Can anyone advise how deep problem might be?  Sound went quieter (from mid) and indistinct, then with soft "bump" lapsed into silence while playing aux/CD. Power telltale remains lit, but both channels absolutely silent whatever vol/bal from tuner/aux i/puts, and all to outputs. No obvious external fuse but perhaps internal (o/p) fuse? or power supply fault, affecting L & R together?  (And, any possibility of finding spare tapedeck drive parts, which diagnosed "sick, no spares" in UK some years ago?) 

Hi and welcome to Beoworld !  

Your Beocenter is probably in need of a good service after some 30 years.
It's very common to see that capacitors dry out and that can cause the
strangest things.
There are fuses inside the Beocenter but if one has blown, the cause will
have to be investigated. Voltage readings etc. is needed to get further into
a diagnose (f.e. power supply readings for the preamplifier) so I suggest you take it in for repairs.
Capacitor kits are available and, depending on your exact needs, it may be possible
to locate good used parts for the Beocenter. I can probably help you with factory new belts too.

Martin

Thanks Dillen - had noticed "recap" often while looking through existing threads - sounds very likely the cause, or necessary anyway. I know fuses age sometimes, but obviously unwise to simply replace without checks......... As far as tape goes, problem was poor sound quality due to tape speed irregularity..reason unknown except reported to require "unavailable" spare(s). Shall bear your offer in mind......