Mid-70's Big Muff Pi Problem

[stratocaster68]

I pulled this out for the first time in about 3 years. The volume controls works fine but scratchy. The real issue is the Tone control. It does not seem to make much difference in the sound when the whole way up or down. Could this be a loose wire, failed component. I'm quite capable with a soldering iron so that won't be a problem.

Also, it's been so long I had forgotten the characteristics of the Sustain control. Seem to work more like a volume control than affect the sustain. The whole way counter-clockwise and I get no sound. The other way the sound is fine. Just trying to jog my memory.

Thanks!

[soundguruman]

Right now just clean the controls with some stuff like deoxit D 100.
Seems like a lot of the issue may be dirt. Then test again.
There may actually be some part non working, but get the controls functioning before you decide that.

[stratocaster68]

Right now just clean the controls with some stuff like deoxit D 100.
Seems like a lot of the issue may be dirt. Then test again.
There may actually be some part non working, but get the controls functioning before you decide that.

Thanks soundguruman! I'll give it a try. What's odd it that it worked fine about 3-4 years ago. But again, it's pretty old.

[J M Fahey]

What's odd it that it worked fine about 3-4 years ago.

So ?

Sh*t happens !!

[jrfrond]

Yeah, cleaning is the first step. The controls on the BMP have radical extremes, so if you cannot hear them working, there is definitely something wrong. I know it was in a closet for 4 years and worked before hand, but electronics doesn't age like fine wine.

[Enzo]

Old guy's observation:

Everything works until it doesn't.

[jrfrond]

Old guy's observation:

Everything works until it doesn't.

Exactly! A tangent to that is: "Everything is good, until it breaks".

[soundguruman]

Yeah, cleaning is the first step. The controls on the BMP have radical extremes, so if you cannot hear them working, there is definitely something wrong. I know it was in a closet for 4 years and worked before hand, but electronics doesn't age like fine wine.

Yup, pretty much "anything" you can clean first, dirt is the #1 cause of problems, then test again.

[Mark Hammer]

1) The Sustain control IS a volume control; it controls the level of the signal hitting the clipping stages. Ideally, there is a small-value fixed resistor between ground and the ground leg of that control so that you can never turn the "volume" completely off.

2) The Tone control pans between two simple filter sections: one which passes primarily mids and highs, and the other which passes primarily low mids and bottom. Since the entire signal is coming out of the wiper of that pot, at least two of the solder lugs/connections to that pot MUST work for you to be able to hear anything at all. If the issue with the pot is that one of the outside lugs is iffy, then you would hear either a thin nasal sound OR a boomy mushy sound, instead of the stock variable combination of the two.

If that is the case, then you can either replace the pot with another 100kB linear taper pot (B=linear), or you can attempt to "rehabilitate" the existing pot. If you are able to, de-solder the pot and remove it from the pedal, making notes about what gets reconnected to where. Gently pry up the tabs that secure the back of the pot to the part with the contacts and resistive strip, and remove the back. Using either some alcohol or a more appropriate contact cleaner, clean the gunk off the resistive strip with a suitable cotton-tipped applicator. Gently pinch the rivets that secure the solder lugs to the resistive strip with some needle-nose pliers (removing the back will let you get at them more easily), replace the back, and pinch the tabs shut again. Re-solder the pot into place.

[guitician]

1) Using either some alcohol or a more appropriate contact cleaner, clean the gunk off the resistive strip with a suitable cotton-tipped applicator.

I used to think that alcohol was a good cleaner for pots until I saw what it does to rubber some time after you clean it. It will completely dissolve the stuff, making it into a goo. Since the carbon film trace most likely uses some kind of binder which may be dissolved in a similar way, I never use alcohol on any films of unknown composition.