The Iskra 122 is a 1970s-80s Russian industrial & scientific calculator which uses nixie tube displays. I’ve repaired and restored a couple, but one PCB I’ve been struggling to fix. Since the board live in a densely packed cage inside the machine, and since the chips are difficult to test in isolation even with a powered board, it makes diagnosis almost impossible unless you see a specific failure which has been recorded before.
You can use board swapping if you have a known good unit to isolate which PCB is faulty, but from there? It’s difficult. I spoke to Sergei Frolov who runs the Soviet Digital Electronics Museum (link here: http://www.leningrad.su/museum/) about this problem a while ago and he showed me a video using a transit board – essentially a straight extension that lifts the problem PCB up so you can get an oscilloscope on it. Logic probes are no good here as all the chips operate on -27v!
I had a brief search but couldn’t find anything that would work, so I finally on Christmas eve, sat down with KiCAD and designed one. The pitch of the boards is 4mm which is no longer a common type and edge connectors with 58 pins in this pitch are completely impossible to source. However, there’s a 3.96mm part from EDAC which fits the boards snugly enough to allow that 0.04mm per pin to not matter too much, so I made it 4mm on the calculator side, and 3.96mm on the board side. It’s important that the PCB is the correct side to fit in the cage and guides, and fortunately my first attempt was correctly measured.
Here it is in place, the machine in the photo runs so badly that pressing any number causes it to crash. It didn’t used to be that bad and only crashed on certain calculations – so my hope is that the chip in question has become bad enough to be easily spotted now. Previously I’d changed 3 chips which are common failures, with no change whatsoever. Maybe I will have this one fixed soon.
The part number for the connector is an EDAC 315-058-500-208, these are available from Mouser and similar suppliers.
The PCB design was uploaded to PCBway.com, you can buy them from there at a low price or download the gerber files for use yourself:
https://www.pcbway.com/project/shareproject/Iskra_122_board_extender_transit_board_fc7aeb14.html
