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Thread: Commodore PET restoration help needed

  1. #1
    Join Date
    Apr 2020
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    Basel-Stadt, Switzerland
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    Default Commodore PET restoration help needed

    Hello good people of the Vintage Computer Forums,

    I bought my first non working Commodore PET and am a bit at a loss regarding the lack of resources i can find online.

    I have a pretty clear issue but no amount of googling has given me any useful results.

    When I turn on my Machine, all i see is a "#" symbol on the top left corner. What could be the cause for this.

    One real problem with this machine is that i see at least 3 custom modifications (reset button for example), so its going to be even harder for me to actually trouble shoot.

    If anyone has an idea of where to start the trouble shooting, i would highly appreciate it!

    I will provide pictures later but i need a camera that actually works for these kind of pictures. my shitty smart phone camera just doesnt work.

    Thanks a lot!!

  2. #2
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    Apr 2020
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    Default

    I finally have pictures to show!

    But yeah, first order of business is cleaning the machine

    https://imgur.com/a/dqFy3in

  3. #3

    Default

    What model of PET do you have?

    You can find schematics and other resources on Zimmers here http://www.zimmers.net/anonftp/pub/c...pet/index.html

    A single # in the corner is unusual. The rest of the screen is blank?
    One of the first things the startup process does is clear the screen of the random values in video ram, so it seems the startup process is getting that far at least.
    If the video ram was bad (which is common) you would normally see all of the same character repeated over the whole screen. If the character rom was bad, the # character wouldn't look correct or the whole screen would be white or green as if a reverse space was covering the whole screen.
    If the system ram was bad, I wouldn't expect the startup process to get as far as clearing the screen and you'd likely be seeing random characters over the whole screen.

    If you have a cassette drive for your PET (or if one is built in), listen when you first power on for the sound of the cassette motor turning for about a second and then it should stop.
    The motor is not controlled directly by the buttons on the tape drive. When you press play on the cassette drive, the computer senses that you have pressed a key and turns on the motor.
    If the motor turns on and off like it should, that means the CPU is running the kernel routine that polls the sense line and turns the motor on and off.

    My first guess is that you may have a bad Basic ROM. The edit ROM seems to be clearing the screen and it wouldn't get that far without the Kernel.
    The # character is the first thing that is normally printed in the top-left corner for Basic v2.0 ### COMMODORE BASIC ###
    So possibly the basic rom is degraded and crashing after the startup process jumps to basic.

    If you can, post (or link to) some photos of the PET and the "custom modifications" you have questions about.
    Last edited by Hutch; May 20th, 2020 at 03:56 PM.

  4. #4

    Default

    I see the photos now. What you have there is a 2001-N.
    ROM upgrade kits and option rom socket often leave one or more ROM sockets empty.
    I'd lookup and double check the ROMs you have there to make sure you don't have one missing or an extra one in an option rom socket.

    Yeah, that's a reset button. Looks like it's connected to the 555 power-on reset chip.
    Try pressing the button and you should get a repeat of the power on sequence but with the video ram already cleared.

    The CPU is definitely running the startup routines. Something is crashing it before it starts basic. Most likely a bad ROM.

  5. #5
    Join Date
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    Default

    So how many ROM slots are taken up by default? Does an upgrade mean they were all replaced or that some were added? My question more or less is if i can reverse the upgrade by taking out the added ROM chip? Like, i assume the one with the yellow sticker on it is after market. taking it out would leave 3 slots empty, and i do see pictures online of this mainboard with 3 empty slots.

    Sorry, i want to be very careful here and not just do things that i dont know the effect of.

  6. #6
    Join Date
    Apr 2020
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    Basel-Stadt, Switzerland
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    Default

    So how many ROM slots are taken up by default? Does an upgrade mean they were all replaced or that some were added? My question more or less is if i can reverse the upgrade by taking out the added ROM chip? Like, i assume the one with the yellow sticker on it is after market. taking it out would leave 3 slots empty, and i do see pictures online of this mainboard with 3 empty slots.

    Sorry, i want to be very careful here and not just do things that i dont know the effect of.

  7. #7

    Default

    Quote Originally Posted by SkyCaptain View Post
    I finally have pictures to show!

    But yeah, first order of business is cleaning the machine

    https://imgur.com/a/dqFy3in
    That's a 2001N motherboard, the second revision of the CBM PETs after the 2001. It uses dynamic RAM.
    Your issue is likely depending on dirty ROM socket(s) or bad ROM(s). The CPU can go as far as clearing all the video RAM, that's done by the uppermost ROM. Then it gives control to the Editor and Basic ROMs and that's where things go bad.
    I would also remove those three diodes close to the 74154, since they were clearly part of a bigger mod. They can't do much in the way they're left now, but also there's no reason to leave them connected.
    Do you have some test equipment and a way to read ROM/EPROMs?

    Frank

  8. #8
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    Default

    Quote Originally Posted by iz8dwf View Post
    Do you have some test equipment and a way to read ROM/EPROMs?

    Frank
    Hi Frank

    I do not as right now but its on the list of items to buy. I was looking at one of these: https://www.ebay.ch/itm/USB-TL866II-...704214ef0bdfa6 but im not sure yet if it can read an eprom too so further reading is necessary on this end too. Im gonna need a multimeter too. Im not sure about removing the mods just yet even though that was my first idea too as other people on the 6502 forum said that this was a bad idea because some of these could actually be from the factory.

    Emmanuel

  9. #9
    Join Date
    Jun 2012
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    Default

    Welcome to VCFED.

    You will need a multimeter and a logic probe as a minimum to perform fault finding.

    To repair any faults you find will generally require soldering and desoldering skills.

    The fact that the screen clears is very good news.

    Gut feeling tells me that you may have a faulty ROM, or it may be something to do with the butchery around the 74151.

    Yes, a list of the ROM numbers would be good.

    Two further things we may need is a NOP generator (this can be made yourself from two 40 pin IC sockets and some resistors) and a copy of my PETTESTER code to burn into a 2716 EPROM to replace the EDIT ROM. This provides a series of tests to checkout the basic machine operation. Either you can burn your own EPROM or ask someone nicely to send you an EPROM and recover their costs.

    Where are you in the world?!

    Dave

  10. #10
    Join Date
    Jan 2013
    Location
    Washington State
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    Default

    Before your test equipment arrives you can try reseating the socketed ROMs by carefully lifting them from their socket and pressing them back in.

    It looks like you have BASIC 2.0 with a couple add-on roms. The rom with the yellow sticker and the one with clear tape on it are obviously add-ons, I can't read what they are from the pics.
    Maybe take a closer pic of the back third of the motherboard.

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