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Thread: Commodore PET 4016 - REPAIRED!

  1. #21
    Join Date
    Jun 2012
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    Many years ago I came across IC sockets that had been 'life expired' in the factory by firmware changes before releasing the items to us. We had some problems so I had to do debugging. I was using a Z80 ICE - so generally I was quite happily debugging the hardware. When I removed the ICE - the darn unit wouldn't work. Twisting the ROM in the socket - and voila, a working unit...

    Anyhow, I was thinking of swapping the two tests for Page 0 and 1 RAM around. My first test forces $55 and $AA into the RAM and loops if I don't get the correct data back. The problem with this is that you can't see what the data was that was read in error. The second test fills the RAM with $00, $01, ... $FF and checks the result. Whatever is read back (on an error) gets displayed on the VIDEO screen.

    If you have a RAM fault (in the case I now have) you are none the wiser. In the proposed 'test swapped' mode - you would stand a chance of identifying the faulty RAM by looking at the displayed character, converting it to binary and (from a knowledge of the address) hopefully identifying the fault.

    Thoughts?

    Just 'easing myself' back in after a holiday (and disassembling some 6800 code whilst in the hotel - so my brain is now in 16 bit X register mode...).

    Dave

  2. #22
    Join Date
    Feb 2009
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    Quote Originally Posted by MikeS View Post

    FWIW, from the Commodore docs: "Resistance shall not exceed 1 Kohm over life of contact (10 million operations)."
    Mike,
    1 K Ohm is an interesting spec and brings up an idea. On the main board there is a 10K Ohm pull-up network for the eight Column lines to be read by the PET while it strobes the row lines. If the impedance of the keypad was 1K, then the voltage at the Pet PIA input due to the voltage divider effect would be 5 *1/(1+10) = 0.45 V, a good logic LOW. Any higher switch impedance would cause a questionable logic LOW and lead to a NO key detected.

    What if the resistor network (14 pin package) was changed to a 100K Ohms? Then it would take a much higher switch impedance of of over 10K Ohm before the keyboard failed.

    Of course the RC time constant when the key was released to when the input is pulled up to a good HIGH might lead to double characters on the screen depending on the de-bounce logic in the PET scan routine.
    -Dave

  3. #23
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    Feb 2009
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    Quote Originally Posted by daver2 View Post
    I was using a Z80 ICE - so generally I was quite happily debugging the hardware.
    Z80 In-Circuit Emulator! I remember. What a fun way to debug. Setting break points, traps, and complex triggering, etc. I hope you had a Symbolic Disassembler as part of it to watch the code run in Symbolic instead on 1's and 0's. Too much fun and getting paid for it!

    Anyhow, I was thinking of swapping the two tests for Page 0 and 1 RAM around. My first test forces $55 and $AA into the RAM and loops if I don't get the correct data back. The problem with this is that you can't see what the data was that was read in error. The second test fills the RAM with $00, $01, ... $FF and checks the result.

    Whatever is read back (on an error) gets displayed on the VIDEO screen.
    Yes! this would be a good screen output for troubleshooting. You could then probably skip the alternating pattern test.

  4. #24
    Join Date
    Jul 2016
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    Ontario, Canada
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    If it's any help: my SuperPET keyboard appeared totally dead when I first got it. However, I cleaned all graphite contacts with alcohol and this revived about half of them. Some of the keys required hard presses or many presses to even work (after cleaning). Then after a few days of use I noticed the keys started becoming easier and easier to use again, until they all were back to normal (worked on first press).

    I find that if I don't use they PET for a while, some of the more troublesome keys start to get tough to use again, but after a while they "soften up" to normal again. Weird but it works.

    That's what I would have suggested before using the contact restorer. The 8-bit guy on YouTube had success with the contact restorer though (although I think it would have been easier try the above method first).

  5. #25

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    Hi all!

    Just checking in to let you know where my PET stands right now with the keyboard. After lightly sanding off the MG Chemicals adhesive, I cleaned everything with alcohol and reassembled the keyboard. All but 13 keys worked. I guess that's an improvement! Still, this is very frustrating. I really appreciate everyone's input with suggestions for repair. To be continued...

    -Mark

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