Since the problem persists whether you’re using a 9V battery or a power supply, it’s likely not a power-related issue. You mentioned that your guitar is shielded and that other pedals don’t produce as much noise, which is a good sign.

Considering you have some experience with electronics, there are a few things you can try to repair your Russian Big Muff. First, check the pedal’s connections and make sure all cables are properly plugged in. Sometimes loose connections can introduce unwanted noise. Next, inspect the internal components for any signs of damage or loose parts. If you notice anything suspicious, try resoldering the connections or replacing faulty components.