Troubleshooting Cramer RV power supply

[madan1]

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 disconnected from chassis/ground/anode and gate and at some point someone has soldered a resistor between the cathode and the gate to keep them non stop open(?).
I connected the thyristors as shown in the schematics (gates to gate connectors on the mainboard and anode to chassis), but now I'm getting some strange voltage readings (Vac).


When using chassis for reference with disconnected gates:

T1
Cath. - 17.1 Vac
Anode - 0
Gate - 16.6

T2
Cath. - 1
Anode - 0
Gate - 1

When using transformer winding midpoing for reference (this is the V+ used for charging the batteries) with disconnected gates:

T1
Cath. - 12.34
Anode - 9
Gate - 12.34

T2
Cath. - 12.3
Anode - 9
Gate - 12.3

I suspect the thyristors went bad and that is why they were disconnected.
Is there a way to test them with only a multimeter?
On the board, both gates show 3.3Vac 50Hz and 1.3-1.4Vdc so probably there is some switching going on there..

[madan1]

I got lucky and found the schematics.

[CapLeaker]

The lid sez there is a 4A fuse for the battery charger. Check it. Also there is a switch for the charger.
Btw. It does tell you to NOT operate the unit on the power grid without batteries being connected.

I did find a manual of this thing but it is in Dutch. There is supposed to be a charger relay?

[CapLeaker]

Could also be whatever drives the gate of the thyristor is bad.

[madan1]

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, which I'm still trying to understand. Looks like it is controlled by Vref and B2 ( the power rail for charging the batteries ), but no input from the main controller. Hmm, now writing that, this circuit might not work correctly without the batteries smoothing the rail.
Well, I just ordered the thyristors and expect them to arrive by the end of the week. Then will test it again, but this time installed in the vehicle.

[CapLeaker]

This circuit has been obviously modified for a reason. Fix that PSU properly up like the fuse holder, thyristor and go from there.

[madan1]

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.