HP 9815

[pdaguytom .]

Hi Tony,

Yes that was top of 1st things to do list, was make an extension cable.
I've double checked that I have +5v, -5v and 12v on the cpu board side of
the plug.  Yes I left the i/o cables unplugged. The cpu board actually is
pretty well kept, didn't notice any corrosion or physical damage.   I'll do
more tracing this weekend to see if I can find the problem.

Tom
On Feb 5, 2016 7:06 AM, "tony duell" <ard at p850ug1.demon.co.uk> wrote:

>
> > 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
> lines.
>
> 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.
>
> -tony
>