#1
So my friend has asked me to have a look at his acoustic guitar which is acting up. Basically, when plugged in, the two E strings are coming through fine, the A and B strings are then considerably quieter, and the D and G may as well be silent altogether.

It's got an under-saddle piezo pickup, is it likely to be the case that the pickup itself has just somehow become damaged and needs replacing? I'm not too familiar with the preamps in acoustic guitars so I don't if there is likely to be anything else going on with it..

Any advice would be great, thanks!
PRS SE Chris Robertson
Yamaha SF1000 (Both of 'em)
Fender Stratocaster

PRS SE Angelus Custom

Laney L20H Lionheart
Marshall 1936 w/ Eminence

Rather large pedalboard..
#2
It might be poor contact between the saddle and the piezo - some USTs are very sensitive to this. There are a few fixes:

Level the bottom of the saddle
Ensure adequate string break angle over the saddle, by cutting ramps between the pin hole and saddle if necessary.
Put shims under the quiet strings. I use thin shim brass
Cut slots in the saddle between the string to make if flexible, like the Fishman fishbone:
#3
Quote by Tony Done
It might be poor contact between the saddle and the piezo - some USTs are very sensitive to this. There are a few fixes:

Level the bottom of the saddle
Ensure adequate string break angle over the saddle, by cutting ramps between the pin hole and saddle if necessary.
Put shims under the quiet strings. I use thin shim brass
Cut slots in the saddle between the string to make if flexible, like the Fishman fishbone:



Do you think poor contact is likely to be the case if it's something that has recently occurred, and not in conjunction with a string change? It seems unlike to me that anything might have moved while it's been under consistent pressure from the strings. Of course, I don't know, hence my asking :P
PRS SE Chris Robertson
Yamaha SF1000 (Both of 'em)
Fender Stratocaster

PRS SE Angelus Custom

Laney L20H Lionheart
Marshall 1936 w/ Eminence

Rather large pedalboard..
#4
I don't know what would cause that kind of failure, you would expect it all to work, or the whole thing to stop working completely from a broken wire.

Something else to try is tap test, which I forgot to mention first up. Take out the saddle, exposing the piezo strip, and plug the guitar in and gently tap the piezo at each point where the string would go over. The tap should sound the same at each point, if not, it would indicate a bad piezo element.

Another possibility is bad earth contact on the piezo strip. I had this happen on a Fishman UST years ago. The tech fixed it by installing a thin brass strip under the UST. This was earthed by drilling a hole in the other end of the saddle slot and running another earthing wire through it, soldered to the brass strip.