Fender Keyboard Amp Buzzing

[dpm309]

This must be the week for solid state amp problems for me. Pulled the chassis on my  bandmate's Fender Keyboard 200 to clean the pots and when I put it back in, I am getting a loud buzzing (120Hz) sound. There are 3 sets of speaker wires (12" woofer, 10", and tweeter) that I reconnected after I tried cleaning the pots. Fired it up and noticed the loud buzz. When I reach in and hold all 3 sets of wires together, the buzz disappears. The sound also goes away when I touch or move the tweeter wires around. As soon as I release the wires from my hand, the sound comes back. Could this be some kind of grounding problem caused by a break in one of the wires? Getting sound from all of the speakers. Also it appeared that all 3 sets of speaker wires were twisted together when I removed the chassis.Any ideas? I have attached the schematic.

[shooter]

speaker wires were twisted together when I removed the chassis

did you straighten them out?
IF so twist 'em back up, maybe even take a piece of wire make a dozen or so loose wraps around the bundle and ground ONE end (use insulated wire so no more gremlins get in:)
the schematic is to crappy to view, I'd suspect ANY ribbon cable you had to mess with
I'd agree, you let ground gremlins in

[dpm309]

Yeah, I straightened them out when I pulled the chassis. Will twist them back together and see if that helps. I chop-sticked all of the ribbon cables and other components without any change in the buzzing noise so it has to be the wires. Will also check the continuity to make sure I didn't break a wire.

[dpm309]

Checked all of the wires for breaks, etc and twisted them back together and am still getting the hum/buzz.  When I hold the black/white pair that goes to the bottom speaker, the hum completely disappears. As soon as I let go, it comes back. I am wearing shoes so I am not acting as a ground. Cleaned all of the jacks in case there was in issue there with no luck. Any ideas?

[shooter]

wasting time, but maybe;
If you have a dummyload R, "replace" speaker with DL

my guess, since the speaker signal is BIG, something's picking it up, when you grab the wires you're "decoupling" the induced signal.
try and isolate them, wrap them with insulated wire and ground one end

[sluckey]

This must be the week for solid state amp problems for me. Pulled the chassis on my  bandmate's Fender Keyboard 200 to clean the pots and when I put it back in, I am getting a loud buzzing (120Hz) sound.

Surely you checked it before you fixed it. And?

[dpm309]

From what I remember jamming at his house, it did have a slight buzz that you could barely hear. The buzzing disappears by touching the bottom speaker wires only. No effect on the buzzing level when I touch the wires on the middle speaker and tweeter. Will try hooking up the bottom speaker to my dummy load and see what happens.

[sluckey]

Sounds like something has happened since you touched it. Maybe a direct hit. Maybe accidental collateral damage. Retrace your steps to see if any clues pop up.

[dpm309]

See picture. Started playing around with a grounded insulated wire etc. but this arrangement completely eliminated the buzzing. Wrapped some insulated 18 GA stranded wire  like a coil near the speaker connection. Connected a jumper cable to the strands of the wire and first connected the other end to the ground with no reduction in buzz. I then disconnected the jumper from the ground and laid it on my bench, no more buzzing. If I disconnect the jumper from the insulated wire, the buzzing returns. Must have created some kind of coil which is blocking whatever interference was causing the buzz. Go figure!

[shooter]

will that test "repeat" with any speaker

[dpm309]

Swapped the 10" and 12" speaker leads and the buzzing returned.  Swapped them back, no buzzing. Spliced another 24" of insulated wire to the one wrapped around the speaker leads and will reinstall everything back into the cabinet and hope for the best. Wondering if there is something with the 10" (bottom) speaker that is picking up interference. I have attached a better schematic of this amp.

[shooter]

Wondering if there is something with the 10" (bottom) speaker that is picking up interference.

the schematic is MUCH better
the 10" is acting as a horn/tweeter, there's a crossover network attached to it, maybe something there 

[dpm309]

Could be.  Reassembled the amp and it is working great with no buzzing noise. Need to stick with tube amps in the future.