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Thread: Commodore PET restoration help needed

  1. #111
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    SkyCaptain,
    What happened with the EPROM programming? We could send you a programmed 2532.

  2. #112
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    Hey All, im very sorry about the late updates, the project has not stopped. I failed building a second adapter and at this point i do feel like its the programmer that notices that something isnt right and refuses to write to it. I have a neighbour though that has an old burner that might be able to do this. I will keep you all posted.

    Dave_m, thanks for the offer, it would be all 4 ROM chips, and i might come back to you if my neighbour cant write to the chips either.

    Thanks again for all your patience and progress updates will come.

    SkyCaptain

  3. #113
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    If your friend can not help with the programming, I will send the programmed 2532 EPROMs to you. It would take a while in the mail from America but we must get your PET working.

  4. #114
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    Hello everyone, I am back with an update!

    Sadly, my neighbour did not come through with the Eproms. Dave_m though did and i am extremely thankful for that! So thank you again.

    I installed all chips from D9 through D6 in the labelled order and started the machine. At first, the screen stayed black and nothing happened. I was kinda disappointed but started pressing buttons just for the hell of it. With each button press, more of the start text started to appear until finally it read the whole lines ending with something in the 15k RAM and displaying "ready". The cursor did not blink. It was solid. or it went away. The text sometimes switched from all caps to all small. I think i have to take a good look at the buttons of the kexboard too cause while trying to write "10 print "hello world"" i had quite the hard time. Sometimes the computer wouldnt update the screen and the second press would add the character a second time. Sometimes i do think its just dirty keys. Anyways, by the time i hit return to write the second line, the machine stopped responding completely.

    So we fixed so far:
    16K of RAM that looked really burnt removed, now the machine displays 16K correctly.
    Replaced the ROM with eprom, now making the machine somewhat functional.

    I hope these symptoms ring a bell to anyone of you
    SkyCaptain

  5. #115
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    Quote Originally Posted by SkyCaptain View Post
    Sometimes i do think its just dirty keys.
    Sky,
    This is good news that there is life in the old PET. Old key contacts getting non conductive is normal. Clean the keypads with isopropyl alcohol (at least 97%). If that does not work, coating them with silver conductive paint or circuit trace repair stuff will work. Two wires on the bottom of the keyboard assembly will have to be unsoldered. They have to do with the shift lock. The little key assemblies do not have to be removed (so not to lose any springs), just clean the pads. Remember to replace and tighten all the tiny screws on the key bottom of the circuit board so it will not flex under keyboard operation.

    The intermittent operation may be do to bad connections perhaps in the ROM/CPU sockets. What kind of test instruments do you have? A voltmeter? A logic probe?

    EDIT: Look up MG Chemicals Rubber keypad Repair Kit # 8339 for information on keypad repair kits.
    Last edited by dave_m; September 16th, 2020 at 08:47 AM. Reason: added MG Chemical info

  6. #116
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    dave_m,

    I have a mulimeter, and i have a logic probe.

  7. #117
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    Good. That should be enough to find the problem.

    One thing to do easily: with power off, gently and partially lift the CPU chip a little from one end of its socket to be able to rock it back into place to rub the oxide off the pins and make a better connection.

  8. #118
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    Alright ive dont that. I have removed and replaced all of the 65XX chips before when i checked all the resistances and lines.

    Further investigation of the behaviour reviled some following thing:
    The machine basically runs. I will clean the keys and i will check if i need to use that kit. Im not sure. Im more and more able to make each key show up. but...
    the experience is still really choppy. Generally speaking, the screen doesnt update unless a button is pressed. Sometimes only a single symbol would update, sometimes the whole screen.
    i was able to run:

    10 print "hello world"
    20 goto 10

    And it filled the screen, and it seemed to be running through without the lag that ive seen before. Until i stop the program and the stutter/lag returns.

    SkyCaptain

  9. #119
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    When the program is running, the data characters come from the main RAM, and when in direct mode when typing, the characters are held in the screen memory until the interrupt handler looks at them every 60 Hz. I believe screen scrolling is done in the interrupt handler. I hope there is not an issue with the screen refresh logic as that circuitry is a little complicated. We need some input from daver2, MikeS and the rest of the PET gang.

  10. #120

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    I'm not a Pet expert but the fact that is waits to see a key pressed to bring on the signon, leads me to think the computer is seeing a constant key press or something. Maybe related to an interrupt. One of the Dave's might understand better.
    Dwight

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