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Old 07-07-2007, 03:12 AM
phe phe is offline
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Join Date: Dec 2006
Posts: 48
Default debugging the protect circuit

I'm trying to figure out why the flip flop in IC L on the memory board is not changing state. I've checked the voltage on lines 20 and 70 of the bus when I flip the switch to protect and unprotect. Both lines go high when I flip the switch in either direction (protect or unprotect). With the system powered off, pins 20 and 70 are open (infinate resistance). So there is no short between them.

I've checked resistance across the three pins from the protect/unprotect switch. With the system off, the top and bottom pins of this switch show zero resistance, so something is shorted across those two pins. I would not expect this to be correct.

Anyone have any ideas of further places I could check to find out where the problem might be. I can't image it is a bad switch - that would be nearly impossible wouldn't it? But shouldn't these pins show infinite resistance between them? I'd like to find other signs that point to a bad switch before I desolder and remove it.

Paul
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