One possibility: Suitcase pianos of this era are prone to intermittent failures caused by cold solder joints where plastic Molex connectors are soldered to the amp boards.
Take a look here and in the other threads mentioned in that discussion: https://ep-forum.com/smf/index.php?topic=10048.msg55968#msg55968
If this is the problem, a soldering iron will fix it, and using a little hot glue to keep the connectors from coming loose again will keep the problem from recurring.
Alan
Take a look here and in the other threads mentioned in that discussion: https://ep-forum.com/smf/index.php?topic=10048.msg55968#msg55968
If this is the problem, a soldering iron will fix it, and using a little hot glue to keep the connectors from coming loose again will keep the problem from recurring.
Alan