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" <[email protected]> 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 >
