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Thread: CBM 8032 - What does the startup chirp actually mean?

  1. #11
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    Quote Originally Posted by MikeP View Post
    Regarding the display - absolutely nothing, not even that static feeling that you can feel near the surface. I did however look at the back and it has the orange glow.
    Don't worry about that yet. The 6545 CRT Controller is not being initialized so there is no Horizontal Sync to generate the high voltage for the CRT. The orange glow is the filament that is driven by 18 VDC on the video board.

  2. #12
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    Quote Originally Posted by dave_m View Post
    Mike,
    That is correct. The Reset is OK. It will be inverted at UD15-pin6.
    HI Dave,

    Are you saying that this behavior on pin 3 of UD16 is normal?

  3. #13
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    Quote Originally Posted by MikeP View Post
    HI Dave,

    Are you saying that this behavior on pin 3 of UD16 is normal?
    Yes, my message #6 was wrong. I looked at the Reset schematic and thought I saw two inverters. Actually there is one inverter and a non-inverting buffer.

  4. #14
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    Reset looks OK.

    Check (with your logic probe first) to see if there is any activity on connector J7 (from the PET mainboard to the monitor).

    Disconnect the monitor at J7 and leave it disconnected. Video signal faults on the PET mainboard can damage the monitor if you are not careful. We should only reconnect the monitor when we see appropriate signals on J7 (to be on the safe side).

    Use your logic probe to see if you can see any activity on pin 1 (/VIDEO), pin 3 (V DRIVE) and pin 5 (H DRIVE).

    Tell us if the signal(s) are:

    (a) Permanently high ('1').
    (b) Permanently low ('0').
    (c) Appear to have pulses on them.

    I am not expecting to see anything - but it would be a start to isolate the initial fault to the PET mainboard or the monitor.

    If you can see some activity - we will need to go through a training exercise with you to start using your oscilloscope. What make and model is it by the way (so we can download the documentation)?

    Dave

  5. #15
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    Hi Dave,

    I disconnected the monitor as you suggest.

    Here is the pin status:

    1 (Video): Active (Logic zero with positive single pulses, according to Elenco LP-560 Logic probe)
    3 (V Drive): Low
    5 (H Drive): High

    Best regards
    Mike

  6. #16
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    This is OK. As soon as we get the program running, the CRTC chip will be initialized and the video syncs will start. It looks like the video data may be running already. As daver says it may be bad low RAM, a bad ROM chip, etc that is keeping the CPU from running in program.

    Daver2's diagnostic EPROM should help in detecting what is the issue.
    -dave_m

    EDIT: I will burn the part today.

  7. #17
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    I am not too convinced that the CRTC will burst into life when you put my PETTESTER in though...

    The CRTC is initialised within the EDIT ROM - in the same place where the chirp is - so there would have to be a localised fault to prevent the CRTC being initialised. Possible, but not probable (in my opinion).

    Is the CRTC in a socket - or is it soldered in (just in case we need to replace it)?

    Dave

  8. #18
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    I have seen a case where the chirp was happening but the CRTC was not being initialized and the cause was RAM.
    This happened to Tim Colegrove with an 8032.
    I suspect the chirp may rely on an RTS and the stack is corrupted causing a crash or maybe the CRTC initialization code is relying on some instructions that require Zero page.

    IIRC in that case, he was seeing some pulsing from the video output lines but they were at the wrong frequency. There may be some default output from the chip before the registers are set with the correct values.

    Oddly, when he was troubleshooting, he swapped the edit rom with one from a 4032 and with that ROM the CRTC was initialized, albeit with the wrong values and the picture was not stable but there was a picture.
    This sent us off on a troubleshooting tangent but the final cause of the problem was the RAM.
    Last edited by Hutch; November 5th, 2020 at 08:03 AM.

  9. #19
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    Hi Dave,

    My CRTC (6545-1) is not socketed. I do have the tools to de-solder it and put in a socket, however I have only done that once before, so I think I would want to do it a few more times before I touch the PET's board. I have a dead C64 board that I can practice on.

    Mike

  10. #20
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    Hutch - good point.

    Yes, the hardware is initialised as a subroutine with a <RETURN> and then a further call to initialise the CRTC.

    Yes, page 0 is used to hold the 16-bit address of the CRTC initialisation table.

    Interestingly, the CHIRP noise is itself a SUBROUTINE and I would expect to have heard a 'strangled' CHIRP in this case as it is possible that the <RETURN> at the end of the first CHIRP <CALL> would have gone wrong?

    Perhaps someone ought to post an audio clip of what a 'correct' PET 8032 chirp sounds like for comparison...

    Dave

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