What came in
A Tanglewood Odyssey electro-acoustic, with a simple enough story: the preamp had stopped working. No output through the EQ-790 system, no reading from the onboard tuner, nothing. The reported cause was a leaked battery, and that turned out to be exactly right, though not in the way you'd first assume.
What we found
The battery itself was the easy bit: swap it out and move on. The real damage was downstream. The leak had crept along the wire running from the preamp board out to the output jack, and by the time the preamp cavity was open, that wire had failed completely right at the plug end. No continuity, no signal out, regardless of what the preamp itself was doing.
It's a reminder that battery corrosion rarely stays where it starts. Once it's on a wire, it works its way along the copper under the insulation, and the failure can show up somewhere well away from the battery compartment itself, in this case, at the jack rather than the board.
What was done
The last inch of the affected wire was cut back, past the corroded section, to clean copper, then re-soldered to the output jack. The volume pot had picked up some grime over the same period and got a clean with contact cleaner while the cavity was open. A fresh battery went in last.
How it left
Preamp working normally across the board: volume, EQ, and tuner all functional. Guitar back together and playable.
This is a small job with a bigger lesson in it: a dead 9V left inside an acoustic preamp for months is doing more than just not powering the electronics. Once it starts to leak, it's slowly corroding whatever it's touching (wire, pads, the inside of the battery clip) and that damage doesn't announce itself until the day you plug in and get nothing. If a guitar's going in the case for an extended spell, pulling the battery takes ten seconds and it's cheap insurance against a job like this one.