Marshall JCM900 - loud hum on LOW power option (back switch) - burnt PCB

[spunko]

Hello friends. This amp came on a sunday morning and in a hurry. Super loud hum on low power but on high power was good, said the owner with a video of proof.
First thing I did was took it out of the cabinet and saw the burnt spot. Inspected the PCB and it was sad 

Was it an arcing problem what caused this?

The switch tested good, so I cut the area because all of it was conductive, replaced R8 (don't worry about the pic it was taken before soldering), soldered back everything and it worked great.
All tubes were matched, all volatges ok. I tested it for a couple of hours, and it gigged for hours that night. So far is playing great.