Hum problem on active bass

Uncle Krunkus

I've got a bass with an active pre-amp which has a nasty hum problem.
The pre-amp module is a "Ming Shi #9020 2BAND EQ (T.B) Made in China"
I found the wiring diagram for it and re-wired the whole thing.
I built a faraday cage out of thin brass sheet to completely surround the pot-bath. I even added shielding braid to cover the wires right up to the pickups.
Still,..... no joy.
Now, it could just be particularly noisy, nasty, Chinese pickups being amplified. But I don't know how you could make a pickup so poorly as to create this much hum.
I've just noticed that if I touch the potting mix in the back of the pre-amp module, (and nothing else) the noise goes way up, even more. This doesn't make sense to me, unless the potting mixture has become conductive over time. (like the problem with the voice modules in a Roland MKS-30, and other synths)
Does anyone else think this is likely, or have I missed something?
Any help would be greatly appreciated.

diablojoy · Posted: Fri May 10, 2013 2:26 am Post subject:

Uncle Krunkus · Posted: Fri May 10, 2013 3:13 pm Post subject:

Thanks for the response diablojoy.
Sometimes it's inspiring just to know that someone else is thinking about the problem. Smile

I replaced the treble pot, which was obviously losing contact with the wipers, and made sure it was the value recommend by the wiring diagram, but I'll double check the others. That's a good call.

The string ground is still in place, so I suppose I should lose that first?

The owner is happy to get rid of the pre-amp, and go to a passive setup, if that fixes the problem. But that would be a shame if it's not warranted.

So,... I'll get rid of the string ground, and see what happens.

Thanks heaps again for the collaboration. Cool

I'll let you know how it goes.

Andrew

diablojoy · Posted: Mon May 13, 2013 3:46 pm Post subject:

DES · Posted: Mon May 13, 2013 8:21 pm Post subject:

Have you completely ruled out the jack? A lot of inexpensive basses with active pickups use Switchcraft style clone jacks....usually used for end pin jacks in acoustics, and the internal contacts tend to break very easily. Typically the jack will be a stereo jack with the 'ring' contact providing the switch to turn the electronics on. This is done by connecting the negative lead (or common) to ground when a guitar plug is inserted into the jack. Make sure the jack is wired properly too....I've seen them wired incorrectly as well. Try bypassing it completely if you're not sure and want to rule it out....

Here's a link to Switchcrafts site and the 'real' items:

http://www.switchcraft.com/productsummary.aspx?Parent=884

Wobuse · Joined: Dec 07, 2011 Posts: 34 Location: Lyons,il

Kind of a long shot here ,but try turning your soldering iron off.
I was building a spring reverb,and testing all it's sub circuits in the chassis ,on top of my work bench .i spent hours reworking the ground scheme ; trying to get rid of a horrendous hum. Finally figured out that my adjustable solder station emits a prodigious EMF field. Turned it off & it was all quiet .

Uncle Krunkus · Posted: Fri May 17, 2013 3:32 am Post subject:

Yeah, but this is noisy anyway. Way before I turned the soldering iron on.

Wobuse · Joined: Dec 07, 2011 Posts: 34 Location: Lyons,il

Well ... Just for yukes , try puting an insulator over the potting ( like a chunk of plastic from a milk jug) , then wrap the thing in a sheild ( like foil or metal tape)
Try it grounded & un grounded .
That might help.

z3r0k3lv1n · Joined: Sep 27, 2013 Posts: 1 Location: Brasil