Buzz with guitar volume off (TB Mk1.5/GeFF)

[Spexicola]

I've just completed a Tone Bender Mk1.5 build, and it sounds great, the only problem is that when I turn the guitar volume off to kill the signal, I get a nasty buzz. It's hard for me to tell if it's 60 or 120 by ear. It goes away immediately when I nudge the volume up just a bit. I've tried multiple cables and guitars.

Any ideas? I'm pretty new to this, so I'm likely missing something basic. I used the schematic linked below to build.

http://1.bp.blogspot.com/-IVgQG0XRTCw/T2mwj3f-BMI/AAAAAAAABKk/AiZ72qURaLw/s1600/mk15schema.jpg

Thanks!

[willienillie]

120Hz is between the 1st and 2nd frets on the A string, so if you can bend that B-flat up to match, there you go.  60Hz is an octave lower obviously.  120Hz usually only occurs after 60Hz wall juice is full-wave rectified, so not found in a battery-operated pedal, and not the hum single coils pick up from their surroundings.

I was getting a noise of some sort with my Fuzz Factory clone, using an old (ProCo?) input cable that otherwise worked fine, and only when I turned my guitar volume all the way down.  Very similar to what you're describing, though I don't recall the nature of the noise, hum or whine or whatever.  It confused the hell out of me, I thought something had gone screwy with my Fuzz Factory, but I switched to one of my George L's cables and the problem disappeared.  So I quit using that other cable, but I still have no idea what could have been wrong with it.

[Spexicola]

I read where putting a resistor <50k or trimmer in series with the input cap may help this, but I'd rather diagnose what's wrong with my circuit.

[willienillie]

Yeah I'm interested to see if you figure it out.  I couldn't come up with even a guess about what I experienced, which may or may not be the same thing you're dealing with.

[PRR]

> putting a resistor <50k or trimmer in series with the input cap may help this

That may be the correct answer. If it is not exactly 50/60/100/120Hz then it is probably self-oscillating through the RC at Q2 emitter and the RC at Q1 base.(*) About 5k in series with the input cap spoils the resonance.

(*) Yes it takes three R-C to tango and we see only two. However all circuits are full of high-cuts from stray capacitance and they can accumulate to be a "third pole" even when nowhere near the first two poles.

[Spexicola]

> putting a resistor <50k or trimmer in series with the input cap may help this

That may be the correct answer. If it is not exactly 50/60/100/120Hz then it is probably self-oscillating through the RC at Q2 emitter and the RC at Q1 base.(*) About 5k in series with the input cap spoils the resonance.

I noticed that I'm picking up AM radio in the noise. A lively ranchero station to be specific.

I series a 25k into the input and with a little bit of resistance it cleared right up.

Is there any way to fix this without adding impedance to the input? I'd like to keep the component count stock if possible.