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SYS11b MPU repair

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  • SYS11b MPU repair

    Been messing around getting a spare MPU going for a recent purchase.
    This was a completely dead board that had been partially ratted out for parts.
    After replacing all the missing parts, all was going well and I had taken the board in and out a few times after initially getting it running with a PC power supply on the bench.
    I was chasing a small issue with two coils firing at the same time and all of a sudden nothing, the diagnostic LED was blinking 3 then 5 then 6 then 7 flashes.
    According to the chart this meant that 4 of the PIA's were no good, (large rectangle outlined in picture below):

    Click image for larger version

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    I started by replacing 2 of them but nothing changed.

    After installing Leon's test rom:

    I followed his procedure for checking the outputs going from high (5V) to low (0V) and I could see that all four PIA's were inactive on pin 3 (PA01)
    Click image for larger version

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    I was having some trouble with intermittent readings and I also noticed by comparing the board to another working board that the blanking LED was not full brightness and seemed to be slightly blinking aswell.
    This turned out to be a bad 555 timer in the blanking circuit.

    When I replaced the PIA's I installed sockets for future replacement and this meant I was able to bend up the leg of pin 3 to see if anything changed once it was disconnected from the board and it didn't make any difference.
    Going buy Leon's guide you would think that the PIA was bad but it wold be very unlikely that all four would be the same so I started looking for something common to all PIA's.

    If you look at the pinout of the PIA there are 8 data lines and these are tied together for all PIA's.
    Checking these with my logic probe I could see that at all four non-working PIA's data line 1 (pin 32) was dead but on the fifth PIA (small rectangle) it was OK.

    It turns out that on these boards the working PIA (IC38) is the start of the daisy chain for the data lines and if there is a break between the first and the second PIA (IC41) then PIA's 2-5 (IC41, IC42, IC51, IC54) will not work.
    When I tested with my multimeter I found the break, added a jumper to bridge it and the board started

    So it turns out the the Williams diagnostics was right all along how it was doing the multiple sets of blinks indicating the four different faults that was actually one fault causing four chips to fail the self test.
    The two coils firing turned out to be a solder bridge between two pins of the 7407 IC

    Hope this helps out someone else trying to repair one of these boards.

  • #2
    Awesome explanation good on you for persisting to rectify the faults, that should help others out


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