In the process of troubleshooting a faulty Drake TR-4C, years ago, the root cause was traced to a non-working PTO.
A visual examination of the mint-condition PCB revealed nothing. Component checks with a multimeter also showed nothing amiss.
Then 'Eureka'! A continuity test showed that the PTO coil was isolated from ground.
A closer examination of the PCB, with a magnifying glass, revealed a dry-solder connection at the ground end of the PTO coil.
Just one solder touch-up and the old workhorse was back in business!
_______________________________