System 7 MPU unstable

Started 1 year ago by Spudgunman in forum TechTech: Generic.

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System 7 MPU unstable

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  • Started 1 year ago by Spudgunman
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    1 year, 11 months ago

    So I have a BK, it has a problem where it locks up in attract mode, a solenoid will kick then it locks.. on reboot it will sometimes blow the solenoid fuse.

    a power cycle normally fixes and sometimes it wont...

    sound works all the time, I can diagnose and it plays sounds like it should, till reboot.
    +12 +5 look ok from the power supply, checked on testpoint TP1 and TP9 for +12,+5 respectively

    I pulled the MPU, hooked up to my bench power 5v and booted the board up.. jumpered the (test points)TP8-TP8 to simulate the reset circuit ...

    here is where things get wacky... about 10% of the time the board will reset with no issues, '0' on boot... jumper TP8-TP9 '0' clears out and everything good. I can hit diagnostic and things all good (all good = nothing on LCD and nothing on LED)

    about 85% of the time I get a 0 for CPU lock, I then need to reset MPU and try again... (remember I am bypassing the reset-circuit and jumpering test points TP8-TP9 using nothing but 5v clean from a trusted source)

    sometimes it will throw a random number error about 5% of the time

    so any help would be awesome

    1 year, 11 months ago

    I did hook a scope up to TP6 and it looks ok and TP5 looks ok (I see oscillating signal)

    If I hook 12v up the TP8 goes High quickly (faster then the documented 1second) but I also did this by hand jumping the 9-8 and no difference in success rate

    is My IC1 faulty... do they die?

    1 year, 11 months ago

    i did just check pins 38,39 of the cpu and see nothing interesting when viewing on a scope. the variation of the signal could just be static for all its worth.. I expected to see a 1.5-3v dip in the oscillation?


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