Broken 70s muff fuzz

[HeavyFog]

So a friend of mine gave me his 70s op amp muff fuzz to see if i could fix it and i'm running into a few problems. First, someone "fixed it" before me and did an awful job at it since apparently, he wasn't able to make it work. The battery leads were poorly soldered back in, a few traces on the pcb were ruined and bridged, and there's a mess of flux and solder blobs everywhere that was touched up. The pedal works to a degree, but the sound is very weak and gated and you can only hear at all on the low e string if you really hammer it hard. I suspect it might be a problem with the power supply, considering that's where new wires were attached but in mot sure since i measured 9v on the lead and on the trace. Just want to be sure before i take the soldering iron to it.

The parts and values on the pcb also aren't matching up with any schematics iv'e ever seen for an op amp muff fuzz. I was expecting the values not to match up but i wasn't expecting to see an extra capacitor and resistor, both of which do look like they belong there. Any help? 

[HeavyFog]

Forgot to mention that the on off switch and the pot both work (the pot bring the volume down) and i do get a bypassed signal. Il try to get a picture up as soon as i can

[Kipper4]

Post up a potential schematic please.
Usually but not always. That I have to hit the lowest note possible really hard and even then it sounds gated thing. Is a biasing issue.
The fact someone's already messed with it might not help.
Rich

[HeavyFog]

Here's a schematic



And here are some pictures of the pcb

Top (there's a resistor under the switch and a visible green capacitor just to the side)



Bottom (keep in mind i haven't soldered anything yet)

[GibsonGM]

Know how to use a meter, Fog?    Can you post voltages at each pin of the chip, as referenced from ground? (battery - terminal)

Have the unit on and plugged in but no guitar playing into it.

Dot on chip upper left side is the #1 pin, then they go down 1234, jump to right side and then it goes bottom to top 5678.....

Voltages will tell is if there is a bias issue (R3, R5)  as Rich suggested (a good deduction)

[Kipper4]

Post up some voltages.
I hope you didn't do that solder job.
There's a lot of flux on there. It would probably be beneficial to clean that up with some alcohol and a tooth brush.

[HeavyFog]

Il get some voltages on the ic pins, i have an old meter lying around here somewhere.

and no that soldering wasn't me

[Kipper4]

and no that soldering wasn't me

Phew my thoughts might have insulted you else.

[HeavyFog]

Alright here are the pin voltages:

1-5.28v
2-5.17v
3-4.46v
4-nothing
5-4.46v
6-5.16v
7-5.28v
8-9.37v

Not entirely sure what to make of these values but i have a feeling something isn't right herre

[GibsonGM]

They're not far off. 

Do you have access to a capacitor, so you can make an audio probe?  I would just probe pin1 and pin 7 while someone plucks notes to see if it still seems "gated".     you should have an audio probe for this work, anyway    If your audio is 'normal' thruout the circuit, then something else is wrong (vol pot?)

Read on, and post your results:

http://www.diystompboxes.com/pedals/debug.html

[j_flanders]

Hi, it's great to see someone with an actual IC Muff Fuzz. I'm trying to build one at the moment.

The parts and values on the pcb also aren't matching up with any schematics iv'e ever seen for an op amp muff fuzz. I was expecting the values not to match up but i wasn't expecting to see an extra capacitor and resistor, both of which do look like they belong there. Any help?

I also tried to build one a few years ago but, as a lot of people have reported, it sounded gated, did not clean up with the guitar volume and sounded nothing like the transistor version of the Muff Fuzz or the single Muff1 in the the Double Muff.
Back then I used the same schematic you posted. And I guess most people did, since it's the only schematic found online together with this one:

Most suggested mods are adding a resistor and/or capacitor parallel to the clipping diodes.
You then end up with a schematic like the one below, which I currently think is actually a Muff Fuzz.
It does sound like the transistor version of the Muff Fuzz and cleans up almost equally well.


But as far as I can tell from the resistors in your picture it's not entirely the same. But that isn't too uncommon for EHX effects.

Would you be so kind to list the values of all the components or take more clear pictures, or just tell the colour bands on the resistors and markings on the caps and diodes?

[HeavyFog]

UPDATE
For whatever reason its actually working now but its very quiet even when the volume is all the way up. I'm getting a signal that sounds like a muff fuzz, just a quiet one with a bit less gain. Maybe its the pot? Bypassing it with wires doesn't seem to do anything

[Kipper4]

And if you short the diodes?

[HeavyFog]

Tried grounding them (if that's what you meant) and it just killed the signal all together. I'm starting to think its the pot since it crackles quite a bit and because its so old. Its just odd that suddenly its working but i haven't done a thing to it yet, so maybe i wiggled the pot in such a way that it works? Had the same issue before with my russian big muff.

(edit) it definitely doesn't have as much gain as it should

[Kipper4]

That's not what I meant.
If you think it's the pot audio probe it before the pot. It should be loud.

[HeavyFog]

Probing it didn't give a signal that was any louder so i don't think its that. I heard from my friend that it worked fine, then the battery lead broke, so he gave it to someone he knew to fix it, and after that i didn't work at all, so i suspect now that the source of the problem has something to do with the attempted repair. It's not the battery leads since i'm getting normal looking voltages on the ic pins but perhaps its something else

[Kipper4]

And if you probe it after R4 (- input of U1b)

[HeavyFog]

Nope still no difference. I think what il try to do is trace the circuit to see if there's a cut of something touching that shouldn't be, since it starting to look like that's the case

[j_flanders]

Tried grounding them (if that's what you meant) and it just killed the signal all together.

To ground them,you need to connect them to Vbias (4,5V) or if you want to connect them to 'real' ground (0V) you need a capacitor to block the DC otherwise you 'kill' the signal.
But since these are diodes in the NFB loop of an inverting op amp they are already tied to 'virtual' ground (Vbias).
And jumpering/shorting them would create 0 resistance in the feedback loop thus gain of zero giving zero output.
If you want them out of the circuit you'd really need to clip at least one leg of each. I wouldn't bother.

Maybe its the pot? Bypassing it with wires doesn't seem to do anything

You still need a suitable load, so if you really bypass/jumper the pot you should add a 50k resistor as load instead.

UPDATE
For whatever reason its actually working now but its very quiet even when the volume is all the way up.

Check with a magnifying glass for bridged solder connections or traces.
Maybe a silly suggestion but check if your guitar volume is fully up. A properly working Muff Fuzz cleans up very well. Depending on your pickups and volume pot it can go from clean on 8 to overdrive on 9 to fuzz on 10. But even the cleans shouldn't be "very quiet"
Not related to your problem but, like a fuzz face, this pedal has low input impedance and doesn't like another pedal in front (unless it's unbuffered and in bypass)

1-5.28v
2-5.17v
3-4.46v
4-nothing
5-4.46v
6-5.16v
7-5.28v
8-9.37v

Still a considerable difference between -IN and +IN pins. I measured my pins based on the Muff Fuzz schematic posted above:
1-4.40v
2-4.55v
3-4.55v
4-nothing
5-4.55v
6-4.55v
7-4.40v
8-8.97v

BTW I'm still very much interested in the component values of your pedal.

[HeavyFog]

Il write down all the part values next chance i get. If you have a picture of the bottom of the pcb on your muff fuzz (if its an original) that would help as well.