I have a Faith guitar which came with a Shadow 4020 preamp fitted. The preamp worked beautifully for a few years, but at some point over the last year (I think!) it started to misbehave. Specifically, the level of the output changes by very large amounts (haven't actually measured it, but tens of dB) intermittently. A typical example might be that over the course of a few hours, I might soundcheck it at one level, pick it up to play a while later, and find that it is essentially inaudible, then possibly after another hour it might suddenly return to its original volume (deafening anyone in front of the PA).
The unit appears fine in all other ways -- adjusting any external settings does not cause any interference/pops/changes in output, tuner works perfectly, etc.; I've obviously changed batteries repeatedly, deoxidised the battery contacts, and even opened up the unit to inspect and clean a little dirt off the circuitboards. Nothing has made any difference.
Should I just accept that the unit is probably damaged at the level of individual electronic components and replace it? (If so, does anyone have any recommendations? The guitar is definitely out of warranty by now, but I think I am able to get an identical replacement for the preamp unit itself for about £100 from the manufacturer.)
