[jubilado]

Thanks again for the reply.
The 626 Z20 is slightly different from 621 I think - the mains fuse is not mounted on the board but attached to the chassis underneath and there is no voltage selector. But there is no difference components wise I suppose. I've added a pic of mine here. Measuring across the pink and blue wires highlighted (pins B1 and b2 ) the AC voltage varies between 57.9v and 58.5v. Measuring voltage at the AC outlet shows a steady mains voltage of 240v

I've cleaned up that pot on the Z20 board
and
Measuring across the resistor 232 with volume at zero gives a steady voltage of 60.2 volts at one end of movement and 57.8v at the other. Guess there's a problem somewhere on the regulator.