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HP 4192a power up

Question asked by ACProgrammer on Aug 3, 2012
I've got an HP4192a low frequency impedance analyzer that doesn't seem to be able to get past the POST and I was wondering if anybody here had some pointers for debugging this thing.

The symptoms are that on power up, the device blinks the front panel lights/LED display (it appears to be walking patterns across it) but how it does this is very inconsistent.  How it behaves is not entirely consistent but seems to follow some general patterns:
1) Sometimes I power it up and all it does is turn on a group of the unit indicators and the GPIB status lights.
2) Other times, it walks a pattern across the various indicators/LED displays very quickly, and still other times also walks the pattern across the lights/displays slowly.  However, when it walks the pattern across the display, it often gets stuck showing something that looks like .y in 1 or 2 places on the display.  More rarely, I've seen it just seem to loop walking the front panel patterns.
The odd thing about all this is that I can cycle power and it will behave differently, sometimes subtly so (but never right).  Also, pushing any button other than power on the front panel has no effect.

Based on the service manual, this appears to be a problem with the digital control board (board A6 according to the service manual).  So far what I've done to troubleshoot is pull and clean the leads on all the socketed ICs on the digital board, particularly the CPU, EPROMS, and RAM chips.  I've also reseated all the connectors I can find and tried removing battery power from the RAM chips.  I've checked voltages and everything seems right.  I even pulled the logic board and looked for dry/broken solder joints, but everything looked fine.  One time, I think I bent a leg on the RAM IC when I put it back.  After blinking the front panel a bit, the analyzer showed an E-20 on the front display, however, it appeared to be scanning the display segments rather slowly since the display flickered alot.  Fixing the bent leg restored the analyzer to its previous malfunctioning state.

So, does anybody have ideas for what to look at?  The service manual isn't a whole lot of help since its procedure requires a 16343 logic test box and special eproms for signature analysis, both of which seem to be nearly impossible to find.