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

  1. #61

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    Quote Originally Posted by dave_m View Post
    I would be suspicious of oscillations in RESET circuit. Check the 555 timer (A2) and capacitors C66, C67 and C68. This PET has some fascinating issues.
    Nope, I've looked at that super carefully. The 555 side of the inverter looks perfect. I'll break out the USB microscope soon to look over the traces.

  2. #62

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    Quote Originally Posted by dave_m View Post
    This PET has some fascinating issues.
    That's what comes of 30 years marinating in mouse urine. It's clearly not good for electronics.

    That said, it appears that the waveform matches pin 3 on UC3, but has a difference in magnitude. This could be just coincidence, but I don't think so. I see a resistance of about 56,000 ohms between pin 3 on UC3 and pin 12 on A10. It climbs very slowly, so it seems almost certain that a capacitor somewhere is involved.

  3. #63

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    Quote Originally Posted by dhoelzer View Post
    That looked ok. +5 on one side, 200khz signal on the other.
    And the value of R30 is still what is supposed to be, just checking?

  4. #64

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    Quote Originally Posted by iz8dwf View Post
    And the value of R30 is still what is supposed to be, just checking?
    It appears to read as 470 ohms, which is correct.

  5. #65
    Join Date
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    Quote Originally Posted by dhoelzer View Post

    it appears that the waveform matches pin 3 on UC3
    That C3 (LS244) output is Buffered Address 1 (BA1) which should be similar to the input on C3-pin 17. The source there is A1 from the CPU (C4-pin 10). One of these signals is being coupled to the Reset signal.

  6. #66

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    *Major progress!*

    I have now got enough of the system working and repaired that iz8dwf's RAM/ROM replacement board will successfully run diags and boot to BASIC... Still, there are some bits and bobs that aren't quite ok yet.

    First, I'm not sure I've resolved the BA1 coupling issue. That remains to be seen.

    Second... There's something seriously up with the cursor and something in the timing somewhere. Screen and image is rock solid... I can type... but the cursor will go from a nice, calm, sedate blinking to blinking like mad. When it does this, the keyboard is also ultra-sensitive, making it nearly impossible to type single characters. Finally, while it's doing the fast blink, it's also very slow to respond with errors and such...

    Thoughts?

  7. #67
    Join Date
    Jun 2012
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    Look at the CPU /IRQ line with an oscilloscope whilst the cursor goes mad. Does the /IRQ line go mad at the same time?

    I suspect the problem is the keyboard PIA/VIA or whatever device it is. Not able to have a look at the schematics at the moment - but I will have a look later.

    Dave

  8. #68

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    Quote Originally Posted by dhoelzer View Post
    *Major progress!*

    I have now got enough of the system working and repaired that iz8dwf's RAM/ROM replacement board will successfully run diags and boot to BASIC... Still, there are some bits and bobs that aren't quite ok yet.

    First, I'm not sure I've resolved the BA1 coupling issue. That remains to be seen.

    Second... There's something seriously up with the cursor and something in the timing somewhere. Screen and image is rock solid... I can type... but the cursor will go from a nice, calm, sedate blinking to blinking like mad. When it does this, the keyboard is also ultra-sensitive, making it nearly impossible to type single characters. Finally, while it's doing the fast blink, it's also very slow to respond with errors and such...

    Thoughts?
    Cursor blinking and keyboard scanning depends on the MOS6520 in UC7 position. It should produce the IRQ with a stable period. You can try swap the two 6520 and see if things get better or worse. There might also be another chip making unwanted IRQs but that can be only the other 6520 or the 6522.

    Frank IZ8DWF

  9. #69
    Join Date
    Jun 2012
    Location
    UK - Worcester
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    +1.

    UC7 is the beastie...

    Dave

  10. #70

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    Quote Originally Posted by iz8dwf View Post
    Cursor blinking and keyboard scanning depends on the MOS6520 in UC7 position. It should produce the IRQ with a stable period. You can try swap the two 6520 and see if things get better or worse. There might also be another chip making unwanted IRQs but that can be only the other 6520 or the 6522.

    Frank IZ8DWF
    Looks like I have more work to do. I have six other 6520s, all of which were previously known-good. They all behave the same way... about 3 good cursor flashes and then we get fits of fast-flashing.

    I'll have to look the schematics over... We're talking about anything that can hit the CPU IRQ line, right? As you know, this board has turned out to have a lot of damage that wasn't readily apparent.

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