Mk 138 not working

Heres one for you:

I bought three thermostat kits which are great but of these and one isn’t working for me.

At the moment I am getting 1.21 volts at the + / - input.

All boards seem to be wired up the same and he solders all seem ok.

I ave replaced c3 and c4, the relay, zd1 and the trimmer but still getting 1.21 volts rather than the expected 12v.

Anyone any advice on what to check for.

Thanks

We assume the 12V power supply is coming from a lab. power supply.
Probably, the unit is drawing excess current because of a short somewhere.