Fuzz Factory style pedal problem

[Aber335]

Hi,
I just completed my build of a Filth Fack Pedal from Fuzzdog and the effect only half works. The sound is muted and the knobs don't do much. They respond a bit but not much. Kind of a weak signal. I have tried to figure out the issue and went back through my build. I was careful with the transistors and used a heat sink. I have tried the effect in and out of the closure to see if maybe that was the issue. I did remove the 3pdt switch and believe the problem to be either in the circuit or in the pots. I tested the pots in the build (not before they were installed) and they are a bit strange. there are 5 pots. I believe 2 of the 5 worked completely when both testing both sides of the pot. The other 3 didn't exactly work as the other two with one side not showing change when the knob is turned. Maybe I am missing something about pots connected to the circuit but it seems like they weren't completely working. Is it likely that 3 of the 5 pots I received have an issue? Could it be that I damaged them somehow? I cannot see how that would happen but maybe. Any advice would be helpful.

Here is the doc to the build.

http://pedalparts.co.uk/docs/FilthFack-Wired.pdf

[idy]

Three pots are used as "variable resistors"; the middle lug is connected to one of the outside lugs and those two will always be shorted out. Stab, Comp, and Gate are like that.
And no, bad pots are an extremely unlikely problem...unless you stressed them and broke the lugs or soldered onto the rivets (instead of the lugs) or just baked them with the soldering iron.

That being said, you generally want to attach the wires to the pots so there is some mechanical attachment that soldering makes solid. With these units (not intended for this kind of use, I know this is a kit, but...) you would at least wrap the wires around the leg before soldering. Probably not worth doing over now, if they seem attached.

What is extremely likely is that the transistors are in the wrong way. We would all check data sheets for each and then test to be sure before soldering them in (or socket them.) You cannot trust the picture on the pcb, and even a vendor-painted dot on the collector may lead you astray.

[Aber335]

Thanks for your response! Helpful to know the pots aren't so sensitive. I checked the transistors but can't see a mistake. There is a dot on the AC128 transistors which according to the kit is meant to be the collector. I made sure that is in the C socket. On the smaller black plastic transistor (2n3904) I installed how I knew how.

[idy]

Couple of things to try. You know this thing is supposed to sound whacked and requires some tweaking to get a "normal" fuzz tone...(google "fuzz factory settings.")

Double check the polarity of the electrolytic caps.

Double check the color codes on the resistors.

Reflow solder (except maybe on the Ge transistors). Blobs mean not enough heat. When the leads are hot enough the solder "sticks" and "wicks" up the leads. Some say like chocolate chips.

Then make an audio probe (just a cable with a cap in series with the hot lead) and trace the sound. It should sound the same only hotter after the first transistor, then fuzzy after the two other transistors. Where does it become weak?

[Aber335]

Thanks for your input. This is quite the learning experience. I haven't ever built an audio probe but will try to do that. I really appreciate your suggestions. I will def try to reflow solder to see if that helps. Might be a loose connection. Ill also check the polarity again on the caps.

I am wondering if maybe the resistor might have been wrong. I received them and checked them but they are really hard to tell apart. I will try to recheck those colors and see if maybe I made an error.

[idy]

Oh yes, they are hard to tell apart. Those little color bands. Older and sadder hands test them with a meter before soldering.

Who can tell brown from purple in this light?

Which side is which... is that brown black red brown or brown red black brown?....etc.  (that example is not very good, one of those is mot used, but still...)

And the time I got a baggy of mislabeled resistors (47k for 470k if I recall) and discovered this by going over a large project one resistor at a time.

Resistors are sometimes hard to measure in circuit. But with this project it might work, i.e. they are nor usually in parallel with each other. Hard for me to say if the transistors will conduct when you test a R. Maybe try reversing the leads (red and black) to make sure you aren't turning a transistor "on."

[Aber335]

Hey again,
So I'm back to the drawing board. Not sure what i learned but I tried lots of stuff and still trying to figure out my problem.
- I looked over to verify the polarity of the caps which were correct. No issue I could see there.
- I reflowed solder throughout the pcb (with the exception of the transistors). I didn't reflow solder on the pots themselves. Maybe I should.
- I got real up close and personal with each of the 6 resistors. I tested what I could and all read close to the resistance numbers except R3. It was reading something like 1k8 or something so I swapped it for a 47k transistor (which I happened to have). It turned out it was just having the resistor in the circuit that was messing up my reading because I tested it before putting a new one in and as soon as it went in it also dropped to that same 1k8 reading.
- then I build an audio probe. Kept touching different points but it wasn't obvious to me what didn't work right. One problem is that the pedal works. It just doesn't work well so I got signal on most spots. It seemed like R6 had sound on one side and not much if any on the other. It is a 220k resistor. Does that mean it drops the signal more? I also noticed that the black 2n3904 transistor had sound on one pin but not much on the other side. Wasn't sure if the transistor did something strange to the signal.

I am assuming the pots don't give sound out of every pin. The ground on some wasn't really making sound. Also the gate knob only one pin seemed to work.

Any ideas what I am missing?

[idy]

R6 goes from the output to ground. On one side you hear fuzz, on the other, nothing. Most of the pots go one side to ground or +9v. Check the schematic. You don't need to know everything to follow a little bit.

Gate and Volume go one side to ground. Comp and Drive go one side to power, which may be not quite 9v because of Stab, which "starves" the voltage a little.

You would expect to hear a nice signal going into the Base of Q3, and a nice boosted signal coming out its Collector.
It should go through C2 and into the Base of Q1, and come out the Collector, probably fuzzy.
From there directly into the Base of Q2, where it should come out the Collector ....Super Fuzzy.


You also might post voltages for the pins on each transistor. We would expect the bases to be a "diode drop" away from the emitters. On Q3 (silicon) that should be .6v difference, on the other two (germanium) around .3v. Maybe a little less.


On the schematic you may notice that the Q3 has the little arrow pointing outwards, it is NPN (Never Points iNward) while Q1 and Q2 have little arrows pointing inward because they are PNP, (Points Inward Perfectly.) The arrows follow the direction of "conventional current;" if you imagine current flowing from the + of the battery towards the -. In real life this is backwards, electrons are negative and flow from - towards +, but its a "convention." Much of the instability of this thing comes from the first half and the second having different power supply orientation.. the Q1 and Q2 are "upside down" from Q3. The PNP fuzz face was designed as a "positive ground" circuit, and the booster before it (an LPB) is "negative ground."


The two most common ways we see transistors the signal comes in the B and if you take the output from the E you are getting a "buffer," same voltage but better impedance, it can provide more current to drive something. If you take output from C (like here) you are getting an inverted signal but amplified, more voltage. Notice you are going to need a resistor between the power or ground and the leg you are tapping into or you will get DC only.