This is a short page about a nice calculator from 1975 I got off a local flea-market. It came complete with warranty-certificate, but unfortunately only the "4"-key worked properly.
Fix was simple and boring (cleaning contact-pads on PCB), but look at the nice pics. :-)
A quick google-search finds info about this calculator here, here, here or here (last page in Dutch).
Front of main PCB after opening up the enclosure:
Yes, that's a vacuum fluorescent display there.
This main PCB was soldered to a keypad-PCB. Bending them apart a bit, showing the back of the main PCB...
... and the keypad-PCB:
Now, keys looked like carbon squishy bit meets gold-plated PCB-pads. Probably, either the squishy bits were worn out or dirty, or the PCB-pads were oxidised or dirty, or both, or something else was wrong.
Trying the squishy bit from (working) key "4" on PCB-pads for other keys didn't seem to work, and squishy bits from non-working keys on the PCB-pad for key "4" seemed to work. Therefore, it was likely that there was a problem with the PCB-pads.
Cleaning with a bit of white gasoline and re-assembly resulted in all keys working fine:
Warranty-certificate with serial-number:
BTW, I'm not planning on collecting / hoarding calculators...