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Thread: Help a sick PET

  1. #1

    Default Help a sick PET

    Hello everyone. I recently bought a commodore PET from a nice older gentleman who said it was from an elementary school he was a principle at. When I first turned it on it showed the garbled screen I've seen in photos before but after turning it on again it showed this screen.



    The screen isn't frozen but there are obviously lines on it and the keyboard is unresponsive. Any help would be appreciated. Thanks!

  2. #2

    Default

    If there are any socketed chips first try reseating those just in case it's something super simple like a bad connection. There are some really smart PET experts here that can give you some better clues but in the mean time search the forums too and you might come across a similar trouble shooting session. Sometimes it's a bad CPU depending on what the screen looks like (you can test that with another 6502 chip) not clearing the screen and initializing the BASIC ROM.
    Looking to acquire: IBM 5100, Altair 8800

  3. #3

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    I'm not a PET expert, but I have some experience with the 6502-based Apple II's. I'd say it's bad RAM. There's probably a bit stuck somewhere in one of the RAM chips and it's causing garbage "!" characters. I had the same problem with my Apple //c when I first got it, except instead of ! being displayed, it was $

  4. #4
    Join Date
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    Västerås, Sweden
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    Default

    So every 17th position has a ! (ASCII 33) instead of a space (32). If it is a video RAM issue, wouldn't the second "O" in COMMODORE and the "Y" in BYTES also get corrupted? If you clear the screen with CLR/HOME, do the exclamation marks remain?

    For your information, the screen memory starts at location 32768. It means you could try some POKE 32768,85 to get an "U" in the upper left. You could even make a loop:

    FOR I=0 TO 199:POKE 32768+I,85:NEXT

    which would fill the top five rows with "U". This pattern in binary happens to be 01010101, and its complementary pattern you might want to try is 170 (a graphics symbol). All this is assuming you freely can use the computer, despite the unresponsive keyboard. If the exclamation marks are in your way, you should at least be able to manually type various characters on screen and see what the outcome is.

    Since the CPU executes a fair chunk of code to get to that state, including calculating available memory, it would not be my first suspect.
    Anders Carlsson

  5. #5
    Join Date
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    Sevenoaks, Kent, United Kingdom
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    Default

    Maybe I missed it but what Model of PET is it ?

  6. #6
    Join Date
    Dec 2012
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    São Paulo country, Brazil
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    Of course reseating all socketed ICs is a good idea. Also extracting one IC at time and verifying if they have rusty pins (and sanding pins a little if needed) could help.

    Another good behavior, if the board is dirty is: wash the board. Use hot water and a toothbrush, then dry it with an hairdryer, leave it under the sun for some hours to be sure everything is dry before to power it. I recently fixed the same problem you have simply changing rusty sockets and washing the board
    (see http://www.vintage-computer.com/vcfo...-on-the-screen )

    If the keyboard doesn't work, the culprit could be one of the two 6520 (if I'm right and your PET is a 30xx -like 3016-, it should be UC7).
    You also should tell us the model: is it a 16k or a 32k ? Because if it's a 3032, the ram amount is wrong.

    of course it could be a video ram problem (2114 chips, that luckily are still available and not too expensive on eBay).

    -- Giovi

  7. #7

    Default

    Thanks for all the help everyone. I know very little about the PET so all the help is appreciated. The model of the pet is model 2001 with the more traditional style keyboard and the external tape deck that connects from the back. I also have the tape deck and some school learning programs but I can't seem to get any response from it. I don't know if load has to be typed on the computer before power is sent to the tape drive or if mine is just dead. Thanks and I'll try out some of these solutions later this evening!

  8. #8
    Join Date
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    Default

    So a 2001N that in some parts of the world would be known as a 3016 IIRC. Non-CRTC, is that Basic V1 or V2 with the ### message?
    Anders Carlsson

  9. #9
    Join Date
    Dec 2012
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    São Paulo country, Brazil
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    Quite strange, because AFAIK the PET 2001 should be 4k or 8k, and the boot screen should be

    *** COMMODORE BASIC *** (Basic 1)

    and not

    ### COMMODORE BASIC ### (Basic 2)

    So it could mean the PET was upgraded, maybe with a newer board (30xx series)... Can you post a detailed picture of the board? It should help to understand what is the board.

    -- Giovi

  10. #10
    Join Date
    Dec 2005
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    Toronto ON Canada
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    7,176

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    Quote Originally Posted by giobbi View Post
    Quite strange, because AFAIK the PET 2001 should be 4k or 8k, and the boot screen should be

    *** COMMODORE BASIC *** (Basic 1)

    and not

    ### COMMODORE BASIC ### (Basic 2)

    So it could mean the PET was upgraded, maybe with a newer board (30xx series)... Can you post a detailed picture of the board? It should help to understand what is the board.

    -- Giovi
    No; the original PET using static RAM would be known as a 2001-4 or 2001-8 depending on the amount of memory, and each came in four versions depending on the type of RAM and ROM chips.

    This was replaced by the 2001N which used dynamic RAM chips, either 16 or 32K, which was also known as a 2001B, 3032, 4032, etc. depending on where it was sold, how much memory and what type of keyboard it had and which version of BASIC, but they all used the same basic circuit board until the 8032 and "universal" boards came along.

    This is presumably a 2001N/3032 with BASIC 2 (or 3 depending on how you count ) and 16K (although as mentioned it could be a 32K with bad RAM).
    Last edited by MikeS; August 20th, 2013 at 09:30 AM.

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