Yamaha G50-112 oscillates

[spy]

I have a Yamaha G50-112 with a very weird problem. When you plug a speaker the amp seems to work perfect but when you turn up the volume at high levels it starts to oscillate in a low frequency. Things got worse when you turn up the Distortion pot. When you turn up the reverb pot there's a high pitch oscillation!

With an 8 ohm resistive dummy load, things are worse. Oscillation is there even with the volume pot at zero. When you turn it up a little it disappears but if you continue it appears again but at much higher frequency. Same behaviour here with the Distortion and reverb pot just worse than before with the speaker.

With no load at all the amp is working just fine.

Then, I tested the preamp and the amp separetaly by disconnecting the orange wire (IN). Both the preamp and the amp even with a resistive load were working just perfect!

What is happening over here?!

Schematic is over here,

[Enzo]

Sounds like motorboating to me, and one likely cause would be lack of power supply decoupling. So make sure all grounding conections between assemblies is good, and watch the power supply rails for sign of the issue.

[spy]

Thank you Enzo! There was a poor contact between the pcbs of the psu and the output stage.