Hello
Yes, it’s not the speakers; anyway, just swap the left-right connections on the amp, and if the problem changes speakers, then it’s the amp.
I hope it’s not a tube amp because there are very high dangerous voltages present, even if it has been unplugged for a while, due to the capacitors.
I can hardly advise to touch it. There are insulating electrician gloves, but do not handle it without unplugging it.
With old devices, chemical capacitors swell or leak as they become faulty, but mainly with devices that are 50 years old because the quality of capacitors was not good. Ten years ago, Yamaha amps could have good components, and in audio, often the choice of good components improves the sound. Normally, this should happen less often, except that motherboards for PCs less than 10 years old have failed in whole batches due to cheap chemical capacitors that quickly became defective. So we can check. It’s mainly the large capacitors that are more stressed near the amplifiers or transistors in particular. This would usually produce a poor sound.
So, it is visible, and the right side would be worse than the other. Good lighting and a magnifying glass if necessary.
On an old device, behind the adjustment knobs, there are often variable resistors that oxidize, changing their resistance, although more often, crackling occurs when turning the knob.
There are also sometimes adjustable variable resistors used for adjustment on printed circuits with a small indentation or slot for a screwdriver. Often, a dab of glue blocks the adjustable part; the very precise setting should not be touched.
The same oxidation problem and modified resistance as behind the volume knob.
Inside relays, if there are any, and switches, oxidation can also occur.
For these two oxidation problems, repair technicians say that a spray of special deoxidizing cleaner for electronics that penetrates through the component holes solves the problem.
After 10 years, humidity has had time to penetrate and oxidize all contacts, all non-soldered connections, crimp connectors on wires, and contact resistance can quickly become problematic, especially if the voltage is low and the current is high. It can be useful to disassemble and deoxidize all these contact surfaces but to reassemble everything exactly as it was; simply moving a wire a few centimeters in an audio amp can create hum.
We can check the large resistors that heat and cause the printed circuit to heat up, as well as damaged solder joints and tracks.
All these “common” problems with old devices can be checked, but I don’t know the possible link with an improvement when hot.
For intermittent failures, some do not hesitate to “torture the device to reproduce the fault.”
So this forces one to work on the device while it is open and functioning, which goes against all usual safety advice; there is 230 volts AC, which after rectification provides a voltage of around 350v even if it’s not a tube amp. Let’s not forget that these are professionals describing these practices, trained and used to minimizing manipulation errors and preventing consequences.
They say they use a “cold spray,” an aerosol designed for this purpose and sold by suppliers for electronics.
So, once hot, the amp works well; cool down one component after another, especially the amplifiers or transistors fixed to heatsinks, which are metal parts, the switches, or relays too tight to let the deoxidizing cleaner penetrate. The fault should reproduce when the defective component is cooled.
By random chance on the internet, the first page I find presents products that match the two descriptions:
http://www.manutan.fr/protection-et-produits-de-nettoyage-electroniques_c_Main400F002Q.html?&fromSearch=false&filter=price,float,5.0-20.0;brandname,list;price,float;&finalCatString=8519304225180834008;8519304225180834060;8519304225180834081
