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Thread: PET 2001-8 (Blue PET) needs HELP!

  1. #1

    Default PET 2001-8 (Blue PET) needs HELP!

    Hi,

    I have a sick blue PET (The one with 6540 ROMS and 6550 RAMS)

    it started out with the famous random Character screen, and today after switching it on i got no display at all ...

    I do have an oscilloscope, so i tested the Video out pin and it just stays High ... VSYNC and HSYNC are there and seem OK ...

    I also tested the Powersupply (transformer) and i measured 14V AC, 7V AC and 14V DC, there is also 14V AC going to the Monitor
    the 14V DC is going to all 4 7805 Chips and i have +5 on all the 4 Outputs ...

    Is 14V DC OK? it seems very high ...????? and the 7805´s are getting pretty hot ...

    Greetings TOM from Vienna

  2. #2
    Join Date
    Mar 2010
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    Florida, USA
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    Too funny. I was just coming here to start my own thread about a Blue Pet 2001-8 I rescued from the recycling bin at the local dump last week Mine has the random character display, and seems to run fine in the 5-minute intervals I've left it on. I really should pull out the multimeter and test further voltages! My screen also jumps slightly up and down along the vertical axis (the whole display, not just a single/group of rows/columns). And one of the stems on the keyboard is broken (as well as missing about a dozen keycaps, sigh)

    I'll refrain from starting my thread for now and see how your troubleshooting goes, making my own thread if need be
    ---
    Currently seeking:
    * Roland MPU-401/AT (with daughter card header)
    * Magitronic K-156 Keyboard (5pin DIN w/ XT-AT switch)
    I also collect PC and C64 Sierra On-Line software!

  3. #3
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    Apr 2011
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    I had a random character screen problem on another PET, and it turned out to be bad capacitors in the reset circuitry.
    You might want to check the reset pin on the 6502 - pin 40. It should be low for about a second, then go high and stay there. A reset pin stuck low would be a possible explanation for the random characters.

  4. #4
    Join Date
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    Toronto ON Canada
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    Quote Originally Posted by scouter3d View Post
    ...I also tested the Powersupply (transformer) and i measured 14V AC, 7V AC and 14V DC, there is also 14V AC going to the Monitor
    the 14V DC is going to all 4 7805 Chips and i have +5 on all the 4 Outputs ...

    Is 14V DC OK? it seems very high ...????? and the 7805´s are getting pretty hot ...
    Hello Tom, and welcome!

    No, assuming you're measuring correctly, 14VDC is definitely twice what it should be. Are you sure the power connector and/or the transformer is/are wired correctly?

    That PET uses a centre-tapped full-wave circuit which should give you roughly 7.5 to 8V per leg; sounds like you've got them both in series instead of parallel somehow.

    The monitor should connect to terminals 7 & 8.
    The AC centre-tap (terminal 5, ground, black) should connect to pin 3 (the centre pin).
    The two AC legs (terminals 4 and 6, brown) should connect to pins 1 and 5 (the two outside pins).
    The DC to and from the capacitor (red) should connect to pins 2 and 4.
    Orientation of the connector doesn't matter.


    I think you should get video of some kind even if the CPU is halted; sounds like maybe the clock isn't running at all. Try removing the character generator IC and see if it makes any difference.

    In case you didn't know, there are schematics for all 4 versions and the monitors here:

    http://www.zimmers.net/anonftp/pub/c...001/index.html
    Last edited by MikeS; September 30th, 2011 at 10:07 AM. Reason: Add wiring detail

  5. #5

    Default

    @ MikeS : Thank you for the voltage information! I think some preowner accidently switched 2 wires on the transformer!!!!!

    I also think i should get Video even with the CPU haltet, or the video ram removed, ... removing/changing the character generator makes no difference

    Thanks, i have allready loaded the shematics ...

    Now i will switch back the wrong wires on the transformer ... and start checking signals again ...

    PS: i have made a internet page with pictures of the repair process here: http://www.8bit-homecomputermuseum.a...t/bluepet.html

    Greetings TOM

  6. #6
    Join Date
    Dec 2005
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    Quote Originally Posted by scouter3d View Post
    ...PS: i have made a internet page with pictures of the repair process here: http://www.8bit-homecomputermuseum.a...t/bluepet.html

    Greetings TOM
    An excellent writeup! You have been having fun with that baby, haven't you...

    Those are the worst (or the best if you enjoy challenges ) - the ones where someone before you has tried to fix it and added a few unknowns in the process...

  7. #7

    Default

    Yes! i have lots of fun and learning! with my little PET!!!

    I have switched the wires on the transformer, now all the 7805´s run much cooler ... (on 8V instead of 14V)

    Still no Video on PIN 1 of the Monitorconnector (only static High) ...


    Question) would the PET generate video without the 2 PIAs and the VIA (or with a defect PIA/VIA) ????

    Reset Signal on CPU seems OK (goes high after 1-2 seconds and stays there)
    Clock on CPU is OK also Phi2 is OK
    But there is no pulsing on the Address lines ... (maybe bad rams/ bad roms)

    But the PET should generate Video even without the CPU running ...

    So where to look next???

    Greetings TOM

  8. #8

    Default

    Hi,

    I solved the "no video" problem: it was a defect 74LS74 (C1) !!!

    Now i have Video

    But the CPU seems to start only every 10th bootup ...

    Greetings TOM

    Follow my progress here: http://www.8bit-homecomputermuseum.a...t/bluepet.html

  9. #9
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
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    Quote Originally Posted by scouter3d View Post
    Hi,
    I solved the "no video" problem: it was a defect 74LS74 (C1) !!!
    Now i have Video
    But the CPU seems to start only every 10th bootup ...
    Excellent! Have you tested the RESET circuit? How does it behave if you manually reset it at the 555 (DE9)?

  10. #10

    Default

    I will have a look at the RESET circuit ... but i think the RESET signal is fine because i can see a change on the Address and Datalines everytime i RESET ...

    Does the PET need all ROM´s present to boot? or can i exclude one or another?

    How many RAM chips need to be present?

    Can i use Ruud Baltissens PET test rom with my 6540rom.com adaptor? and in which ROM slot? Alone (without other ROMs)? With howmuch RAM? Without RAM?

    Greetings TOM

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