1. Acquisition

I found this calculator in one of the back rooms of the Book Art Museum (Łódź, PL), where I worked for five years. As a lover of old technology (and a new kid on the block in retro computing), I was more than happy to bring it to my lab and start restoring it to a working order.

2. Cleaning

I did it in my old lab at the museum. Prior to firing it up, I cleaned the Soemtron with compressed air already at the museum; it collected mighty amounts of dust, so I did the blowing outside. This also involved pulling the twelve PCBs from the arithmetic/logic unit cage, blowing air on each one of them, with extra care not to damage the delicate components like signal transformers or the ferrite core memory.

3. Initial Power Up Attempt 

Connected the calculator to a variac and powered it up, bringing the voltage gradually up to nominal value (240V). Nixie tubes lit up showing zeros. Attempts to enter numbers were failed - nothing changed on the display when doing it as the documentation specified. At some point all tubes went dark and had intermittent contact, suggesting the power rail going down somewhere. Time to call it a day...

4. Transport

I took the calculator to my lab in Gdańsk on my return trip from the museum.

5. Debugging the Nixie display issue

First I powered the thing up, then measured the voltages, made sure they're within spec. While the Nixie positive and negative voltages were present on the PSU, the anode driver didn't get the positive. Reflowing solder on the connectors seems to have helped - almost certainly a cold joint. I reflowed other connections on the motherboard too, as they're under some mechanical stress from inserting and removing the ALU boards from their sockets.

6. Debugging the keyboard

Some keys were sticky, so I decided to take the keyboard apart and make sure that the switches work properly. The keyboard uses mechanical micro-switches in SPDT configuration, where all the NC contacts are wired in series to the ground (detecting the inactive state of the keyboard), and NO contacts are routed to a number decoder and function decoder logic. These decoders translate each key to BCD-coded numbers.

7. Still not alive...

Despite the keyboard now working properly, the calculator still doesn't even want to show the number I just entered. The function and number decoders proved OK after some testing. Documentation study and more work will be necessary - I'm suspecting a faulty ferrite core memory driver, or the memory itself. Got to take a deeeeeeeeeeeeep dive...