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Playing my own guitar fitted with an SC Laboratories piezo UST + line amp and (half-way through a set!) I suddenly got a loud mains frequency hum, as though an earth lead had come adrift.  I discovered that if I touched the guitar's jack plug chassis at the same time as touching the strings, the hum disappeared, so I finished the set with an old string wrapped around my right wrist and wedged into the jack plug.  Next day, I took the transducer, line amp and battery pack out of the guitar and plugged it into an amp to find the loose connection, but couldn't find it and couldn't reproduce the hum.  I re-flowed all the solder connections on the PCB and tightened all the cable connections, put it back in the guitar and it was fine for 10 minutes playing, when the hum started up again.  Connecting the jack plug earth to the strings (through my body) stopped the hum again.  Why should this be - the strings aren't electrically connected to anything - the (Graphtech) saddle should be insulating them from the piezo casing.  Anyone know what's happening here?  Is the transducer breaking down in some way?  I can't get any help from SC Laboratories - I think they might have gone out of business - they're not answering emails and there's an answerphone on the landline. Any ideas, guys?

Every now and then I will see the ground to the UST have an intermittent short. It is ok when out of the guitar but shorts when there is string tension. The problem is generally right where the wire hits the UST. I have fixed a couple but it is easier to replace the element with a Fishman or other style UST and just wire it to the existing preamp.
Hope this helps
Evan

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http://www.NewYorkGuitarRepair.com

Thanks for that, makes sense.  The bit I don't understand is how come my body plus the guitar strings seems to act as a replacement earth?  Another of life's little mysteries?  It's a shame I can't get another transducer from SC, my one sounded really good and I don't think a Fishman will give me the same quality.  I might try the B-Band electret UST and preamp - I've heard good things about them.

Check out the K&K web site before you make your decision. I have installed two of them with great results.

 

Mark

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Wisdom is justified by her children

[QUOTE=Max1157]

Check out the K&K web site before you make your decision. I have installed two of them with great results.

 

Mark

[/QUOTE]
Have you installed the UST?  This is my personal gigging guitar and I've got bad playing habits like hitting the top with the plectrum and muting quite agressively with the right hand - I always imagined the under-top transducers would pick up too much of that.

Latham

I probably have the same bad habits but I really like the quality of the sound from these pickups. The owner of the company took time with me to help with the first installation. I would also recommend the pre-amp he sells.

 

Mark

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Wisdom is justified by her children