Pig Butt - Effect cuts out (to silence) when guitar volume turned down

[somnif]

I'm honestly not sure if this is a problem with the effect or my guitar (recent model Fender Telecaster standard). This is true on both pickups and full tone pot sweep.

The effect is very.... "Wooly" for lack of a better term on the lower side of the scale (an open E power chord, for example), but fine on the higher strings. But when I turn the volume knob on my guitar down to a certain point, the effect cuts out completely. Not even bypass clean tone, just full stop silence. I turn it up a little and the sound returns.  This is with maybe an eighth of a turn left on the volume sweep.

Very odd, but strangely enough I experienced a similar phenomenon with a veroboard transistor BMP I built a while back, and just assumed I had screwed something up.

I'm not even sure where to start in diagnosing this. My guitar? My cables? My guitar sounds fine when run to the amp directly. Or is this just a theme with 'Muffs, requiring a certain level of input to get over the hump, so to speak.

[Bret608]

I've had this happen twice before. Both times, it happened with transistor-based fuzzes where I had one of more of the transistors oriented the wrong direction (one a Harmonic Percolator, one a Tonebender MkIII). But I'm not sure what could cause this on an IC Muff. In this case I'd recommend checking all your resistor values as a first pass.

[somnif]

Ok just took voltage readings, and the 741 is acting odd.

4558:
1-4.52
2-4.51
3-4.27
4-0
5-4.08
6-4.52
7-4.51
8-9.17

741:
1-2.7mV
2-4.66
3-starts at 4.60 and slowly drops to 3.42
4-0
5-2.7mV
6-4.70
7-9.15
8-0

(keeping in mind the 4558 and 741 have different pin orientations, at least assuming the specs I read were correct)

Will check my resistor values shortly.

edit: All resistors match the BOM.

[somnif]

Just for fun I swapped the 741 for a TL071 and I still have cut out at low guitar volume. Very odd.

[m-Kresol]

But when I turn the volume knob on my guitar down to a certain point, the effect cuts out completely. Not even bypass clean tone, just full stop silence.

is this with the effect on or off? If your effect is off and you don't get bypass signal when turning the volume down, there's a problem with your guitars pot.

[somnif]

This is in a test rig, so the effect is constant on.

But....

I actually did the smart thing and decided to check my guitar straight into the amp, and guess what, it goes silent before the pot is fully turned. How did I never test this before, argh!

And its a fairly new guitar too, damn you Fender 

So, yeah, problem exists between player and effect, no fault with the circuit!

[m-Kresol]

more or less good to hear.
what I would do with your guitar is to check the pot. Measure resistance between lug 2 and the lug going to the output jack (usually lug 3) and see if the change is constant and what resistance you get when the signal cuts out. Replacing the pot yourself might be a lot easier/faster than bringing it back to the place where you bought it.

[Bret608]

Glad to hear it's figured out, anyway! +1 on swapping that pot yourself being quicker/easier.

[jtaormina]

Ive experienced some noise from a fender guitar before. I thought it was the circuit but was the guitar as well. Poor grounding. It's a squire that I have in the shop for testing.

[somnif]

Huh, how odd. It has a 50nF tone cap. I thought Fenders all had 47nF. Learn something new every day (Also didnt know they made ceramic disk caps that big that werent MLC!)

Measuring the resistance of the pot is tricky (lug 2 is output, for what its worth), but assuming the guide I read is accurate, it seems to have the appropriate sweep, it just zeros out a little early. And I have no idea why. I could get a better reading if I were to pull it out of the circuit, but I'm not willing to do that quite yet.

[m-Kresol]

it seems to have the appropriate sweep, it just zeros out a little early.

well, that sounds to me to be the problem you were describing. I'd switch out the pot for another one with the same nominal value.