EAR 834P - Sale or try and repair...

I have an EAR 834P and the volume is decreasing on the left channel. I opened the case and the tubes appear fine. However, there is some browning on the breadboard on one wire (coming from the toroidal power transformer), to the single phase bridge rectifier, and resistor (breadboard: P5, D3).

I have done some simple soldering (ex., DIY bottlehead amp) just not sure about diagnosis this issue.

• Does anyone have any ideas on a possible repair?

Otherwise, I plan to post it on the forum for sale. Just hate to take a loss on the price if it is a simple part replacement.