HP9825 internal ROM?

Gregory Beat w9gb at icloud.com
Thu May 5 13:12:48 CDT 2022

Curious Marc has a complete set of YouTube videos on his restoration
of the HP 9825, due to Power Supply failure (no crowbar) damaging a number of ICs.

The Last video of the series shows fabricated of the PC Board for 
it’s missing Crowbar Circuit.

HP 9825 Crowbar Protection Circuit Project (Schematic, Parts, Gerbers, etc.

You can Order the PC Board from PCBWay

Date: Wed, 4 May 2022 16:40:13 -0400
From: Chris Zach
To: CCTalk mailing list
Subject: HP9825 internal ROM?

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?


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