5 Feb
2016
5 Feb
'16
7:03 a.m.
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