[turretslug]

That's a neat looking amp- I'm something of a fan of these '70s/'80s receivers, generally well-built and long-lived, though they latterly got derided as behemoths. Is there a possibility that a strand of outer braid has bitten through the inner insulation on one side, overloading one amp channel without necessarily muting it?- some coaxial cables have quite soft inner insulation and twisting up the outer braid into a pigtail can cut into it. A more exotic possibility is that the amp might be going unstable with an unusual cable like this- most amps were designed to cope with a wide range of loading permutations but there's a limit to how much extraneous capacitance/inductance some could cope with. This was an Achilles' heel of some fancy audiophile amps that cut back on compensating components for purist motives- unlikely with a mass-market amp like this, but an outside possibility if the basic checks don't show up something like a partial short.