Hello! I am Andre from Brazil, I got a used Russian Big Muff that sometimes sounds awesome and quieter with barely any noise at all, (quieter than my Maxon tube screamer when engaged). But somedays it is sounding like this:

I dunno what can be causing this noise, sometimes I insist on playing and the noises seem to fade away, but the majority of the time, it is unplayable.

the problem persists either on 9v battery or powered by a power supply.

The guitar is shielded, and with other pedals, it barely makes any noise. (You can see when I turn in the tube screamer with a compressor. the noise suppressor was off for the purpose to show the noise my rig has.

So, any ideas on how I can repair my Russian big muff? I know a little about electronics ( been repairing and modding old video games for some time).

It is really hard to find this pedal here. it costs a month of work in Brazilian Reals :/

So, please I am kindly asking for some directions to try to repair it. This is my first pedal from Electro Harmonix the sound is amazing when it is not involved in a wall of noise.