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. https://youtu.be/gIfJ30fPAOw HP 9825 Crowbar Protection Circuit Project (Schematic, Parts, Gerbers, etc. https://www.curiousmarc.com/computing/hp-9825-scientific-computer#h.xrov16yr03br You can Order the PC Board from PCBWay https://www.pcbway.com/project/shareproject/HP_09825_66552_Crowbar_Protecton_Circuit_for_the_HP_9825_Vintage_Calculator_a244bde6.html greg chicago == 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 problem. If so is it possible to build a board that can contain a more modern ROM with the system code on it? Thanks! Chris
