Troubleshooting piezo pickup

MartinBush

Hi all

I have borrowed a Crafter mandolin. Seems decent based on what I know about mandolins (very little) but there is no sound from the pickup. I've borrowed it with a view to buying and the owner is OK with me investigating the problem.

Is there a standard way to troubleshoot and work out what's wrong?

I know the lead and amp work.

There's no battery in the pickup or control section as far as I know. I have tried running the controls back and forth in case there's any dirt, but I don't think that's it. 

Martin

TheMadMick

You might like to check that the pick up is piezo. The only one I had was like a mic on the mando top. If so the output may be very low and need a pre-amp.

MartinBush

According to one of the only pages on the web I can find with much detail it's "passive" and has enough output to drive the writers amp. https://jakewildwood.blogspot.com/2014/08/c2010-crafter-m70e-acoustic-electric.html#:~:text=The pickup is (thankfully)%20passive. 

More than that I don't know sadly.

BillDL

Is the output jack a combined endpin-socket or does it have a standard jack socket fixed to the outside of the body's side with a washer and nut, like this?

If it's like that, then it is entirely possible that the nut has come loose and, while tightening it, the jack spun and broke off a wire.  Obviously if you remove the nut the jack is going to fall inside, and if your hand is too small to fit into the soundhole you won't be able to push it back into the hole.  It's best to use an improvised tool to do this.  Many years ago I stripped down a 1/4" plug from one of those cheap plastic guitar cables so that I just had the plug, and soldered a length of copper wire to it. It looks something like the fishing tool shown at 1:27 in this video, but used for a combined endpin socket or the hand-made tool shown on this blog page. The greatest diameter of the plug is still small enough for the washer and nut to slide over.  This allows me to insert the plug, release the nut and washer, and push the socket into the body towards the soundhole for inspection and repair (if the wires are long enough), then pull it back through the hole.

If the wires are still securely soldered to the lugs on the socket then at least you have eliminated one potential cause.  It is a wise move to remove the plug from the socket while it is accessible through the soundhole, slide a toothed washer over the threaded collar of the socket, then insert the plug again and pull it back through the hole.  If you have enough thread on the collar on the outside of the hole for the nut to wind on fully after with the additional thickness of the toothed washer, pull it back through again and apply a dab of superglue to secure the washer to the body of the socket.  The washer will grip the inside and should prevent the socket from spinning as you tighten the nut again.

Combined endpin-sockets often have a very small socket on the end inside the instrument and the pickup wires are connected to a micro plug (2.5 or 3.5mm diameter) that pushes into that socket.  Cheaper made endpin sockets can often fail.  If somebody has lifted the under-saddle piezo element out of the saddle slot it can tug the plug out the socket or break the connections on the micro-plug.

Be careful not to tug on the wire that presumably goes up through a hole in the saddle slot to a piezo strip.  They can be quite fragile where they connect to the pickup element.

If you can unscrew the plate for the slider controls, carefully lift it out (wires may be short) and look for broken connections.

MartinBush

That's invaluable info @BillDL - had you not said that I'd have unscrewed the jack and realised my error when it was too late

The jack is as shown in your picture. I will go carefully as the instrument is not mine yet. It seems a good one but I do like the idea of having a working pickup as it will make recording easier.

One other thing of note is that I think, when using a certain setting on my Spark, I briefly heard some sound when I moved the volume. Not the sound of the instrument - a sort of rustling. If that did happen and wasn't my imagination or some sort of RF interference is it meaningful?

MartinBush

Quick update.. it's now working.

I started by taking out the control and from there I could see the jack - no loose or disconnected wires. 

Next I slackened all the strings and gave the bridge saddle a prod. Then I tapped the strings and... Sound! 

I presume the piezo wasn't seated correctly.

Bad news is a string snapped when I tuned it back up, but I'm taking this as a win overall

BillDL

Even if the piezo strip wasn't seated properly I would have thought that it would still have made some sound.   I would guess that the section that goes down through the hole in the saddle slot may have a dodgy connection.  Some of the piezo elements have a very rigid spike encapsulating the wires from where they emerge at the underside of the transducer:

If that is bent it can break a connection.  Glad it's working now though, and I hope it holds up.

MartinBush

We will see how it goes @BillDL the good thing is that I know that the jack and controls work. Getting at the piezo, if I need to, would be a challenge I expect. It does seem that slackening the strings and tapping the saddle did the trick.

I will be restringing it soon so will see how that goes.

ICBM

Yes, the piezo most likely has a break in the wire, probably where it goes through the bridge. The good news is that it works now… the bad news is that it will probably stop again at some point. It may be possible to fix it by soldering up the break, but if not then it will need a new pickup eventually.