HP 9845 C with 280 option startup failure

In which case, start pestering Dave Colver for the schematics :-).

Yep, I traced out schematics of the PSU too. Be warned this is _by far_ the most complciated supply you're likely to meet. it's on 5PCBs, there are 17 chips on the control board (counting the chopper transsitor modules for the secondary-side regulators). And 4 maine chopper transistors. A rough breakdown of what is on each board : PSU mainboard (09845-66520) --------------------------- Mains rectifier Startup transformer/rectifiers 2 main chopper transformers +5, +/-12V output (09845-66517) ------------------------------- Output filters (and energy storage inductors for the LV switching regulators) for the logic supply outputs +/- 22V output (09845-66519) ---------------------------- Output filters, etc, for the tape drive and printer power supplies Chopper driver transistors Crobar for the printer supply (this is the only crowbar in the unit!) Chopper (09845-66518) ---------------------- 4 indentcial chopper circuits with transformer-coupled drive (drivers for that are on the -66519 PCB). Wired as 2 totem poles across the 400V rectified mains to drive the 2 main chopper transformers PSU Control (09845-66534) ------------------------- Startup/cotnrol supply regulators Output rectifiers Chopper control for the 2 main choppers Switching regulator control and choppers (PIC6xxx series devices) for the +12V, +7.5V and -12V supplies Overvoltage and overcurrent shutdown Init/PowerOK circuit The PIC6xxx chips are basically a darlington pair chopper transistor, the flywheel diode and a few resistors in one can. I think I managed to find data for them on the web somwehre.

The LEDs monitor 4 supply lines in the unit : 1) +Vdrv (+ve supply to the chopper driver from the startup supply) 2) +5V sense (main +5V output) 3) VPrtSense (Printer supply) 4) -8V (-ve supply for the control circuit) All should be on if the supply is working ccorrectly. The mono monitor doesn't have its own mains supply, it runs off assorted voltages from the CPU box supply. Of coruse it does generate the HV for the CRT from the flyback transformer as usual.