HP9825 internal ROM?

Antonio Carlini a.carlini at ntlworld.com
Wed May 4 16:56:22 CDT 2022

On 04/05/2022 21:40, Chris Zach via cctalk wrote:
> 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 problem.
> If so is it possible to build a board that can contain a more modern 
> ROM with the system code on it?

CuriousMarc did a crowbar on youtube: 

The circuit is available on his website: 

The whole series was fun to watch (even though I don't have an HP9825)


Antonio Carlini
antonio at acarlini.com

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