assy No. 250425?

  7. #7
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    Here you go: http://personalpages.tds.net/~rcarls...MATICS/250425/

    However, this might not suit your needs. If I find another link, I'll post it on here.
    Current retro systems:
    2x Commodore 64 Breadbin 250407 Rev. B and C
    Packard Bell Pack-Mate 28 Plus, aka, the Ultimate Sound Card machine
    iMac G3/600 Graphite, Mac OS X 10.4.11 Tiger, Lubuntu 16.04.3 LTS
    iMac G4/800 Lampshade, Mac OS X 10.4.11 Tiger
    YouTube channel: https://www.youtube.com/c/RetroPCUser

  8. #8
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    Quote Originally Posted by retro-pc_user View Post
    Here you go: http://personalpages.tds.net/~rcarls...MATICS/250425/

    However, this might not suit your needs. If I find another link, I'll post it on here.
    Thanks, that was fast.

  9. #9
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    Ok made some progress. I determined I wasnt getting a clock pulse so I pulled the MOS 8701 out of a working machine and tried it and I got something for the first time. A textless screen with border. u1_2.jpg

    Well one thing at a time, does anyone know where I can source a cheap replacement to the MOS 8701 or know of a modern replacement?

    My next step is to desolder the ROM IC's U3 and U5 and try them in another unit as well.

  10. #10

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    Quote Originally Posted by VERAULT View Post
    Ok made some progress. I determined I wasnt getting a clock pulse so I pulled the MOS 8701 out of a working machine and tried it and I got something for the first time. A textless screen with border. u1_2.jpg

    Well one thing at a time, does anyone know where I can source a cheap replacement to the MOS 8701 or know of a modern replacement?

    My next step is to desolder the ROM IC's U3 and U5 and try them in another unit as well.
    That symptom (no text on a correct border/background) is almost always a faulty Basic ROM.
    You should have watched one of my videos to spot at the first sight a faulty 8701, the black screen isn't black, there's just not even the sync there. On a CRT monitor the difference is quite large. A working 8701 produces a flash and sync lock on a CRT monitor. A dead 8701 produces no change in the black picture at power on.

    Frank IZ8DWF

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