I have an acoustic guitar - a Cort MR750 dreadnought, from about 1998 - with a Fishman pickup system that came pre-installed in the guitar. I haven't been able to identify the type of pickup or preamp, although I have emailed both Fishman and Cort in an attempt to get more information.
There's a continuous hum coming from the guitar, that mostly goes away when I grab the metal end of the cable jack, making me think that this is a grounding issue. Changing cables, mixers, amps - none of this makes a difference, the hum is still present.
It's also worth mentioning that the problem came upon me quite suddenly, in-between two gigs about two weeks apart. The guitar was subjected to no unusual stresses in that time, aside from being carried in the trunk of a car in the cold (but it was very well protected, in a hard case with nothing on top of the guitar).
The preamp has no identifying marks, other than saying "Fishman Equipped". (Or, if there were such marks, they've long since been worn off.) But here are pictures of the preamp and of the pickup:
So I've narrowed the problem down to the guitar electronics. None of the wires inside are loose (I tightened up a cable that screws into the preamp, and that didn't make any difference), and all the soldered connections inside seem solid. I've swapped the battery and made certain it's connecting solidly, neither made a difference.
How I can go about diagnosing the problem? I'd like to replace as little of this system as I can. (I don't want to order a pickup, only to find the problem is the preamp or the wiring.)
Even better, could this simply be a shielding issue? Can the problem be corrected by shielding the electronics? How does one even do that with an acoustic guitar, by sheilding the interior cables?