Fishman Preamp malfunction

mje · #1 02-05-2007, 07:57 AM

This is more of a BG than a DB question, but (1) it's applicable to DB electronics and (2) when I tried asking on the BG side, I found myself in a crowd of clueless thrashers, one of whom suggested I should ask Ken Smith ;-) So this is directed to fdeck, DURB and our other electronics mavens.

I bought an Olympia acoustic BG with a Fishman Classic 4 preamp a week ago that exhibits a couple of problems:

1. Hum, which goes away when you grab the shield. There's a good signal along with the hum, but I don't know if there's enough gain.

2. When inserting the cord into the endpin jack, as soon as the barrel contacts the ring connection, completing the battery circuit, the power LED flashes and goes out. Removing the cord and reinserting repeats this. I pulled the jack and manually shorted the contacts and got the same effect, so it's not a jack malfunction.

Pulling the preamp out, the wiring appears to be all intact. No visible shorts where the coax attaches.

I don't know what the tolopolgy of the Fishman preamp is, but I'm guessing this might be a shorted cap. One thought was to check the current draw on the battery, which is a bit difficult but can be done.

Any ideas?

(If you're wondering why I bought a BG with electronics problems: It was only $100 with the HSC- how could I resist? That, and the seller didn't tell me about it. He did say "I only play it acoustically" which suggests in retrospect that he knew damn well about it, but it was still a good deal)

robgrow · #2 02-05-2007, 09:18 AM

I'd suspect the jack. Can you temporarily connect a known, good-quality, simple 2-conductor jack as a test? (and bypass the battery switching function completely for the time being.) I could be way off, but, in my experience, mechanical problems are at least as if not more likely than an electronics circuitry problem in a situation like this.

Good luck.

bloodyjack · #3 02-05-2007, 11:11 AM

I am not familiar with that fishman pre-amp but my platinum pro pre-amp also flashes the power LED on jack insertion and then goes out, it comes back on when its time to change the battery. This is a current saving feature it is meant to do that. I just looked on there web site and there current on-board stuff does this too.
So do not discount a bad jack because of the LED.

mje · #4 02-06-2007, 11:24 AM

Quote:

Originally Posted by robgrow

I'd suspect the jack.

Already been ruled out. See above.

mje · #5 02-06-2007, 11:26 AM

Quote:

Originally Posted by bloodyjack

I am not familiar with that fishman pre-amp but my platinum pro pre-amp also flashes the power LED on jack insertion and then goes out, it comes back on when its time to change the battery. This is a current saving feature it is meant to do that. I just looked on there web site and there current on-board stuff does this too.
So do not discount a bad jack because of the LED.

Excellent- didn't know that. So the hum problem isn't related to the LED flashing. That should make things much easier to diagnose. It may be just inadequate shielding.

winnets · #6 02-07-2007, 06:06 AM

Quote:

Originally Posted by mje

Excellent- didn't know that. So the hum problem isn't related to the LED flashing. That should make things much easier to diagnose. It may be just inadequate shielding.

Are the strings grounded at all? that would take away the hum as long as you were touching them. It could be a bad solder joint somewhere along the line too.

mje · #7 02-07-2007, 07:24 AM

Quote:

Originally Posted by winnets

Are the strings grounded at all?

No. They usually don't ground the strings on acoustic BGs.