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Urgent - Help with a piezo bridge

Discussion in 'Pickups & Electronics [BG]' started by eukatheude, Jul 19, 2013.

  1. Jul 19, 2013 #1
    eukatheude

    eukatheude

    Apr 2, 2012
    italy, pordenone
    I'm fixing this guitar for a friend of mine. It has active pickups, a Fishman piezo bridge with the Fishman preamp. Some other guy wired the battery ground and the pre output to the ring lug of the output jack, so the battery was discharging even when the guitar was unplugged.
    At first i erroneously put the piezo output on the sleeve, thinking it was a ground; i tried in other ways (both bare and battery ground to the sleeve, pup output to the tip and piezo to the ring and viceversa etc) but in the end the result is the same, even after putting it back to the old configuration: the piezo doesn't work.
    And what's worse i can find schematics for the fishman, but they have nothing to do with what's on the guitar, and i can't find the guitar's diagrams. If there's any knowledgeable folk out there who's willing to lend a hand i'd gladly take it. :D
    I should note that i disassembled the bridge to see how it was made; but after putting it back together everything seems in order, as there's continuity between the saddles and the piezo input on the preamp.
    One weird thing: when testing the output jack, i plugged a mono jack in; i got continuity on ring and sleeve, but on the tip instead i got resistance. What could it be? Just bad soldering?
     
  2. Jul 19, 2013 #2
    ctmullins

    ctmullins Dominated Gold Supporting Member

    Apr 18, 2008
    MS Gulf Coast
    I'm highly opinionated and extremely self-assured
    Don't know much about the Fishman, but basic troubleshooting suggests bypassing the preamp and running the piezo straight to the jack, just to verify functionality. Once you've verified that, you need a way to test the preamp - another piezo if you have it, or you might go to Radio Shack and buy one of their cheap buzzers and cannibalize it for testing.
     
  3. Jul 19, 2013 #3
    eukatheude

    eukatheude

    Apr 2, 2012
    italy, pordenone
    Thanks for your answer. The preamp is a little pcb silicon mounted to a pot, it's filled with surface mounted resistors so i want to touch it the least possible. It should be working because the pups go through it and sound normal. What i don't understand is why it just stopped functioning like that. I will try going to the tip with the piezo output.
    I should mention the wiring is a little weird: there's a 3 way switch going to the pre, but rather than switching the two inputs before going into the pre, the piezo goes straight there, and the pups are connected to one of the switch lugs and also directly to the pre. The rest is ground. That would be something i'd look onto, except it worked fine before i switched those wires.
     
  4. Jul 19, 2013 #4
    walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    the fishman "powerchip" is the pre you have there.

    pickups go through their own switching and controls, then to the powerchip;

    piezo goes straight to the powerchip (hot and ground)

    9V goes to the red wires of both pickups and powerchip...hmm; here's where it gets tricky, getting that 9V to both pickups and preamp;

    i think this is the way: snip off the battery clip right at the head (so all the wire is still attached to the powerchip); splice all the red wires from the pickups, the pre, and the red of a new battery clip together; splice the black of the new clip to the black wire from the old clip.

    i suppose you could keep the original clip, cut just the red wire in the middle somewhere, and re-splice both ends with the reds from the pickups, but i dunno if you'd have enough wire length.

    finally, power chip has i wanna say 4 wires going to the special 9-pin output jack.
     
  5. Jul 20, 2013 #5
    eukatheude

    eukatheude

    Apr 2, 2012
    italy, pordenone
    Unfortunately the setup here is different, there isn't a 9 pin jack but a stereo.
     
  6. Jul 21, 2013 #6
    eukatheude

    eukatheude

    Apr 2, 2012
    italy, pordenone
    Great, now the pot lugs got broken. I'm trying to desolder the broken tips but the solder pads just won't melt, while the epoxy around them boils.
     
  7. Jul 21, 2013 #7
    walterw

    walterw Supportive Fender Commercial User

    Feb 20, 2009
    alpha-music.com
    you've got too weak an iron, and/or no fresh solder to tin the iron tip!

    you want to run a little fresh solder into the old to "wake it up" so it melts, and you want a strong enough iron to melt it quick before wrecking the circuitcard.

    you also need a solder-sucker to get the old stuff out of the holes in the circuitcard.
     
  8. Jul 22, 2013 #8
    eukatheude

    eukatheude

    Apr 2, 2012
    italy, pordenone
    The iron is a 75W. I have a solder sucker. I'll try adding some solder when I get back from work. I'd try desoldering from the other side of the board but it's a wirey mess.