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Thread: PET 2001-32N - Diag works in UD9, but with Kernal it fails to start

  1. #1

    Default PET 2001-32N - Diag works in UD9, but with Kernal it fails to start

    I'm a bit stumped on this one. I'm waiting on some LS TTL parts to come to replace some marginal components in the video signal circuit on the motherboard, but I have things working well enough that I get a pretty solid composite signal out. I've also decided to move forward with replacing the 9" CRT, which has been nothing but problems, with a 9" LCD mounted in its place (hence the composite conversion).

    My problem now is that with the typical PETTEST ROM in place of the Kernal, I had been getting screens of Gs and character sets, no problem. It worked every single time. When I replace the PETTEST with the 901465-03 KERNAL ROM, it fails to start. Strangely, it had been mostly starting (COMMODORE, etc.) a few days ago. Now, when I put back in the PETTEST, I'm getting what you see in the photos, which feels like a bad RAM somewhere? Yes? No? Suggestions? Would that cause Dave's TEST ROM fail to run in the KERNAL slot? I wouldn't have thought so.

    At this point I've verified and/or replaced all of the ROMs with freshly written 2516s... The ROMs in there at the moment verify correctly for:

    UD6 - 901465-01
    UD7 - 901465-02
    UD8 - 901447-24
    UD9 - 901465-03

    I'm stumped as to where to look next. Here's a couple of photos, if it helps.

    KERNAL installed:
    IMG_4158.jpg

    This can vary a bit in that the @s can be Gs if I have just recently swapped the test ROM in or out.

    Clearly wrong test ROM. This was correct this morning, now it is alternating Bs and Gs:
    IMG_4160.jpg

    And the other screen of charsets, which also looks wrong.. I don't think it should be alternating caps:
    IMG_4159.jpg

    Thanks folks!

  2. #2
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    Quote Originally Posted by dhoelzer View Post
    At this point I've verified and/or replaced all of the ROMs with freshly written 2516s... The ROMs in there at the moment verify correctly for:

    UD6 - 901465-01
    UD7 - 901465-02
    UD8 - 901447-24
    UD9 - 901465-03
    I assume you are using 2532 4K EPROMs except for the EDIT EPROM which must be a 2516 or 2716 2K EPROM?

    The pettest shows bad RAM in the first 1K. This will prevent boot.

    Try daver2's pettest V3. It must be run in the EDIT ROM socket (D8 ). Leave the kernal ROM (D9) in place. You can find it here: https://drive.google.com/drive/folde...Bqa231ZXItOQWP

    If the RAM is good, it goes on the verify the checksums of the ROMs and then lets you test your keyboard.

  3. #3
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    Yes, it looks like you have an alternating data bit fault. The bad/good indicates page 0. The video display indicates screen memory. Unless you have two faulty RAMs? The other alternative is an address to data bus short (or some other type of interaction).

    Yes, my test ROM runs in the EDIT socket...

    Dave

  4. #4

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    Quote Originally Posted by daver2 View Post
    Yes, it looks like you have an alternating data bit fault. The bad/good indicates page 0. The video display indicates screen memory. Unless you have two faulty RAMs? The other alternative is an address to data bus short (or some other type of interaction).

    Yes, my test ROM runs in the EDIT socket...

    Dave
    Yes, sorry, I mis-typed that. I was trying your tester in the EDIT slot with a Kernal in place; it shows about the same screen and fails to start.

  5. #5
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    As a 'space' is hex 20 and a '@' is hex 40, this shows problems with the D5 and D6 bits. A 'G' is hex 47. As daver2 mentioned, perhaps bad display RAM is confusing the issue.

    Thinks to consider before having to replace the eight lower RAM chips:

    NOP Generator to check out Address circuitry.
    Replace or piggyback the two video static RAM chips (F7 and F8 ).

  6. #6

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    Quote Originally Posted by dave_m View Post
    As a 'space' is hex 20 and a '@' is hex 40, this shows problems with the D5 and D6 bits. A 'G' is hex 47. As daver2 mentioned, perhaps bad display RAM is confusing the issue.

    Thinks to consider before having to replace the eight lower RAM chips:

    NOP Generator to check out Address circuitry.
    Replace or piggyback the two video static RAM chips (F7 and F8 ).
    Thanks for this. I actually replaced the video RAM with sockets about 2 years ago and have socket new chips in there, so I donít think thatís the issue.

    Thanks for the pointer on 7 and 8. I couldnít easily see which RAM chips were at issue, so I started replacing from the bottom up. Iím six DRAM chips in with no improvement so far. Iíll skip ahead when I get back from the beach, but I may just move forward with replacing or at least docketing all of them.

  7. #7

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    Great progress. I've now pulled out all of the old DRAM and replaced with sockets and replacement chips. I have two problems at this point:

    The keyboard is iffy. I've pulled it apart to clean the carbon contacts, but have discovered that even shorting the contacts is spotty on most of the keyboard and often is the wrong character. I'm assuming this is the 6522? I've tried swapping them but that showed no measurable difference; it may have been worse. I've proactively ordered some Rockwell 6522s.

    Here's the test starting:

    IMG_3133.jpg

    The second problem is the DRAM test. One test resulted in:
    IMG_3137.jpg

    The screen reads: MEM FAIL 3 1 1CEC 01 FF!

    After a power cycle and letting it sit, I came back to find it here:
    IMG_3138.jpg

    (That reads 01......... PASS 000004.)

    I let it sit quite a bit longer and found it at pass 8. I think I may have to sit and watch it to make sure it's not halting.

    So, the issues are, The $ hanging around on the screen and the failed DRAM test. If I replace the EDIT ROM, it seems to start fine and no dollar signs. I can't really test keyboard function because of the above, so that remains to be seen. The second issue is the failed DRAM test. How can I translate that output to a chip to try reseating/moving?

    The board after the DRAM work:
    IMG_3135.jpg

  8. #8
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    Good work so far.
    For some reason, the way you are attaching the photos are making them small even when you click them.
    The keyboard chip is C7 a 6520 PIA. Is that the chip you are swapping?

  9. #9

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    Quote Originally Posted by dave_m View Post
    Good work so far.
    For some reason, the way you are attaching the photos are making them small even when you click them.
    The keyboard chip is C7 a 6520 PIA. Is that the chip you are swapping?
    Ah.. For some reason I thought it was one of the 6522s. I have to see if I have a spare 6520 around.

    I noticed that. Not sure what I did differently. I'll try again in a few.

  10. #10

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    Well that's interesting. First, it was definitely freezing during the RAM tests.

    I managed to get a picture of it running tests. You can see the $ $ in the bottom.

    Then I got a failing test... Not sure how to trace this back to a chip...

    But since then I am back to failing to start. I just get a random screen and the KERNAL fails to start the test EPROM in the EDIT slot, so that seems to be a DRAM problem again...??? I went through and swapped every chip, no good. Good it be the 74153's? I tried piggybacking, but no change.
    IMG_4162.jpg
    IMG_4163.jpg
    IMG_4161.jpg

    Last one... After this, it once again fails to start even after progressively swapping every DRAM chip.
    IMG_4164.jpg

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