Reading further through service manual #1 section on 2ND LO tuning failure - there is an onboard calibration procedure (SHIFT 0 1 (calibration) 4 (2nd LO) - typo in the manual misses the 1 step). This provides prompts to adjust several tuning control potentiometers (4 or 5 total in sequence). In each step a measurement (made after a few seconds) was out of tolerance and prompt was offered as to which potentiometer to adjust. As I don't have the unit open, I just bypassed each of the steps. It appears that calibration of this type is manged through potentiometers rather than DAC outputs and the microcontroller, so the unit will be opened as soon as bench space is cleared for it.
I will update once I've completed (with success or failure) the calibration steps noted.
