I'm not sure the previous guy knew what he was doing... for instance the transformer auxiliary power coil was connected to the gate connectors.
But, yes, once the psu is working, I will fix all the small things like fuse holders, connectors, solder joints, etc. As mentioned, I'm on holiday now and away from my solder irons, dmms, parts and so on.

The fuse is bypassed, because the holder broke, when opened it to test the fuse (the fuse was ok). The switch works fine.
I guess it should not be used without batteries because they are also used as capacitors and if there are no batteries, the power would be so chopped that it might damage the consumers. Looking at the schematics, I do not see any other reason.
The gates are driven by T1 and T2, which test fine. Both transistors are controlled by comparator LM 339 and if I get it right - the output signals are same, but with reversed phase. There is some signal pull down circuitry,...

I got lucky and found the schematics....

Good day.
I'm currently on a vacation and a friend from the camp site asked me to check why his camper power supply does not charge the batteries when hooked to the mains.
The PSU is cramer and from what I've found the same mainboard is used in both cbc-b7 and bcc-b7. The control module/interface panel is Cramer D-5750.
So what I've found so far - the transformer seems to be working fine and the board has no visually damaged components. I tested most of the diodes and transistors with a multimeter (the only tool I got here) and they read fine.
The two thyristors have been...

Probably a stupid question, but have you checked the tweeter and the woofer? Are they shorted?