I have yet another GE here that I got in fairly stripped-down condition (three out of the four G4s I managed to find are all GE's!). So I swapped out it's "front panel board" (thank you service manual) and it's working fine. So I guess the original one has gone bad. I'll take a closer look at it at some stage and see what the problem might be. This is what the faulty one looks like (it's a different model from the one I replaced it with).