Thanks, Joerg.
I think this item pre-dates a lot of this digital stuff; I doubt
there's a single eprom in it - and no custom chips, either!
It was built in about 1980 and doesn't go through any self-testing
routines. One has to carry out manual tests oneself. This wonderful
simplicity should hopefully enable me to keep it running indefinitely,
once I can get it fixed. Serviceability is one of my main criteria when
choosing an item of test equipment.