N Cook wrote:
It has one of those green blobs on the back with also a "Post Office
Certification" sticker but whether they were transfered by Dellboy I've no
way of knowing

In any device that old, I would start by checking or replacing any
relays. chances are the contacts have been tarnished with exposure to
air, arcing or just plain wear and tear. I recently had to clean up a
few on one of those early 80s sharp 'vz-' vertical turntables.
-B.