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Thread: PETunia's Repair Log

  1. #1

    Default PETunia's Repair Log


    I'm new to this forum (although not new to computers ) and have recently purchased a not working Commodore PET 2001 which I hope to repair...


    I have video which is good and characters are nicely formed so that is good

    I am working through the ROMs (these are 6540s)

  2. #2



    ROMS installed...

    H7 EMPTY
    H6 UVEPROM adapter installed upside down! Contains image slightly MODIFIED rom-1-d000.901439-02.bin and vanilla rom-1-d800.901439-06.bin. Should be in H2?
    H5 MOS / MPS 6540 / 012 4278A -> OK
    H4 MOS / MPS 6540 / 016 4278A -> OK
    H3 MOS / MPS 6540 / 015 4378A -> OK
    H2 MOS / MPS 6540 / 018 4478A -> OK but should be in H7????
    H1 MOS / MPS 6540 / 019 4178A -> OK

    I removed them all and read the contents.

    MOS / MPS 6540 / 012 4278A from H5 runs very hot and returns only 00s and 08s... -> DEAD

    I'm slightly concerned about the $D000-$D7FF region of MODIFIED rom-1-d000.901439-02.bin

    I think if I make a replacement for H5 which is MOS / MPS 6540 / 012 4278A aka rom-1-c800.901439-05.bin, put the MOS / MPS 6540 / 018 4478A from H2 into H7 and put the UVEPROM adapter the other way up into H2...

    ... need to create an H5.

  3. #3
    Join Date
    Feb 2009
    Southern California, USA


    My reading of the schematic indicates the following:

    C000 H1 901439-13

    C800 H5 901439-14

    D000 H2 901439-15

    D800 H6 901439-16

    E000 H3 901439-17

    F000 H4 901439-18

    F800 H7 901439-19

    Also check ROM images : It's a little confusing for these 2K ROMs.

  4. #4
    Join Date
    Feb 2009
    Southern California, USA


    These should be the part numbers and dash numbers for the 6540 ROMs. The parts themselves may just have the dash number such as MPS 6540-013 the rest of the number is the manufacturing date.

    You appear to have a mix of BASIC 1 and BASIC 2 ROMs (if I am seeing a -012. The photo is not clear). They may not play together. Note that Zimmer's calls BASIC 2 "BASIC 3". They call the Revised BASIC 1 "BASIC2". Very confusing.
    Last edited by dave_m; July 24th, 2020 at 01:23 PM.

  5. #5
    Join Date
    Feb 2009
    Southern California, USA


    If it turns out you have a bad mix of ROMs and/or have to replace a bunch of them, consider replacing all of them using The ROMulator. For your configuration (original 2001) you would set the dip switches to SW1 on, SW2 off, SW3 on, and SW4 off.

    In any case, good luck with the repair of your PET.

  6. #6


    I created a ROM replacement which I named a "Omni ROMulator" which I now realise clashes with the partial name of the product you replied with! Great minds think alike. I will rename my device at the next rev.



    I replaced $C000-$CFFF with a 4K Omni at H1 (also replaces H5) images rom-2-c000.901439-09.bin & rom-1-c800.901439-05.bin
    replaced $D000-$DFFF with a 4K Omni at H2 (also replaces H6) images rom-1-d000.901439-02.bin & rom-1-d800.901439-06.bin

    I am using the original MOS / MPS 6540 / 015 4378A at H3
    the original MOS / MPS 6540 / 016 4278A at H4

    I then burnt the tester into a 2716 and put it in the Skyles's adapter at H7.

    I also replaced some bad 6550 with parts supplied by Little Diode

    I get...

  7. #7


    Running the tester..


    Now the Skyles adapter is really designed for 4K so I think there is bus contention for $F000 and that's why the checksum of H4 is wrong.

    So... removing the Skyles adapter...

    I put the MOS / MPS 6540 / 018 4478A which arrived in H2 in its correct location H7

    I'm 80% sure I have a complete set of ROMs and good 6550s

    And turned on...

    I get garbage for a second and then the screen clears to Black

    No idea what to do next!

  8. #8


    I created a proper tester ROM for H7 today and tested again...


    So I now have a complete and working set of H1 to H6

    I also tested the original MOS / MPS 6540 / 018 4478A by putting it in H6 and get...


    which I think proves that works and so my moving the MOS / MPS 6540 / 018 4478A into H7 and using the other ROM/Replacements tested above that should give me a complete set.

    So now it boots to a black screen!

    One thing I did notice is that the tester ROM doesn't always give the same results... mostly it does but sometimes I get differing answers... maybe that is a clue.

    Apologies for the lag in these posts... I am a new member and all my replies have to be moderated.

  9. #9


    So this is what I currently have...


    Is it OK to have rom-2-c000.901439-09.bin in H1? I believe that is that the parts list at is implying... i.e. 6540-019 ROM at H1 which is 901439-09 ?

  10. #10


    So now it boots... first garbage and then the screen clears. I think I have good ROMs and OK RAM...

    Is there a way to test the PIAs and VIA? What should I expect if I boot with these removed?


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