I have a background in electronics but not in audio amplifier.
I have a Sony Audio amplifier, a 25 years old model. I turn on the
power and it takes 20 to 40 minutes to hear the music. Just before I
hear the music, There is a ZZZZZZZZZZZZZ then a CLICK. Each selection
(Tuner, Aux or Tape) works or does not. Once the amp works, it works
until I turn it off.

On what stage, from the power supply to the audio output, I should look
at?
What type of component I should look for ( capacitor, relay,
transistor)?

Do you have a suggestion of a web site?

The purpose of this posting is to understand how an audio amplifier
works to repair it.