By some way the most comprehensive and helpful advice on the internet for repairing chess computers is provided by Gerardo Mateo (aka Berger) (link). His website is now available in English, as well as Spanish and Catalan. Patience is required to explain repairs in detail and to take pictures as you are going along, so many thanks to Berger.
Included on my website are a few mostly minor projects. For general advice on repairs to reed switches and LED replacement try my Conchess repairs page.
I am no electronics expert, and only carry out the sort of simple repairs you see in these pages to my own chess computers. I do not attempt to mend anyone else’s. My own experience is that over half of chess computer faults consist of broken or corroded reed switches, faulty LEDs or control buttons, loose wires and badly soldered joints, loose or split ribbon cables, corroded battery compartments and similar problems which can often be repaired with limited knowledge and experience.
Our throwaway age does not encourage the repair and renovation of old equipment, but to keep the chess computers of the 1970s and 80s in good working order there is usually no other option than to do it ourselves.
To help keep the old chess computers going I have a stock of Fidelity spare parts which are listed (here).