Can a bad pot cause ground hum?

goran · #1 05-20-2010, 03:32 AM

I have a strange problem with my Skyline Bob Glaub:
It hums until I touch any metal part on the bass. My first thought was: it is not grounded and maybe there is a loose wire that I can identify by eye. I inspected it closely, but everything seemed fine.
I took bass to my friend that is handy with soldering iron. We checked all connections, made sure that bridge is grounded. We even replaced the stock Fralin pickup with Curtis Novak one (which, to me, sounds even better than Fralin) and it still hummed. Only thing we haven't done yet is connecting pickup directly to the output to bypass the pots.

We'll take another look next week, but in the meantime I was wondering if any of you has had similar problem. Can a bad pot cause bass to hum like crazy?

Thanks, Goran

PS
@Moderators: if you think this should fall uner some other category, please move it. Thanks!

Chasarms · #2 05-20-2010, 08:23 AM

P-basses usually are not noisy.

Typically, if the noise stops when you touch the strings, it is grounding issue. Ground noise is usually because one part of the circuit has a much better path to ground than another.

You may want to try a "star" ground that patches all the ground points to a single point.

**line6man** · #3 05-20-2010, 08:27 AM

A pot is merely a resistive band with a wiper across it to vary the resistance. It cannot cause such issues as 60Hz hum.

You might consider a proper shielding job, if you haven't done so already.

Quote:

Originally Posted by Chasarms

You may want to try a "star" ground that patches all the ground points to a single point.

That's nonsense. You can't get a significant ground loop in a bass. You are just adding more wire to the bass.

okcrum · #4 05-20-2010, 08:38 AM

Have you checked that the jack is not wired backwards?

**line6man** · #5 05-20-2010, 08:45 AM

Quote:

Originally Posted by okcrum

Have you checked that the jack is not wired backwards?

If the jack were wired backwards, the noise would increase when he touches any metal part, not decrease.

goran · #6 05-20-2010, 08:46 AM

Thanks for the input, everyone!

Quote:

Originally Posted by okcrum

Have you checked that the jack is not wired backwards?

No, we haven't done that, but it makes sense.

I planned to do a full shielding if everything else fails, but yes, it is a P-bass with a simplest electronics possible - it shouldn't hum.

4001 · #7 05-20-2010, 08:46 AM

Bad pot dosen't even give a 'buzz'.

okcrum · #8 05-20-2010, 09:34 AM

Quote:

Originally Posted by line6man

If the jack were wired backwards, the noise would increase when he touches any metal part, not decrease.

Not necessarily in a relatively high impedance circuit like this. Something here's affecting hum when OP touches any part. If he lowers the impedance to ground a lot when he touches something, the hum decreases. So we most likely have a bad connection, or one that's wired incorrectly.

goran · #9 06-14-2010, 06:13 AM

Problem is solved - hum/buzz, whatever you want to call it is gone now. My tech did a great job with this bass - a star scheme (although not 100% star - PM for details) seems to do do the trick. Bass is now 100% quiet!