Noise and weird sound coming out of USA big muff

[joelap]

Hey all, someone is sending me their USA big muff for me to hopefully fix up for them.  He told me the noise is excessive, and wants to see if I can get it to hiss less, and he also described a weird problem with the pedal:

"I've Got a usa bigmuff, and a russian bigmuff. And Of the two I use the russian WAY more, the usa one just sounds horrid, and Its got alot of Noise issues. If the distortion control is turned up past 9 o'clock ya start getting this insanely thick hum/buzz. And the weird thing about the hum/buzz is If you place your finger on the string (without actually playing anything) And just sorta wiggle the string, the Hum/buzz oscilates and The volume and pitch of it Changes. That wouldn't really be a big deal, except If ya turn the distortion control up too High when you do palm mutes and stuff The little big of wiggle your hand creates during the mute can cause the pedal to sorta Act like a tremolo or something. It Makes the volume drop and rise. Its really weird. And basically makes the pedal seem useless to me."

Any ideas?  I was thinking of starting off first by replacing any electrolytics for poly films, and maybe changing some resistors to metal films if I had them.  The weird sound he described, I was thinking of replacing the transistors with something else, but I dont have a big muff schematic on hand... which transistors are equivalent with the USA circuit?

I'm doing it for the guy for free just for the experience of being inside a big muff, and in return he told me I can mod the bejeezus out of it.  So I figured, hey there's two weeks of play for free inside this thing.  So any forewarnings or advice from people who have done this before, I'd appreciate the direction.

Joe

[Meanderthal]

 Watch out for those hookup wires, they just up and fall apart. It sounds like he is suffering from excessive mud for one thing(common complaint), so replacing all the signal path 1uf decoupling caps with .1uf would be a good idea. MPSA18 are good high gain low noise, 2N5088, 2N5089, 2N5133, even higher gain 2N3904 will work well, yes film caps might help, but try the pencil test for a microphonic ceramic cap first, and go for true bypass if it's not already.

[GibsonGM]

2N5089's might offer less hiss. Maybe use a heatsink when you put them in so they won't get damaged.  I'm with Meanderthal, definitely pencil test caps, could have something wrong in there adding to that noise.  The "trem-string" thing could be related to some kind of problem with grounding (as could be excessive hiss and strange symptoms). The hand is acting like a ground or a capacitor, it appears.   Make sure the input/output jacks have good ground connections, that the board is clean/free of beer, etc., and that offboard wiring isn't falling apart and causing all those problems!  Pulldown resistors should be checked.
My 2c... 

[joelap]

Many thanks guys!  I've got some MPSA18's here, and was about to place an order with Aron for some 2N5089's.  I would be socketing these in, so I can mix and match.  Thanks for the info about the hookup wires.  I have a ton of .1 box caps looking for a home thats for sure.

Is there a link to this pencil test?  I havent heard of this before.

[brett]

Turn it on, turn it up and hit random components with a pencil.
If they make a noise via the amp, replace them.

[Yun]

yes, mr.Meanderthal has got that right!  Mr.Pete explained it as "Cheese wire".  when i do mods for the USA BMP, when i disconnect a wire, i automatically replace the WHOLE wire with decent grade wire. 

Check around the power supply "circuit" .  Some usa bmp's have an electrolitic capacitor from the pcb, and EH doesn't solder very well, so, check that first thing, man. 

I've had that problem with 2 usa BMPs so far.  It was solder missing on the traces of numerous solder pads on the PCB!  In other words, it's like that they didn't solder some components to the board.  so- check out that too. 

Another good trick to use when BMP's have the "cycle hissy-hum" is to raise the value from the sustain pot to ground.  You'll looses some gain/sustain, but if you search my posts about the "creammy dreamer"  and my "Super-muff"; you'll get it back plus more, man....