Martin wouldn't SW have a test bed they could plug the motor board into to confirm it's proper operation?
Could they possibly offer you a replacement board on loan to test it?
That's not an unreasonable request and if it fixes your issue you'd obviously be purchasing, or an exchange board?
You yourself would know this is a common type thing at companies who manufacture this stuff though, I do remember when Honda Australia couldn't fix a workmates new car (late 80s and it constantly broke down on the free-way between his home in Wollongong and work at Mascot, costing him most of his wages in tow fees) because they couldn't afford the computer test bed in Australia. Eventually the courts ordered Honda to either fully refund or replace the vehicle with another new car.
I use strong magnification and look for signs of burning on the boards or bad solder but seeing under the legs of surface mount stuff is impossible. The parts are even so small it's not so easy to pick up on a floating component but not everything is detectable like that. Otherwise it's oscilloscope and power and trace everything and I will do that for my own equipment because my time doesn't matter and I can't afford replacement parts but I've long given up repairing stuff for others (other than an original EH Holden car radio I can't get the correct original pot for or the oscillator shunt coil, the unit came theoretically working with half of the internal wires cut and incorrect, swapped components). I'm regretting taking it on for a friend, free.........
I don't know if the following link is any help, I'm guessing you've seen it but I can tell at a glance there's corrosion on the board, probable dry joints and some suspect looking soldering:
https://stargazerslounge.com/topic/3...n-motor-board/
I think the best you can do is see if you can get a loan, known working board which would confirm whether it is responsible.
I don't have one or it would be in the post within half an hour.