Knew I should have at least powered them on before posting, but they were all working a few years ago!
Only find one AM503 system, hours of looking have turned up nothing. Even that system isn't working, getting lots of noise spikes at low frequency, presumably the Hall amplifier.
The one 134 system I have tried is completely dead.
Apologies to those who expressed an interest, doesn't look good.
And I had just repaired my 7613 to test them with, that will be useful at least.