Almost every mysterious symptom in a vintage unit is worth checking against the power supply first, because the supply's failure modes wear disguises.
The disguises:
- Nothing at all. Dead is the honest failure. Check the fuse, then check why the fuse blew; a fuse that blew itself is a symptom, not an answer.
- Protection trips. On many receivers the supply's slow decline shows up as the speaker relay refusing to engage. Small parts around the supply drift with age; on a Sansui AU-717, two 150 ohm fuse resistors on the power amp modules are documented going high and taking the bias with them.
- Hum. Tired filter capacitors let mains ripple through, heard as hum that does not track the volume knob. (Ground-loop hum is its own thread here.)
- One channel weak or dead, or sound that comes and goes. A sagging rail takes everything downstream of it down partially with it.
How we test it, in order:
- Eyes first. Bulged tops, leaked electrolyte, scorched boards. Cheap information.
- ESR screening on the electrolytics. One warning that saves people from bad conclusions: there is no single "good ESR" number. A 1976-era wet aluminum cap measures thousands of milliohms by design, while a modern polymer part measures single digits. Judge each part against its own class and circuit, not a magic threshold from the internet.
- Rails under load. A rail that reads right unloaded and sags when the amp works is a different animal from a rail that reads wrong always.
And the safety line, plainly: the big filter capacitors hold charge after the plug comes out. Discharge them properly before hands go in.
Post the unit and the symptom shape; the power-supply corner of this forum has seen most of them before.