Hoping I can get some assistance repairing a Marantz PM225 amplifier.
About Me: I have a bit of basic electronics understanding, I can solder and OK with component recognition.
I purchase a old CRO yesterday and just getting to know how it works.

symptoms: The amplifier works OK for about 1/2 hour and then starts popping and crackling on its own.
Having no input or adjusting the volume does not influence the popping and crackling.
Turning it off and allowing it to cool down returns it to normal operations for another 1/2 hour.
One of the 4 final power amps was getting very hot. I have replaced all 4 . One still get warm but not hot now.

So where should I start ?

 

Download the service manual ..


https://www.electronica-pt.com/esquema/audio/marantz-audio/marantz-pm-225-audio-13827/

 

Thanks Dodgydave, I should have included that I have downloaded the service manual.

 

Both channels doing it in sync: Start looking at the power supply.
Only one channel: Start with the amplifier for that channel after the volume control.

 

Check the psu on the output transistors, and especially at the centre of the speaker output for Zero volts, any offset will indicate the thermal drift on that output set.