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 Forum index » DIY Hardware and Software » The Repair Shop
Hum problem on active bass
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Uncle Krunkus
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PostPosted: Thu May 09, 2013 7:58 pm    Post subject: Hum problem on active bass
Subject description: Any ideas?
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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.

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diablojoy



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PostPosted: Fri May 10, 2013 2:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
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.


Hi uncle
i tried that once too Laughing wasnt great.
Stewart Mac has some self adhesive conductive tape for shielding which is really good. If you are shielding the cavity dont run a string ground

Quote:
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.

Haha just get me to wind one for you . but probably not the issue

Quote:
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)

yeah that could be it , at least i wouldnt be surprised if it was

one thing that occured to me is on active systems the control pots used are generally around 25K not the more usual 250K/500K but it depends on the system used, was the preamp original or an add on ? have the pots ever been changed ?

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Uncle Krunkus
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PostPosted: Fri May 10, 2013 3:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

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

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diablojoy



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PostPosted: Mon May 13, 2013 3:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

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

Its unlikely to fix the noise issue
Its more of a safety thing ,I prefer to shield cavities properly than have a string ground . with a string ground its very likely in the event of an earthing fault the player may actually become part of the earthing system
say grabbing a mike whilst holding the neck of the instrument,
if there is any potential differance between the two the player becomes
a conductor and sometimes as has happened a very dead conductor.
just better to avoid string grounds whenever possible.

I would be more inclined towards your suspicion of the active circuitry
potting going bad. hmm is there any exposed metal on the active circuitry casing ? if so maybe try earthing that back to the jack ground.
A picture or 2 to might be good.

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DES



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PostPosted: Mon May 13, 2013 8:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

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

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Wobuse



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PostPosted: Thu May 16, 2013 11:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

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 .

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Uncle Krunkus
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PostPosted: Fri May 17, 2013 3:32 am    Post subject: Reply with quote  Mark this post and the followings unread

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



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PostPosted: Sat May 18, 2013 12:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

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.

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Visit my studio ; LimeStoneLabs at: www.Facebook.com/LSlabs
Hear some of my sound track work ,on the web series ,, www.cafegangster.com/16
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z3r0k3lv1n



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PostPosted: Fri Sep 27, 2013 9:23 pm    Post subject: Re: Hum problem on active bass
Subject description: Any ideas?
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Uncle Krunkus wrote:
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.



man, PLEASE, can you send me those wiring diagrams? I REALLY need this.

if you can, please send to my e-mail, its: cidinhodc(at)hotmail(dot)com
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