Hi.

So I've offered to try and fix a friend's guitar and I think I have the fault narrowed down...

It sounds like or even is a dodgy earth connection.

I don't believe the barrel jack's connections to be faulty since wiggling the jack around a bit doesn't produce the problem. I've checked every single solder joint including every one on the pre-amp for dry joints.

I did originally find a few strands of ground wire from the shield to the jack very close to the hot terminal and rectified it and plugged back in and thought I'd solved the problem.

However I've discovered that when I press down on the bridge I can produce the sound of a dodgy earth and pressing it again stops it ( randomly though, might take a few choice firm presses). Also if I move the lead that connects the piezo around inside the guitar a little it makes the buzzing sound. Annoyingly this is encased in shrink-wrap so I can't have a visual inspection but would you say it is a faulty piezo?

If the bridge is pressed just right and the buzzing stops the guitar works perfectly, so it doesn't seem like the actual crystals are damaged or anything. When the guitar is buzzing some audio still does make it through if this is relevant.

I've also noticed that when I touch the shrink wrap on the other end of the piezo cable (the connector going to the pre-amp) it generates an even louder earth hum and touching the casing of the pre-amp at the same time causes the hum to stop?? Also if I press the piezo and cause it to hum and touch the other end and earth myself against the pre-amp, it doesn't stop the hum.

This is a real noodle scratcher and I'd love to fix it if at all possible but I realise it's pretty hard to fix a piezo, but I'd like an expert second opinion that my diagnosis is correct.

I figure if the earth were bad anywhere on the jack then the hum would be consistent.

Any ideas please?

Thanks.

Hmmm... I've taken a few dead piezo saddle strips apart myself for fun. The detail varies, but most are variations on the same theme.

Inside the heat shrink you will find 2 long copper strips running the length of the pick-up, both strips are soldered to a pigtail wire at one end. Sandwiched between the two copper strips is a strip of rubber the same thickness as the piezo elements with holes punched to retain the elements in place. When it is all assembled the piezo crystals are sandwiched between the 2 copper strips, each crystal connected to both strips forming a parallel circuit. One of the copper strips is connected to ground, the other is the output.

I would talk to your local Yamaha main dealership first and make sure that the saddle strip is still available as a spare part. Get a price, find out whether it is in stock and if not how long it is to get a replacement (there can be a 3-6 month lead time for spares not held in stock in the UK which need to be sourced from Yamaha in Japan - most companies are the same). Then decide whether you want to try to fix it, or just go ahead and order the spare. And bear in mind that if you can't fix it, you will have to wait for a spare anyway.

Mechanically, virtually all that can go wrong with the piezo strip is that the soldered connections to the copper strips have failed. Either because of a dry joint or damage through mishandling at some point. You may be able to cut back the heat shrink a little to expose the joints for examination/repair, but unless you are very lucky that will leave too short a section to fit in a heat shrink sleeve without impinging on the saddle slot and impairing the performance. The only other option is to remove all of the heatshrink to expose both strips in full and the elements, make a repair if possible, and then put it all back together with new heatshrink. The problem is matching the old heatshrink so that the repaired strip still fits properly.

Not an economical repair really, but worth a try if you want the excuse to take it apart and see how it works

Andy

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When the going gets weird, the Weird turn Pro.

Thanks for the reply Andy.

I've done a bit of Googling and haven't yet found a replacement part. I will get on the phone to Yamaha tomorrow.

Do you agree then from the symptoms I've explained that it is the piezo at fault? You've stated that mechanically there isn't much that can go wrong with them. It appears to be working fine generally, it's just when you apply additional pressure on the bridge it triggers this annoying hum and when pressed again it stops humming and works fine.

The only other thing I have noticed is that the signal, voltage (if that's the right terminology?) and the ground are squeezed up together pretty tightly and look a little worn where they attach to the barrel jack. I've prodded and twisted the terminals whilst the guitar is plugged in to see if I can get it to reproduce the noise and it doesn't, so am I right to assume that this definitely can't be the problem then? I could try resoldering them on a little neater but only if it seems necessary.

Thanks again

Always a difficult call without being hands on and getting to play with it, but if I was pushed I would suggest a dry/broken joint on the pigtail wire from the earth side, possibly a damaged copper strip on the same side. And what makes me think that is that physical pressure on the bridge makes things change. Physical pressure = mechanical movement in the piezo assembly.

One possible test would be to temporarily disconnect the piezo strip and replace it with a cheap stick on piezo bug. It might not sound great, but it would let you test everything downstream in the socket/pre-amp etc.

Andy

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When the going gets weird, the Weird turn Pro.

Ohhh good call there, I was just routing through my cupboards since I have a few contact mics in there and had a thought... My brother has a Yamaha APX. I believe it's an older APX-4 something model.

My next port of call shall be to borrow that and switch piezos (fingers crossed they're compatible) and see what happens.

Thanks Andy, I would've never have thought of that otherwise.

I tried another piezo from a APX model and the buzzing stopped, so I've also ruled out the pre-amp and the 3 pin barrel jack's connections, it must be the piezo.

I'll ring Yamaha tomorrow, and am awaiting instructions whether to have a go at attempting a repair on the old one. Short of finding a loose terminal, I'm not really sure what else I can do with it though. It might become more apparent when it's opened up.

Well, that's definitely a step forward. Lets hope Yamaha come up with a quick response for you.

My gut feeling is that there is a problem with the pigtails to the copper plates; a dry joint, broken plate, maybe a stray core from the hookup wire making a connection it shouldn't. The real problem is finding a suitable shrink insulation to use. There's not much else to go wrong.

Andy

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When the going gets weird, the Weird turn Pro.

http://www.frost.co.uk/automotive-electrical-tools/black-plasti-dip-brush- on-liquid-electrical-tape.html

Any use in lieu of shrink insulation ?

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Music is it's own reward .

Quote DaveFry:

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http://www.frost.co.uk/automotive-electrical-tools/black-plasti-dip-brush- on-liquid-electrical-tape.html

Any use in lieu of shrink insulation ?

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Good spot Dave, and could be useful to DIYers. But no good in this application I'm afraid, the only thing holding the parts together is the pressure from the rubber heatshrink.

Andy

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When the going gets weird, the Weird turn Pro.

Oh so I've also learned something else then... I just assumed the heat shrink was purely there for a little stability and that it's primary role was for insulation, not to actually hold it together!

There goes my "healthy dose of electrical tape" idea out the window.

Sorry Rhys,

In theory you could do it with electrical tape, but you have to be careful with the wrap, especially the overlaps. The whole assembly has to sit neatly in the saddle slot. If there is extra friction in places along the slot due to the wraps, the piezos closest won't work so well and the strings will be unbalanced.

Andy

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When the going gets weird, the Weird turn Pro.