Yes, other settings and functions are lost too when mains power is disconnected - tuner included. (The unit did produce sound but at very low volume and quality).
The error referred to the model and destination settings, the model number was correct but the destination was blank?
The unit was sold in many countries and with two model names, and all the country codes and both model numbers are still selectable in the unit, thankfully. So wherever that info is stored is still good.
I guess the country codes set the appropriate tuner ranges for the selected destination, among other things - different mains voltages even?
The service manual says that the unit will not function if the destination is not set in the back-up IC EEPROM: IC221, so that would be done before despatch from the factory I guess.
When the unit is powered on, I guess that the back-up IC221 provides the destination and model info which was set at the factory, and the unit "sets itself up" accordingly (??).
It's odd that it only loses the destination setting (and everything associated with it).
So far, with stand-by power maintained, the unit is "remembering" the destination setting. As said, the error occurred after a long mains power loss (7 hours).