I want to give my 2 cents. I would say that problem with Cameca old boards and power resistors is that on some boards they are soldered with common/ normal solder (solidus 183 C) while it should be soldered with HMP solder (high melting point solder, 95% Pb, 2% Ag, 3% Sn, solidus at 300C). HMP is a bit tricky to do correctly, as any contamination by Sn lowers solidus and renders it useless. I got impression that different boards got assembled by different teams, and some boards got less attention to the engineering details. I swear that Cameca assembled some of electronics with eco-compliant (RoHS/ Pb-free) garbage solder on our newest machine - we had tin whiskers causing problems with a stage three years ago. So if it is old machine - de-soldering is very likely, and if it is a new - rather tin whiskers or cracked solder joints due to embrittlement is causing problems.