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Thread: PET 8032 - Blank Screen / Lines at full brightness

  1. #1

    Question PET 8032 - Blank Screen / Lines at full brightness

    Hi all, It's been sensibly recommended that I make a new thread rather then derail an existing one (Sorry!)

    Little bit of backstory - I picked this up from an auction with no real knowledge of what it was, an hour of research later i'm super excited about my new PET



    I've picked up an oscilloscope since it looks like i'll need one, Keyboard and video cables have continuity though i've not been able to overflow the screen by holding spacebar (or maybe spacebar isn't working)

    It boots far enough to beep, I've gone to zimmers to grab the pin-outs for the display output, but at the time of writing it appears to be down


    Code:
    Processor  - SY6205A, But not a MOS chip, (not sure if this is correct). There's two other chips in sockets
    
    MOS - Above the screen connector
    904147-10
    
    MOS - Second row above the serial number ( I'm thinking this is the ROM based on previous reading? )
    904198-01

  2. #2

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    Quote Originally Posted by sabrent View Post
    Hi all, It's been sensibly recommended that I make a new thread rather then derail an existing one (Sorry!)

    Little bit of backstory - I picked this up from an auction with no real knowledge of what it was, an hour of research later i'm super excited about my new PET

    I've picked up an oscilloscope since it looks like i'll need one, Keyboard and video cables have continuity though i've not been able to overflow the screen by holding spacebar (or maybe spacebar isn't working)

    It boots far enough to beep, I've gone to zimmers to grab the pin-outs for the display output, but at the time of writing it appears to be down


    Code:
    Processor  - SY6205A, But not a MOS chip, (not sure if this is correct). There's two other chips in sockets
    
    MOS - Above the screen connector
    904147-10
    
    MOS - Second row above the serial number ( I'm thinking this is the ROM based on previous reading? )
    904198-01
    First of all, turn down the brightness on the monitor, there's a potentiometer on the back! If it doesn't turn black (with or without any other normal display) you need first of all solve the monitor fault.
    The good news is that the mainboard is probably outputting the right vertical and horizontal sync signals.

    Alternate site for schematics:
    https://www.commodore.ca/manuals/fun...t/index-t.html

    If the monitor keeps on high brightness when turning back the potentiometer, this video might have some hints:
    https://www.youtube.com/watch?v=G06PESFu_Zw

    Though the 8032 has a 12" monitor instead of the 9" one shown in the video, so you must get the correct schematic with voltages, test points etc.

    Yes, the SY6502 from Synertek was a common replacement in the good old days. My PET 3032 has an MPS6502 (MOS, plastic package), My PET 2001 has a R-6502 (Rockwell, replaced the original processor sometimes in 1981), My Apple II has a SY6502 looks original as the datecodes of all other ICs are close enough to the CPU's datecode.
    If I'm not wrong, Synertek and Rockwell were the earliest second-sources of the MOS 8-bit microcomputer chips.
    HTH

    Frank IZ8DWF

  3. #3

    Default

    Hi Frank, Thanks for helping

    I've turned the screen down till it's black, Looking at the board layout on the manuals for the 8032, the J7 screen connector isn't in the same place, could this be an issue or would the pinouts be the same?

  4. #4
    Join Date
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    Default

    Hello and welcome!

    If I may make a suggestion: when trying to diagnose remotely details are very important; I suspect the numbers on those ICs are really 901447-10 (character generator) and 901498-01 (50Hz Edit ROM) (not 904147 and 904198 ).

    It's not crucial in this case but typos like that can waste a lot of time when it comes to IC and pin numbers; also, instead of vaguely referring to ICs by their location please use the Uxy designation (x=column, y=row) next to them that matches the schematic.

    Thanks; looking forward to helping to revive another PET.

  5. #5
    Join Date
    Jun 2012
    Location
    UK - Worcester
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    Default

    Welcome to your own thread !

    You will need to get a few more posts (a total of 10 ish) to prevent your posts from being moderated.

    When you say "... it appears to be down", what do you actually mean? There are three signals on the connector to the monitor, the video signal, the horizontal synchronisation and the vertical synchronisation. How many of these three signals actually has a signal. If the H and V synchs. have a signal, what frequency is it?

    This tells us something important!

    Dave

  6. #6

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    Quote Originally Posted by MikeS View Post
    ~I suspect the numbers on those ICs are really 901447-10 (character generator) and 901498-01 (50Hz Edit ROM) ~ please use the Uxy designation (x=column, y=row) next to them that matches the schematic.~
    You suspect correctly, I'm not sure how I misread that, I'm still looking for the board diagram, I've found one on commodore.ca, but the video connector isn't in the same location


    When you say "... it appears to be down", what do you actually mean?
    The website isn't loading for me in the UK, Connection gets reset after about 30 seconds


    In rather bizarre news it's managed to fix it's own video signal after being on for about 15 minutes, and i'm now getting this with the beep, but it only stays for about half a second so I had to snap the picture out of a video
    Discord_tnUd6yg00r.jpg

    Daver - If it's still applicable I can check the video connector with the scope, do you have the pinout on hand as i'm still trying to find it?

  7. #7

    Default

    That is random stuff in the video RAM when it starts. It looks like it is doing the proper initialization of the screen but not bringing up the signon prompt. This would indicate that the problem is likely ( note the word likely ) one of the system ROMs or the system RAM. It could still be other things.
    Do you have a way to read and write EPROMs?
    If you have the scope, you can make a NOP generator and look to see if all the selects are active, as well as the address lines to the devices. That will eliminate that problem. If you have a couple 40 pin sockets, it looks easy to make the NOP generator. Do a search on this MB. There is a recent post showing how to make the 6502 NOP generator. What it does is isolated the boards data bus and lets the 6502 see a constant NOP. The 6502 will then generate every address in a 64K loop. It will tell you quite a bit about the general health of the machine. Do disconnect the video while using the NOP as it will not get useful values anyway.
    Eventually you'll either need to blow a EPROM for diagnostics or have someone make one up for you and send it to you.
    Dwight

  8. #8
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
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    Quote Originally Posted by sabrent View Post
    Daver - If it's still applicable I can check the video connector with the scope, do you have the pinout on hand as i'm still trying to find it?
    What is the PC board number? There are at least three different versions of that board.
    Video J7:
    1 Video
    2 Gnd
    3 Vert. Drive
    4 Gnd
    5 Horiz. Drive
    6 Missing
    7 Gnd

    Also on the user port:
    2 Video
    9 Vert. Drive
    10 Horiz. Drive
    1 & 12 Gnd

  9. #9

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    Quote Originally Posted by MikeS View Post
    What is the PC board number? There are at least three different versions of that board.
    Universal Dynamic PET - 8032089

    Video - Signal appears briefly on startup as expected

    Vert drive
    Vert Drive.jpg

    Horiz Drive
    Horiz Drive.jpg


    Do you have a way to read and write EPROMs?
    Hi Dwight
    Unfortunately not, Is it possible to get a pre-burned diagnostics chip? I'll have a search for the NOP generator thread, though making it might be above my level

  10. #10

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    Quote Originally Posted by sabrent View Post
    Universal Dynamic PET - 8032089

    Video - Signal appears briefly on startup as expected

    Vert drive
    Vert Drive.jpg

    Horiz Drive
    Horiz Drive.jpg



    Hi Dwight
    Unfortunately not, Is it possible to get a pre-burned diagnostics chip? I'll have a search for the NOP generator thread, though making it might be above my level
    Making the NOP generator is much easier then unsolding and replacing a part.
    Dwight

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