Frustrated. Persistent piezo bridge problem

[Butcher of Strats]

My head has not fully absorbed this all but if the unfinished single mag pickup is connected to the volume which is connected to the powerchip, is the mag pickup thus connected to the powerchip?
Not familiar with that preamp presumably designed for the piezo, but I would presume the piezo should connect to the powerchip INPUT, while the mag pickup should connect to the powerchip OUTPUT?

As opposed to both pickups connecting the the powerchip input.

Just taking a guess as I have not used a powerchip or wired a piezo preamp.
Just figuring the mag pickup should not be preamplified by the piezo preamp.

 

[retrobob]

My head has not fully absorbed this all but if the unfinished single mag pickup is connected to the volume which is connected to the powerchip, is the mag pickup thus connected to the powerchip?
Not familiar with that preamp presumably designed for the piezo, but I would presume the piezo should connect to the powerchip INPUT, while the mag pickup should connect to the powerchip OUTPUT?

As opposed to both pickups connecting the the powerchip input.

Just taking a guess as I have not used a powerchip or wired a piezo preamp.
Just figuring the mag pickup should not be preamplified by the piezo preamp.

To my knowledge, the piezo power bridge does not run the mag pickups through the piezo pre-amp, it blends them only and allows you to adjust the piezo's output to closely match the mag output.

 

[Eric_G]

Personally, I don't like this:



You may have overheated the sleeve ground a bit too much and melted the inner shielding of the piezo hot. I would redo this differently to make sure you have a clean connection and your ground is not leaking in your signal.

If the wire is a braded shielding ground, cut the braded wire along the length of the wire (about an inch), twist it to make a independent wire, run it through the hole from under and solder it. That's how I would have done it to avoid melting the hot wire shielding.

You may not have melted it, but it will at least be a safer connection to ground.

Also in the same image, for the copper ground wire, if the PCB is a hole through, I would clean this up.



I would run the copper wire (red circle) from under through the hole, solder on top and nip the end. Just like the other connection (Circle in green).


Also, I'm wondering if you may have created a solder bridge here between the ground and what I think is the pot lug (circle in red)



I hope this help....

 

[Robins]

It seems the piezo is somehow shorted and no sound but just hum.
The piezo is a copper plate then the piezo elements, one per string and then the base plate.
Copper is the positive and the base plate the negative. Voltage is generated by the piezo elements.
A multimeter would have solved the riddle in minutes if done separately on each of the components.
But I have to say it is more complicated than it looks.

All the best,
Robin

 

[dirocyn]

It's gonna get a little more complicated than that. The piezo pad on your saddles isolate your strings from grounding to the saddle which normally isn't a problem because the stop tail piece is the string ground source, but you eliminated the tail piece and opted for string through. With a normal saddle this wouldn't be a problem, just ground the saddle bushing would fix it, but with the piezo's the saddle doesn't ground the strings.
You'll need to ground the strings through the body string ferrules, however you'll have to ground them all or use a one piece ferrule block recessed in to take the place of the individual ferrules. Alternately you could ground one ferrule and install a brass nut to ground the other strings.
However, there is the difficulty getting a wire from the control cavity to the ferrule area.

Grounding the saddle bushing should still be adequate ground. The Fishman power bridge is still mostly metal, it's still electrically connected to the saddle adjust screws and the bridge height screws. The piezo built into the bridge doesn't isolate the saddles from the saddle bushing.



Also, because the saddles and bridge are metal, grounding one ferrule will ground them all--unless you should happen to break the string that's hooked up to that ferule.

 

[Hazard Nova]

These are great suggestions. I will probably start by redoing the piezo wire's connection to the Powerchip. I'll bet I melted the sheathing that separates hot and ground.

 

[retrobob]

Piezoelectricity is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, usually not directly electrical conductive materials. An undersaddle piezo crystal pickups work by being 'deformed' (alternately squashed and stretched microscopically) by string vibrations passed through the cystrals generating a small electric current passed from one side of the crystal to the other. The electricity that's generated by the piezo crystals follows the sting vibration, but generates a different tone with less gain than a magnetic pickup.
Fishman in its instructions #1 states the string ground must be maintained and is not conducted through the piezo saddles.
If only on string ferrule is grounded only one string will be grounded as there is no conductive material between the strings when individual ferrules are used with the piezo bridge. That is why I mentioned a brass nut, it would conduct to all strings, otherwise a ferrule plate would need to be used.

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[retrobob]

In the attached picture if you look closely you can see the isolation between the piezo and the saddles isolating the strings from ground.

 

[Butcher of Strats]

To my knowledge, the piezo power bridge does not run the mag pickups through the piezo pre-amp, it blends them only and allows you to adjust the piezo's output to closely match the mag output.

Right, that is how say a factory would wire those different pickups, one of which requires the preamp and the other which runs passive.
I asked in case there was a mistake in how Hazard hooked them up since he was having problems.
So the piezo goes to the preamp input, while the mag pickup goes to the preamp output when the two are blended.

Not really intuitive if you have a preamp in a guitar, to connect a pickup to the output of the preamp.

 

[Robins]

If the pickups are in any way connected to each other (magnetic to piezo) the piezo is shorted.
That is why a piezo alone works in a guitar but paired with a magnetic you have to make sure they are both isolated from each other.
The piezo goes to the preamp matching the impedance to the magnet (in a way) because the piezo has an insanely high level of impedance.
Then you can add in the magnetic pickup later in the circuit (impedance match).
That is how I understood the wiring and circuit and specs about that whole thing.

You can use the piezo alone in a guitar without the preamp but you have a very weak and wimpy signal depending on the input impedance of the amp - loss of bass and mid frequencies, only shrill treble.

All the best,
Robin

 

[Hazard Nova]

Once again, this is really helpful information. Man I wish I'd had it a month ago! Thanks everyone.

What I'm tempted to do is create my actual guitar body and then apply this information when I transfer the electronics over there. On the other hand I'm really curious to know, if I redo just the attachment of the piezo to the preamp, will that solve the problem...

 

[dirocyn]

In the attached picture if you look closely you can see the isolation between the piezo and the saddles isolating the strings from ground. View attachment 648846

The piezo is isolated from the saddles, yes. It has to be; piezo pickups are fragile and if it were not isolated, adjusting the saddles would destroy it. But the saddles are not isolated from the metal bridge or the bridge posts. Those are all connected together--current will easily pass from the string to saddle, from the saddle to the adjustment screw, from the adjustment screw to the metal bridge, and from the metal bridge to the bridge posts.

And if one string is grounded, the whole bridge is grounded.

Very likely the grounded shielding wire for the piezo's lead also grounds the bridge as a whole. That is not necessarily an adequate ground, but it would be easy enough to test.

 

[retrobob]

The piezo is isolated from the saddles, yes. It has to be; piezo pickups are fragile and if it were not isolated, adjusting the saddles would destroy it. But the saddles are not isolated from the metal bridge or the bridge posts. Those are all connected together--current will easily pass from the string to saddle, from the saddle to the adjustment screw, from the adjustment screw to the metal bridge, and from the metal bridge to the bridge posts.

And if one string is grounded, the whole bridge is grounded.

Very likely the grounded shielding wire for the piezo's lead also grounds the bridge as a whole. That is not necessarily an adequate ground, but it would be easy enough to test.

Not trying to argue or disrespect, however the string rests on the piezo pad and does not touch the saddle at all (piezo is isolated from the saddle), the piezo's are not conductive and therefore do not conduct a ground source to the saddles or the strings.
Since there is no string contact with the bridge saddles metal parts, one string does not ground the others or anything else and why Fishman stresses the original tail piece string ground must be used not the piezo ground to ground the strings.

 

[retrobob]

Once again, this is really helpful information. Man I wish I'd had it a month ago! Thanks everyone.

What I'm tempted to do is create my actual guitar body and then apply this information when I transfer the electronics over there. On the other hand I'm really curious to know, if I redo just the attachment of the piezo to the preamp, will that solve the problem...

No, the strings must be grounded. If you use the ferrule plate and recess it into the body you should be able to use a long drill bit at the correct angle to reach the control cavity (a little risky) and run a wire to the plate area allowing the plate to press against the bare wire to ground it and the strings, this is the easiest way I can see to do it.

 

[Eric_G]

Once again, this is really helpful information. Man I wish I'd had it a month ago! Thanks everyone.

What I'm tempted to do is create my actual guitar body and then apply this information when I transfer the electronics over there. On the other hand I'm really curious to know, if I redo just the attachment of the piezo to the preamp, will that solve the problem...

Anytime I do a pedal, I make sure all my solders point are perfect and clean before even attempting to troubleshoot an issue. Another problem I had in the past was having to much rosin residue on the board, which was creating odd issues.

I now have adopted the habit of clean my soldering with Isopropyl alcohol to remove all rosin left over. Easy to do with a toothbrush. Just be careful of not getting any alcohol in your pots/trimmers....

 

[Eric_G]

Not trying to argue or disrespect, however the string rests on the piezo pad and does not touch the saddle at all (piezo is isolated from the saddle), the piezo's are not conductive and therefore do not conduct a ground source to the saddles or the strings.
Since there is no string contact with the bridge saddles metal parts, one string does not ground the others or anything else and why Fishman stresses the original tail piece string ground must be used not the piezo ground to ground the strings.

I can't argue about right or wrong, but that's really easy to test. String the instrument, use a DMM with one probe on a string and on probe the bridge. If you have continuity, then strings will be grounded by connecting to the bridge post.

 

[Hazard Nova]

I can't argue about right or wrong, but that's really easy to test. String the instrument, use a DMM with one probe on a string and on probe the bridge. If you have continuity, then strings will be grounded by connecting to the bridge post.

Challenge accepted. I'll try and get around to this tonight.

 

[dirocyn]

Not trying to argue or disrespect, however the string rests on the piezo pad and does not touch the saddle at all (piezo is isolated from the saddle), the piezo's are not conductive and therefore do not conduct a ground source to the saddles or the strings.
Since there is no string contact with the bridge saddles metal parts, one string does not ground the others or anything else and why Fishman stresses the original tail piece string ground must be used not the piezo ground to ground the strings.

I'm not trying to disrespect either, you may be seeing something I'm not.

A tune-o-matic could be built using an ordinary under-saddle piezo unit all the way across, but underneath a sturdy (probably metal) shield to protect it from the movement of saddles. I assumed that's how this is built simply because that's what I would try first if I was building a piezo into a TOM bridge.

Possibly instead, this unit has six separate piezo units each built into the saddle. The top of the saddle itself can't be a piezo crystal, those aren't sturdy enough to stand up to string movement and pressure directly upon them. So the piezo crystals would be located under the metallic pads we can see on the top of each saddle--and then connected together by wires. The metal pads on the top of the saddle sure look like they're touching the saddle, and I assume they have continuity with it (and hence to the rest of the bridge). As @Eric_G mentioned that's easy enough to test.

 

[tubedude]

It might be the image but the piezo connection to the jack looks sketchy. I'd retouch that.

 

[Eric_G]

Challenge accepted. I'll try and get around to this tonight.

Sounds good...if you're going to test, might as well do a complete test to answer all your questions.

Set you DMM to continuity and test:

- String to bridge post (Continuity would mean you can ground your strings by connecting to the post)
- If positive above, I would test string to output jack ground and see if your strings are grounded through the piezo ground. (I think I remember you mentioning that was an assumption you had).