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Acoustic-Electric Hum & Crackle Help

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Duncan McLennan:
Let me start by saying that I am by no means a guitar player. I'm a drummer first, and just play a little guitar on the side.

I have an acoustic electric that's been sitting in its case under my couch for months, never really played. It's a Seagull, and I got it cheap because it was a prototype model.  It has an active preamp (labeled Quantum HS1), a piezo pickup in the bridge, and a single coil clamped across the sound hole.

I needed it for a gig, so I pulled it out, cleaned it up, new strings, oiled the finger board, etc., and when I went to plug it in I noticed it has a significant 60 cycle hum that I don't remember being there.  When I touch the connector shell, the metal control panel, the metal pin (output jack) it goes away.

I have been reading about internal shielding and how it is important in an acoustic electric. While I had the strings off I was digging around inside to reattach the jack, which had come loose.

I've fiddled with it all evening now, and can't get the hum to go away. Solder joints look good as far as I can tell. And now it's starting popping and crackling, but with no apparent pattern. It's not like distortion, only when I play. If I set it down and don't touch it, it will randomly start with a mild 'bacon' sound.

The only other thing that changed, was that I put a little dab of white glue on the bottom of the piezo pickup, because the saddle in this guitar is a hair loose, and it was wiggling around. My reading has taught me that you shouldn't glue a pickup in like that (I was too stupid to look this up before, I guess), but could a little glue on the pickup really make it hum like that? Of course the other problem is that I can't get the damn thing out to look at it now that it's making noise.

Any suggestions on how I might fix it very much appreciated.

Walter W Wright:

--- Quote from: Duncan McLennan on February 25, 2011, 08:00:07 PM ---...I put a little dab of white glue on the bottom of the piezo pickup, because the saddle in this guitar is a hair loose, and it was wiggling around. My reading has taught me that you shouldn't glue a pickup in like that (I was too stupid to look this up before, I guess)...

--- End quote ---
yeah, that was not a good idea.

the piezo element could have damaged shielding, or it may just not be getting proper string pressure off the saddle because of the glue. either one could cause weird hums or noise, but the fact that the noise goes away when touching the output cable points towards a bad piezo shield.

you can "fake" a fix by tying a bit of wire (like an old guitar string) around the output jack, then threading it under the strings behind the saddle, so your ground strap contacts the strings between the saddle and the bridge pins. this way, when you're playing, your hands touching the strings will make the hum go away.

Lee Patzius:
If touching any metal part of the guitar makes it go quieter, then that leads me to believe your internal shield wiring may still be good, as it appears that "something" is connected to those parts.

The sheild wiring between the metal parts, and your amplifier chassis my be broken, or not even sheilded at all.

Did you do any resoldering? Is it possible that when you messed with the output jack, you may have reversed the polarity of the output jack?

If all is wired good internally, then it sounds like you have a broken shield in the guitar cable, or no shield at all!

Or, you're using an unsheilded speaker cable. Are you sure you're not using an unshielded speaker cable? That would be noisy as hell.

My first line of troubleshooting would have been to touch the amplifier chassis, to see if the noise went away also. If touching the cable shell at the guitar is quiet, but touching the amplifier chassis is no change, then you got a problem on the shield wiring  between the guitar and amp.
Also, keep the cable clear and far away from stomp box power supplies. They are noisy as hell too.

Duncan McLennan:
I have a feeling I've screwed up the piezo pickup. I took it to a gig and managed to gather some more info, but frankly it's confusing to me. I'm not an electronics expert, but I've done my share of repairs on stuff like this and some things just don't seem to be making sense.

The guitar has two pickups, a piezo under the saddle, and a magnetic pickup in the sound hole. The 60 cycle hum is continuous whether or not I pan to the bridge or magnetic pickup. As I turn down the master volume, the hum gets quieter. The random electrical sizzle/popping noise is also in both channels, but stays at the same volume no matter how loud the master, until it's off, then it's gone!  The piezo pickup has also started with what sounds like clipping, but only when I play on that pickup, and relative to the volume and how hard I'm playing.

So the best I van figure, basically every part of the electronics in the guitar has something wrong with it... and all the problems manifested themselves simultaneously.

I think the best course of action at the moment is just to pull all the electronics, redo the solder joints, and see if I can't the the preamp working quietly.  Then try each pickup on its own.

Is there anything special I should know about working on the wiring in the body? I suppose I can't make it much worse than it is already.

Lee Patzius:
If you could open the back plate and upload a picture, that would help, especially near the output jack.

Speaking of output jacks with active pickups, your output jack most likely acts as a switch, in which they commonly use the sleeve part of a normal 2 wire 1/4" TS guitar cord, that completes the battery circuit, energizing the active pickup.

If you use a 3 wire TRS instrument cord, low impedance power to the active pickup will be go high, from being broken, at the guitar's output jack, and depending on the needs of the active pickup, it may not work properly.  Been there done that!

Since you mentioned nothing about your guitar cord, I'll say it again, make sure you're using a 2-wire 1/4" TS SHIELDED cord.

Of course it could still be a broken signal wire anywhere else in the chain, or even your peizo element, or active pickup, but without pics, and/or testing continuity and shielding of the guitar cord, output jack and guitar electronics, I'm leaning toward the shielded side of the wiring, or a possible disconnect, or even a short, at the jack, for now. It's usually the part that takes the most mechanical abuse, and you mentioned your output jack was loose.


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