electro acoustic issues

[gibson175]

I have a top of the range takamine classical thats causing me grief. Its got a loud hum. The hum is reduced drastically if i touch the guitars input jack with my finger so it seems to be a grounding issue.
I also have a takamine LTD steel string too - that works fine and has done for 20 years now.
I have swapped out the pickup for a new one in the classical...still hums - so its not the pickup.
I have swapped preamps from the two axes...it still hums.
I have resoldered all the contact points.
I think i may have tracked down the problem, but it seems odd...
The shielded wire connnecting the input jack to the preamp seems to be microphonic. If i tap the wire it seems to act like a contact transducer itself....even if its not connected to anything else. So the hum is down that end of the circuit i think. The wire is two conductor plus a shield wire.
Is it possible there is a break in the shiled/earth wire?
Any other things i have not thought of?

[Mr 9finger]

Could be a broken shield wire, but unless it's solid core wire, it's most likely internal to the transducer itself. I'm not familiar enough with piezo pickups to really give you any good advice.

[gibson175]

IM pretty sure its not the transducer for tow reasons...ive replaced it with a new one from the factory, and the hum is present regardless of whether the pickup is plugged in, or even if the input jack wire is plugged into the preamp. Shame there are no piezo experts out there.

[Mr 9finger]

Clip the leads and splice new wires in with a new jack. It very well could be the leads. If you've tried everything else, it's worth a shot.