System 7 MPU unstable

Started 2 years ago by Spudgunman in forum Tech > Tech: Generic.

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System 7 MPU unstable

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By Spudgunman

2 years ago


  • Started 2 years ago
  • 3 posts
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  • Latest reply 2 years ago by Spudgunman

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2 years 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

2 years 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?

2 years 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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