HP-67 a nearly hopeless case

[PANAMATIK]

A long time has passed since I aquired an HP-67 in a very bad condition. It didn't show any numbers, and inserting the new ACT, programmed with HP-67E version, didn't change anything. It was a nearly hopeless case, emphasized by a missing battery contact and very dirty battery case. But the keyboard and exterior looked very nice. Therefore it went for a long time into my drawer, but I didn't forget it.

Last week I tried a new approach to make a repair of this sadly dead calculator. Obviously the display driver chip was defective. I replaced it by a repurposed new ACT hardware, programmed with a HP-67 display driver, which needed to run a rather tricky critical timing. Then the display came to life again. But it turned out, that also the CRC chip was defective. But this is not an issue, because the card reader does not work either with the new ACT. But another problem persists: I cannot drive the exponent minus sign! The HP engineers didn't use the anode driver directly for showing the minus signs, but made a special transistor circuit, because they had the problem to use a 14-digit cathode driver to drive a 15-digit display (see schematic). Their solution was to combine two digits, capable of showing only the minus sign, into one. I still don't have a solution, but I will work on it.



The new ACT and new anode driver placed into the processor board, and now the HP-67 is working again.

As a side effect of my repair efforts, I had to draw a HP-67 schematic, consisting of three separate PCAs. I cannot guarantee it being completely error free, but 99%.

HP-67 Processor board


HP-67 Display and keyboard


HP-67 Card Reader


Does anybody know a proper replacement for a missing HP-67 (Classic) battery contact?

Bernhard