Hi Andrea.
This amp is
very old. Perhaps 1971. It is full of electrolytic capacitors, a lot more than modern amplifiers and these are not everlasting. Over time and the heat inside the case, they dry out and lose their quality completely or even short out, explode etc.
When you buy old electronic equipment, this is what is lurking and causes failure and collateral damage to other parts such as transistors, resistors, even small fires, some period after powering it up again after so many years of neglect. A technician would not simply apply power, they would use a current limited power source or simply replace all capacitors before even testing it.
If there is no sign that much has been replaced, you need to
download the schematic, find the BOM and make a shopping list for the electrolytic capacitors. It won't matter that you have to buy higher voltage parts than shown or even higher capacitance up to 60% more rating, as new parts are smaller. However, never use less than original voltage or capacitance ratings. Don't buy cheap, unknown brands just because that's all you can buy locally. You may regret that and the sound may not be quite what you were hoping for. There are regular quality, industry standard and boutique parts but industry standards like Panasonic, Rubycon, Samwha, Nichicon, Nippon Chemicon. etc in appropriate grades will be fine.
When ordering, check the outside dimensions and type (axial or radial leads) that will fit the space and the lead spacing matches closely or you won't have much success.
Note, these are polarized components, don't confuse, lose, or mix up the correct orientation of positive and negative leads. Mark as (+) or (-) what you see on the cap onto the PCB, if there is no printed layout on it. Do this before removing the cap. Only remove the next 1 or 2 caps as you replace them, to avoid losing your way.
You need to check how to do many things here, beyond one answer and this is not a quick fix like changing a fuse. It's restoration work and when you have finished, don't simply power up until you can test that it is safe by making up your own current limiter; an incandescent 60W lamp in series with the active mains connection to the amplifier. It's call a bulb tester and you can find details by searching this forum.
This is important wiring, critical to your safety, so if you can't wire this up using even a table lamp with a suitable mains socket and wiring, don't proceed.
Hopefully, the amplifier will still be OK after this but recheck for obvious burnt components, signs of heating etc. as you proceed. You are at least at a point where measurements could be meaningful and you can troubleshhoot the amplifier, if necessary.
What happened? what can I do to fix it?
