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A customer has brought me a guitar with a K&K Pure Mini installed, and it's not making any output.  I've even disconnected the piezo's output from the preamp and connected it directly to the cable to the amp and am not getting any sound at all.

Has anyone ever see a piezo fail to the point that there was no output?  There's visually nothing wrong with them (the K&K has 3 piezos), and wiggling the wires doesn't produce any crackling or evidence that there's a short or broken wire in there.  

Since there's no output, that seems to be the source of the problem, but I've never seen one of these fail like this so I'm hesitant to pin it on this.

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The most likely failure is in the jack. Have you tested the three discs individually? It’s practically impossible for all three to fail, and you would get signal if even one was live. I have never had one fail (except my own fault) and I’ve installed hundreds.

There is no sound if one of the discs have a short between the hot wire and ground. I used to bind the cables together with a cable tie, I don't do that anymore. If the cable tie was tightened too much the sound could later suddenly disappear for no obvious reason. When I removed the cable tie the sound was back. Must be a shortage in the cable from the pressure of the cable tie.

worth looking at!

a broken connection (i.e., open) would leave you noise and racket, maybe one or two of the three pads still making sound;

a short from hot to ground would leave you with dead silence and no way to trace it

@Eric Gleason try measuring DC resistance at the jack, if it's close to 0Ω then you have something shorting hot to ground, most likely downstream from the piezo pads themselves

check if you have continuity between hot and ground. if so, one of the 3's legs have shorted.

DING DING DING!! we have a winner!! this is the answer

Was it a cable tie?

There weren't any shorts and the jack proved to be good. Even with the preamp out of the circuit (easy to do since it's all modular) I couldn't get any output from the piezos.  I never did find the problem. It became apparent that troubleshooting the problem was going to cost more than replacing the system, so we installed an LR Baggs Lyric instead.  

That leave us with a short inside one of the cables or discs. The faulty disc/cable can be replaced, making all three discs work.

I appreciate the input, everyone.  It's possible that there are multiple faults here, but the pickup elements certainly aren't sending out any signal.  I disconnected the leads from the jack to remove that as a possiblible point of failure.  The DC resistances between hot and ground is in the mega-ohm range, and there's no output.  Oddly, there's also no crackling or anything when it's plugged in to the amp and I wiggle the pickup leads that would indicate partially broken wires.  

This installation has got to be close to 15 years old, and the Ultra Pure onboard preamp is at least 2 versions old. It predates my time as a K&K dealer.  I had an honest conversation with the customer explaining that I've spent as much time as I'm willing on free troubleshooting, and once I start charging the labor bill is going to rapidly approach the cost of just installing a new pickup system. I don't like to start throwing parts at a problem, and it's a weird failure, but it is what it is and this is her main money-making guitar.  With a full calendar coming up in a week, it's a better decision to just replace it and get playing again.

I have the LR Baggs Lyric system (which I *love*) on hand, so we're going to install that instead.  

I bet it's the "Ultra Pure onboard preamp" that is faulty. I always use K&K Pure Mini, as simple as it gets.

Roger, there may also be an issue with the preamp, but I'm not getting any output from the pickup elements, which are essentially the Pure Mini.

OK.

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