Hi all, my Fender Rumble 40 v3 developed an intermittent problem, where the sound of my bass goes from normal to much lower and distorted then back to normal, then low. I bought this amp used so it is not covered by any Fender warranty. I'd like to repair it myself. I suspect the problem resides on the preamp board and may be caused by a bad solder joint or intermittent op-amp chip. A schematic would be so helpful in the diagnosis of this issue. Does anyone know where I could obtain one? Thanks..