HP9825 internal ROM?
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:
orange 7 volts
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