This comes up on the bench constantly, and the short answer is: your receiver is protecting itself, and the question is what it is protecting against.
Inside almost every receiver made since the mid seventies there is a protection circuit. Its one job is keeping DC voltage off your speakers. If the amplifier output drifts away from zero volts, the circuit opens the speaker relay and you get silence. Trip points are small: a typical relay opens around half a volt of DC, and some designs trip well under that. The click you stop hearing is that relay.
What trips it on an old receiver, in the order we usually find it:
- Aged parts on the driver or protection board. The classic. On a Sansui 9090DB, two 180 ohm fuse resistors on the driver board are documented as the parts that cause most protection faults on that model. Most receivers have their own version of that story.
- Real DC offset from tired output stages. Sometimes the circuit is right: worn bias parts or a failing output device put actual DC on the output, and the relay is doing its job.
- Something that only misbehaves warm. Fine for a few minutes, then shuts off means a part is drifting as it heats. That pattern is itself a clue; tell the thread how long it takes.
What not to do: never defeat the protection circuit or glue the relay shut. It is the only thing standing between a fault and your voice coils.
If you want to look: measure DC volts across the speaker terminals with nothing playing. A healthy channel sits near zero. Post the model and what you measured, and the protection-board regulars can narrow it down.