Pink Triangle Anniversary battery PSU

I have operation problems with PSU battery of my PinkTriangle Anniversary.
In concrete terms, the turntable works regularly when powered by the mains, if the power supply is connected to the batteries the turntable does not work. The good condition of the batteries is obviously beyond question. Probably some passive control component on the battery circuit is malfunctioning, but which does not have the original circuit drawing. The English manufacturer does not have these drawings, and the designer (John Westlake) cannot be reached. I wonder if anyone can give me some useful suggestions.

Thanks
Massimo