HP 9815

pdaguytom . pdaguytom at gmail.com
Thu Feb 4 20:05:47 CST 2016

I recently picked up an HP 9815s ( was originally an 'a' model but was
upgraded with an 's' cpu board and the option 002 i/o long before me), that
displays the "---------------" when turned on.  I've scrounged through
Google and learned that this particular display is generated by the display
board when it has nothing else better to do to direct its attention.  After
checking that the power supply was putting out the recommended voltages and
chasing the +5v around the cpu board and puzzling that for a bit, I've
checked for activity at the cpu and found no voltage at all being applied
at Vcc the cpu.  I've been studying Tony Duell's schematic for the
9815(thank you Mr. Duell), but have not found a reason why this voltage
might not be present.  Is Vcc switched somewhere?  I done some meager
sleuthing of the traces around the 6800 and think I chased Vcc to a
transistor close by, but need to investigate further this weekend.  Anyone
chased this type of failure on the 9815 or traced out wether this Vcc is
applied at power on or switched by some logic at the power supply board?

Puzzled, but determined,

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