The HP-9825 can accommodate up to four ROM cartridges in slots located under the keyboard. Examining these cartridges carefully does not provide any hints as to how they may be opened.
This web page provides information on what to expect to find inside such a cartridge, and how to disassemble it without permanent damage.
The HP ROMs include a feature that allows them to operate at reduced power most of the time and go to full power only when their contents are being accessed.
This is the reason for the external transistor, shown in the diagram below (created by Tony Duell). When the ROM sees an address that it should respond to it switches the transistor so that power flows to the main ROM bank.
Notice that there is space for two ROMs inside the cartridge, each controlled by a different transistor. I wonder whether this means that you can free ROM slots by "doubling" cartridges.
The rest of the 9825 schematics along with many other schematics of HP calculators and peripherals reversed engineered by Tony Duell may be obtained from http://www.hpcc.org/cdroms.
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At each of the four corners of the cartridge there are metal spikes
like the one shown on the left. Two are visible in the first
picture, while the other two are hidden under the label that identifies
the cartridge. The way I removed these spikes is by heating them with
a soldering iron while gently trying to pry the cartridge open by inserting
a flat screwdriver between the edge connector and the cartridge case.
Be patient, once the spike warms up, the two halves of the case will separate pulling the spike out of the bottom hole. Normally the spike will remain attached to the top half of the case. To close the cartridge you can either use a hammer to push the spikes back into position, or follow the reverse process, by heating them while gently pushing them down. |
