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Thread: PETunia's Repair Log

  1. #31
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    Quote Originally Posted by Nivag Swerdna View Post
    I am a bit uncertain about the ROM F checksum.... I am pretty sure the F000 ROM contains... rom-1-f000.901439-04.bin and F800 ROM contains... rom-1-f800.901439-07.bin; if I take those two .bin files and combine them and then put the 4K result into HxD I get
    Checksum-16: 0CA4 which is what appears on the screen?
    You are correct. The two checksums from sockets H4 and H7 will add up to the 4K checksum for the $F000-FFFF address range and will equal $0CA4. Sorry for treating you like a beginner, now I know you are a computer man.

    What is the AC Volts reading across the power connector J8-pin 2 (+9 V unregulated) and J8-pin 3 (ground)? We would expect to see some AC. I will check on my 8032 PET for a comparison.

  2. #32

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    Quote Originally Posted by dave_m View Post
    ... will equal $0CA4.
    So the on page 10 of https://drive.google.com/drive/folde...13&direction=a the value needs tweaking?

    I'm thinking power supply issues at the moment... might try some more detailed investigation.

  3. #33
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    Quote Originally Posted by Nivag Swerdna View Post
    So the on page 10 of https://drive.google.com/drive/folde...13&direction=a the value needs tweaking?
    Oh! I thought you had compared with the manual. So I'm guessing the 0CD4 in the manual is correct and you may have a bad ROM (one of the 2K F0 or F8 ROMs). Let me double check the numbers.

  4. #34
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    I checked the numbers and the correct sum for BASIC 1 for the $F000-FFFF range seems to be 0CA4 so your ROMs are correct, and the manual must be in error.

    Note to daver2: our man has caught a typo in Pettest manual. Error in table listing sumcheck of BASIC 1 $FXXX was 0CD4 and should be 0CA4.

    I used zimmer data and cross checked with MikeS data.

  5. #35
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    Using a digital DVM, with the black probe clipped on the big cap C1 (23K uF) negative (blk wire) and the red probe in J8- pin 2, I get 0.125 VAC and 10 VDC with system connected.

    Looking at typical Vcc pins, I get 0.003 VAC and 5.1 VDC. This is on a 8032 PET.

    With your readings, there is something amiss. But first, with power off, measure the resistance between chassis and digital ground, perhaps you have some bad connections.

  6. #36

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    I would check the AC reading with a capacitor to isolate the DC path. Not all meters will read AC properly with a DC bias. A 1 or .1uf should be enough isolation for an AC reading with a typical DVM having 1Meg input resistance.
    Dwight

  7. #37
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    Hi Dwight,
    Nice to hear from you on the 'The Commodore Channel'. I see your postings on the other forums. My 3 mV AC reading of the Vcc bus seems too good to believe. I think my DVM measure volts peak not RMS. Will that contribute an error with a DC bias? I was going to try with an old VOM, but forgot.
    -Dave

  8. #38
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    Quote Originally Posted by Nivag Swerdna View Post
    So the on page 10 of https://drive.google.com/drive/folde...13&direction=a the value needs tweaking?

    I'm thinking power supply issues at the moment... might try some more detailed investigation.
    Just to confirm, the checksums I obtained were using the Zimmers images with VICE running my PETTESTER in the EDIT ROM socket. It is possible that I mistyped the checksum into the manual. VICE also does a few automatic modifications to the Kernal ROM. I thought I switched all of these ‘improvements’ OFF, but I may have missed one of course. VICE may also ignore me!

    Or you potentially have a BAD Kernel ROM half (as Dave says).

    Dave

  9. #39

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    Quote Originally Posted by dave_m View Post
    Hi Dwight,
    Nice to hear from you on the 'The Commodore Channel'. I see your postings on the other forums. My 3 mV AC reading of the Vcc bus seems too good to believe. I think my DVM measure volts peak not RMS. Will that contribute an error with a DC bias? I was going to try with an old VOM, but forgot.
    -Dave
    Many VOMs are even worse. They just rectify the voltage with a bridge using copper oxide diodes. You connect 5 volts DC to a VOM and you are likely to read about 3 or 4 volts on an AC scale. As for your DVM, there are spikes higher than 3 mv but it is possible that it is just too high a frequency to indicate much. The large reading seen on the other is way too much if it is really not just an artifact of the meter with a DC voltage. Using a capacitor to isolate is the best way on any meter. On a VOM with typical 20K per volt will need a larger capacitor. I would hope that almost everyone would have a 1 or 0.1 uf capacitor laying around in their junk drawer, even if they just removed one from some old board that they don't care about.
    The best way, and right way, is to use an oscilloscope. It is important to also look at the ripple. If it is 60Hz, it means you have a failed rectifier. If it is 120Hz, it means it is likely a bad filter capacitor. You can tell that from a DVM. Over course, without knowing the load, it might even be some capacitor on the board trying to fail or a bad regulator.
    I had a N* with a weak regulator that had about as much ripple on the output as on the input, although, it was clipping at 12V. Again, from the scope, I could see that it had more than enough over voltage on the input. It was just shot.
    Before I'd trust any AC reading on a meter, I'd want to know it was isolated from DC bias.
    Dwight

  10. #40
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    His Dwight,
    Excellent tutorial. I forgot that simple voltmeters only rectify AC signals to drive the meter movement. On my VOM there is an 'Out' jack in addition to the '+' and '-' jacks. Does that jack insert a capacitor in the circuit? Can that be used to remove DC bias voltage?

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