[nigelpritchard]

Thanks to the above I have been able to get the calibration done. Well, to be precise, the cal pot is a bit iffy and at the true calibration point is less than perfect (close to end-of-track). It's just hanging in there at the moment after some IPA spray.

Given that pot replacement is likely to be a non-starter at it's age, the simplest solution might simply be to shunt the pot series resistor R20 (100ohms) with another resistor so that the slider works nearer to the centre of it's adjustment range.