HP 9815

tony duell ard at p850ug1.demon.co.uk
Fri Feb 5 07:04:33 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?

I am sure that Vcc to the CPU is not switched. It is just the system +5V rail. This 
comes in over the 4 wire cable from the PSU board and is then distributed over
the CPU board and via the inter-board connectors to the keyboard/display
interface. The 2 transistors near the CPU are the active pull-ups for the clock

You have re-connected that 4 wire cable when testing? You can leave the 2 ribbon 
cables for the I/O assembly unplugged, but the 4 wire cable, soldered to the CPU
board and plugged into the PSU/printer board in the base of the machine is the
power feed to all the electronics on the keyboard (CPU, memory, etc).

If you have 5V on the CPU board but not at the CPU it pretty much has to be an
open trace or via.


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