HP-33C Pr Error



#4

I got an HP-33C that was in terrible shape; there was major corrosion damage to much of the internal components, and not much was salvagable. I also got an HP-38E that was totally non-functional, but looked OK inside. Both calculators were the type where the chips were not soldered to the circuit board, but were held in place by a backbone.

I took the 2 main chips (probably ACT and RAM) from the HP-33C and transplanted them to the curcuit board of the HP-38E. I then put the boards into the case of the HP-33C. Now the calculator always says "Pr Error" when I power it up, but it otherwise works perfectly. Functionally, it is the equivalent of an HP-33E!

Is there anything I can do to have the calculator save its memory when I power it down, such as adding or replacing a capacitor? Thanks!


#5

The power supply board was different between the "E" series and the continuous memory "C" series machines. The main difference was the "C" series provided the back up power to the RAMs. There was probably also some minor differences on the main board so that the backup voltage made it to the RAM chip. There is probably a fairly easy way to get the backup voltage (I think it was just the battery voltage, mayby protected by a zener diode) to your RAMs, but I have never had a need to work on Spice series power supplies.

#6

IIRC the 2 chips in a 33C are a combined CPU/keyboard interface/display interface and a combined ROM/RAM chip. Something like that anyway.
The main PCB (the flexiprint with the keyboard on it) is the same in all Spice models (-E and -C) that I've worked on, so there's no problem there. But the PSU board (the little soldered PCB) is
different on the -C models. Basically, the power pin on the 'top' memory chip is wired to a special pin on the PSU board.
The -E PSU just links that to the main power line, the -C PSU provides a low-current supply there to maintain the RAM contents when the machine is off. IIRC the difference is a couple of diodes and a 10k resistor, but due to the physical size it's not easy to modify one
into the other. Is it possible to salvage the -C PSU board from the broken machine?


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