HP9825 internal ROM?

Chris Zach cz at alembic.crystel.com
Wed May 4 15:40:13 CDT 2022

Question for the group: I'm working on a HP9825B here. First thing I did 
was take it apart, clean it, unplug all the power connections to the 
board and test the power supply. Supply is good (values below) so after 
checking the boards I put it together and powered it on.

Note: I can see the lack of a crowbar circuit on the +5 line and would 
be happy to install a crowbar circuit. Does anyone have a spare PCB 
board as otherwise I'd need to order at least three of them to build one.

Voltages are still good, however I get nothing on the display. The CAPS 
LOCK does light up the caps lock light and it's cleared by pressing 
Shift but that's about it.

Question: Is there supposed to be a ROM board or cartridge inside the 
unit by default? This one does not have one (the space between the front 
of the CPU board and the 4 card edges on the front). Perhaps that's the 

If so is it possible to build a board that can contain a more modern ROM 
with the system code on it?


Appendix: Power supply voltages:
Blue +25
White -25
Red +5
yellow 11.97
white/red -5
white/yellow -12
orange 7 volts

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