(Topic ID: 230076)

Bally -17 MPU Board Gremlins

By Brentd27

5 years ago



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  • 5 posts
  • 2 Pinsiders participating
  • Latest reply 5 years ago by Brentd27
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    #1 5 years ago

    I am trying to fix a Bally -17 MPU board, and it's becoming a real pain in my backside. It had previously had work done to clean up battery corrosion, but according to the owner, it had been working fine and just quit. (Though I'm told it quit around the time of a lightning storm that damaged some other electronics.)

    When I got it, symptoms were that the on-board LED was locked on. Following standard diagnostic procedures, I determined that the original U6 was bad. Had an internal short to ground on the data line. I made the necessary changes to set the board up for 2732 EPROMS, and popped Leon's Test Rom (V4) in U6 and pulled all other socketed chips. Traced down bad U15 & U16 and replaced them.

    CPU now boots, and I get the expected blinking from LED on U9 Pin 15.

    Added in U10 and U11. On-board LED now blinks as well. Checking outputs of U11, I find several that are stuck high. Checking outputs of U10, none of them are changing. Swapped U10 and U11. The chip that gave no outputs in U10 socket now gives results similar to the original U11 chip in socket U11. New U10 is behaving the same as previous U10. (Should mention that CPU and these 6820s are from a known working board.)

    So it's like something isn't allowing U10 to be selected. I've checked all the data lines D0-D8 with a logic analyzer at U9, U10, and U11, and get the same pattern at all locations.

    And then there's the other weird symptom... If the board flexes at all, the LED will either go back to locked on, or to quick pulses at about 1 second intervals. Whereas the normal Leon test pattern is a steady on-off-on-off at roughly one second intervals, this pulsing is a quick dim flash followed by a about a half second off period, then another quick flash, etc.

    My gut says there's either a bad solder joint or a broken trace somewhere, but I'm not seeing anything jumping out at me. Any ideas where to look?

    #2 5 years ago

    Data lines don't select the chip, that's the CS* lines. Maybe one of the logic chips that drives them is bad?

    I've also got an MPU with the flex issue, haven't been able to track it down at all

    #3 5 years ago

    Well, Looks like I've got a bad U18 (4049 hex inverter) and don't have another one right now. Should have some in in a day or two.

    1 week later
    #4 5 years ago

    Well, I got a bunch of 4049s in. Got U18 replaced. Also got a brand new u9 and a new PIA. Using Leon's test rom, with just the rom and CPU in place, the control LED on U9, pin 15 flashes on and off as expected in roughly 1-second intervals.

    Put the new PIA in the U11 socket, and the control LED goes back to rapid flash on followed by longer period off. The U11 outputs all seem to follow this rapid flash condition. On-board LED blinks in tandem with the control LED. Logic probe shows all outputs changing with this same fast-pulse pattern

    Put a known-good PIA in U10, and nothing changes. All outputs register as no connection. Move the brand new PIA to U10 and known good to U11 and all outputs of U11 still follow the rapid flash. All outputs of U10 still show no connection.

    However with the two PIAs removed from their sockets, the logic analyzer trace looks like all the chip select signals are getting where they're supposed to.

    They're certainly earning the abbreviation PIA. More like PItA.

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    #5 5 years ago

    Found it! Bad solder joint on the CPU socket causing intermittent connection to data bus.

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