Q-switch humming?

[edwardofhuncote]

Just noticed today, the 3-position Q-switch on my Persuader hums a very low pitch (about 60hz) when I touch it and only if I touch it. If I touch the instrument cable too, it goes away.  
 
I can also dial it up or down with the filter. All the way counterclockwise makes it really loud and all the way clockwise makes it nearly inaudible.  
 
Just finished shielding the control cavity a short while back...

[terryc]

Switch not grounded to cavity I would guess but I would check all the electronics to make sure it is all earthed to the cavity.
To eliminate anything else, check amp or plug into another one, cable, power cord etc.
Good luck

[edwardofhuncote]

Thanks terryc. Reading more last night, the trouble is similar to what was going on here:  
 
http://club.alembic.com/Images/394/128932.html?1336003295
 
Lots of good advice but there never was a follow-up.

[edwardofhuncote]

I messed with this a little more last night... I removed the control plate, and checked to make sure all the pots were firmly secured in place inside the cavity. Noticed that if I touch any of the other pots on the inside while touching the switch from the outside, the hum disappears. I think maybe it is an earthing problem.  
 
(getting my dunce hat ready here...)  
 
Could this be my homemade shielding paint failing? I read somewhere metallic enamel and graphite made a good shielding paint. It's possible this hum has been here all along, and I've only noticed it recently, since I very seldom run the filter all the way to the bottom, and attenuate the Q-switch, but I got in this really awesome blues jam the other day... and well, there it was, corrupting my rumble groove.

[keith_h]

My suspicion would be the paint. The paint itself acts as an insulator and will inhibit the graphite particles from touching each other and creating a good shield. If you don't have access to a professional paint I would recommend using copper tape. There are different types. Some have a conductive adhesive and can be expensive while others don't and would require soldering of the joints to get good contact. If you go the tape route you need to make sure it has a continuous edge in contact with the back plate. If not you can end up with leakage.    
 
Keith

[edwardofhuncote]

I figured. I went ahead and ordered a small can of shielding paint from Stewart-MacDonald this afternoon... when it comes, I'll do what I should've done in the first place. (told you guys I had a dunce hat)  =)
 
Will update, but Thanks in advance fellas.

[terryc]

edward..you will have to prepare the surface prior to applying the proper paint to make sure there is a good key and coating.Use a small piece of Scotchnrite abrasive to key the surface.
I would give it 2-3 coats to make sure it is an even surface

[edwardofhuncote]

Well, I had a couple evenings this week to work on this...   After (very carefully) removing the electronics from the cavity and masking around the edges, I used some lacquer thinner and small brass bristle brush to strip all the original shielding plus my homemade coat. Then, I prepped the surface for the new paint (hat tip to terryc) by lightly scoring it. I'll put the 3rd and probably final coat on both the cavity and cover this evening, and hopefully re-assemble the hum-free Persuader tomorrow night. (I have a Sunday afternoon gig I'd *really* like to play this bass for!)  Here's work in progress:    

 

[flpete1uw]

Greg,
  Pardon me for my potential ignorance on this but is the black shown a primer? Every Alembic Bass I've seen,which have only been mine,   will have a silver finish to it.
  I'm impressed with your tenacity with this. I had similar tenacity issues with my Series. It's worth it.
Do not give in!!!!
Peace,
Pete
 
(Message edited by flpete1uw on October 03, 2014)

[edwardofhuncote]

Not at all. It's Stewart-MacDonald's shielding paint... just came in carbon black instead of silver. This bass of course still had most of it's silver shielding from the Mothership, but after I finished plugging, moving, and re-drilling the holes for proper placement of the new electronics, I needed to re-shield. Since shielding paint is $35 a half-pint, well... I got cheap, errr... creative. :hiding:  
 
And yes... tenacity is right. (you might remember) I got burned pretty good when I bought this bass from an eBayer, either twice again as ignorant as me, or diabolically dishonest. Long story short, I opened the case to see my first Alembic, only to find it had been stripped of it's original pickups and electronics, and adding insult to injury, I later found whoever did it made a hash of the control cavity.  
 
 I didn't know any better at the time, had never even heard of the Alembic Club. Thanks to a lot of advice and encouragement here, and the Head Elf working her Magic, I decided to restore this old Persuader rather than swallow my loss and bail out.  
 
So yeah, I'm tenaciously still trying to put right the wrong. I think enough of this bass that I can't imagine having it *not* be right.

[edwardofhuncote]

Shielding is complete and after reassembly the bass is dead quiet. Thanks All!

[flpete1uw]

Greg,
Congratulations! After getting the bugs out you are now free to play without second guessing what you are doing and be free to explore!  
 Rock On!  
Pete

[keith_h]

Congrats .. Glad to hear it is up and running.  
 
Keith

[artswork99]

Nice! Great job!

[terryc]

Glad it all worked out