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Thread: Another dead SuperPET!

  1. #1

    Default Another dead SuperPET!

    Well, you very helpful folks appear to like a challenge with these things. So here's me begging for help!

    The story so far:

    Purchased a dead SuperPET. It's got a 8032090 revision board and the 900007 ComboBoard, so it's a two board model. After purchase and voltage checks out of the transformer, I powered it on. No screen display at all (black, not green). Happy chirp was there.

    I pulled out the daughter board, moved the 6502 to the main board to simplify troubleshooting. Removed and reseated all socketed chips while I was at it. I did still get the happy chirp for the first 8 or so power cycles, then no more.

    Using the Zimmer schematics, I checked what voltages I could with a multimeter. The three linear regulators were all reading a bit low (the 12v was only 10.1v). I replaced all three of those with new (and tested) ones I had lying around. I got the chirp back a couple times, now no more. Voltages on the regulators are still reading low (4.5VDC, 11.1VDC and -4.6VDC), but better than they were.

    I have upstream voltages (like on J8 and the transformer) if that's informative in any way. But I'm feeling like if the regulators are in the ballpark, the upstream stuff is probably okay?

    So awesome knowledgeable and helpful folks, any ideas where to go next with this?

    I do have a nice scope, logic analyzer, frequency counter, signal generator, yadda yadda yadda...so don't hold back!

    Thanks in advance.

    Scott
    67949340_10220783282919758_343511999730155520_n.jpg67799574_10220783280199690_1685277559870193664_n.jpg

  2. #2

    Default

    Voltages are too low and it's strange that all are too low at once. Did you check your voltmeter with a known good reference or another good board with +5V supply for example?
    Do the regulators get hot quickly after power on?
    If your multimeter is indeed good, I'd start looking for abnormally hot chips, you have too high current draw somewhere. I would start with the 4116 RAMs since they are the only chips that get all three supplies. Never substitute parts just for the sake of it (imho).
    You should check pin 37 of the 6502 for a good 1 MHz square wave, then check the J7 video output connector for some life on Hsync Vsync and video outputs, just to understand if you have missing signals or a bad monitor (or both).

    Frank IZ8DWF

  3. #3
    Join Date
    Jun 2012
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    UK - Worcester
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    Default

    csc316,

    Yes, we all like a challenge here - especially when it is a SuperPET!

    Happy chirp is at least good (if only part of the time) - it gives us hope there is nothing permanently at fault.

    +1 for Frank's suggestions. I would do exactly the same.

    Dave

  4. #4
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,641

    Default

    +2 for Frank suggestions. Without Horizontal Drive on J7 at 20KHz, there will be no high voltage generated to CRT. No Horizontal Drive may be due to the 6545 CRT Controller not being initialized by CPU due to a fault, or more rarely a bad 6545. Look for Sync pulses on the 6502 UB14-pin 7 to indicate that the CPU is fetching instructions.

  5. #5

    Default

    Quote Originally Posted by iz8dwf View Post
    Voltages are too low and it's strange that all are too low at once. Did you check your voltmeter with a known good reference or another good board with +5V supply for example?
    Do the regulators get hot quickly after power on?
    If your multimeter is indeed good, I'd start looking for abnormally hot chips, you have too high current draw somewhere. I would start with the 4116 RAMs since they are the only chips that get all three supplies. Never substitute parts just for the sake of it (imho).
    You should check pin 37 of the 6502 for a good 1 MHz square wave, then check the J7 video output connector for some life on Hsync Vsync and video outputs, just to understand if you have missing signals or a bad monitor (or both).

    Frank IZ8DWF
    My multimeter and scope have always agreed perfectly in the past. But now that you mention it, the three voltage regulators I was using as replacements (one new out of the box) were reading a little low in test before I installed them. Not super low, maybe 5%, but low. And my scope is reading 4.87v on the 6502 now. I'll dig up a different multimeter at work and give it another check. Thanks for that tip.

    Here's what pin 37 is giving me. It's right on 1.00MHz, 4.8v p-p but only 4.2v at the top of the wave. So that does seem low, but TTL is triggered way below that isn't it? Now sure how "clean" this signal is, given the era and age of the machine. But it doesn't look terrible to my untrained eye. Picture attached. I'm checking the other pins you suggest now.

    As for replacing parts just because, well, let's just say I've been trained well by Atari on voltage regulators and by Amiga on capacitors. But my insanity stops there.

    Thanks for your help!

    Scott

    6502 Pin 37b.jpg

  6. #6

    Default

    Also, regulators are pretty cool, even the -5v one without any heat sink. No chips get hot.

  7. #7

    Default

    I'm getting 5v constant on pin 3 of J7 (that's the Vsync, right?). Same on pin 5 of J7 (Hsync, right?). That's with or without the rare chirp.

  8. #8

    Default

    Nothing on 6502 pin 7 (0V), with or without the chirp.

  9. #9

    Default

    Another tidbit of info, I have no voltage on supply pin 24 of UD8. Checking traces now...

  10. #10

    Default

    My replies don't seem to post. So sorry if this is a duplicate post.

    Pin 37 shows a decent square wave, shown below. I've got nothing on J7's HSync or VSync lines, whether or not I get the rare happy tone. Nothing on 6502 pin 7.

    A couple of the 74ls244 chips have no voltage on the +5 pins, and appear to have corroded through. I ordered some replacements. But in the meantime, I soldered over those pins to get voltage to the chips, but that didn't help. All the other chips have good +5 pins. But some signal pins where I'd expect to see square pulses or just a high line are showing some sawtooths with V_min = 4.0V and V_max = 4.6V. So that's curious...maybe.

    Scott
    6502 Pin 37b.jpg

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