I still need help or advice on bringing an HP-9815 back to life. It worked fine, then sat for a little while, and now does not work.
When turned on, the display shows all "-------" and has no visible responses to keypresses.
When turned on in Auto-Start mode, a program will load from tape, the User-Interface instructions from the program will print, then when the program waits for keyboard input (even just a r/s) there is no further action.
I don't have a 'scope to check out what's happening and I would like this fine machine to come back alive, I am hoping that there's not that much wrong with it.
Any help or advice will be greatly appreciated!
Thanks, Dan
The HP9815 processor board has 4 select lines coming from it to select one of 16
'peripherals' (things like the display, printer, various sections of the tape drive and so on). The ----------- display
is produced by the hardware whenever the CPU is accessing a peripheral other than the display. But because your machine can still access the printer and the tape drive, this is basically working.
I would start by cleaning the inter-board conenctions on the stack of boards under the keyboard (the keyboard itself, the keyboard/display interface, and the processor. Then
if that doesn't help, start looking at the signals around the 74154 (24 pin chip) in the middle of the keyboard/display interface PCB. This is the chip that decodes the 4 select lines to 1 of 16 lines to select the different peripherals.
I'm not sure how much serious debugging you can do without a 'scope or logic analyser, though.