DEBUGGING SI NPN FUZZ FACE

[wolfy]

Would love to get to the bottom of this.


1.What does it do, not do, and sound like?
Lots of background buzz - I have fuzz but  I have to turn my amp right up to hear a quiet sound. Fuzz pot does not seem to effect fuzz. Volume does effect volume.

2.Name of the circuit = NPN Fuzz Face Jack Orman
3.Source of the circuit (URL of schematic or project) = http://www.muzique.com/schem/fuzzface3.gif
4.Any modifications to the circuit? N
5.Any parts substitutions? If yes, list them. 2n2222a instead of bc108c, 22uf in place of 20uf and 820k instead of 8.2k (to bias q2c)
6.Positive ground to negative ground conversion? It's neg ground by nature
7.Turn your meter on, set it to the 10V or 20V scale. Remove the battery from the battery clip. Probe the battery terminals with the meter leads before putting it in the clip. What is the out of circuit battery voltage? =>  9.57v
Now insert the battery into the clip. If your effect is wired so that a plug must be in the input or output jack to turn the battery power on, insert one end of a cord into that jack. Connect the negative/black meter lead to signal ground by clipping the negative/black lead to the outer sleeve of the input or output jack, whichever does not have a plug in it. With the negative lead on signal ground, measure the following:
Voltage at the circuit board end of the red battery lead =9.52v
Voltage at the circuit board end of the black battery lead =0.00v

Now, using the original schematic as a reference for which part is which (that is, which transistor is Q1, Q2, etc. and which IC is IC1, IC2, C1, and so on) measure and list the voltage on each pin of every transistor and IC. Just keep the black lead on ground, and touch the pointed end of the red probe to each one in turn. Report the voltages as follows:

Q1
C =1.22v
B =0.6v
E =0v

Q2
C=4.64v
B=1.22v
E=0.75v

[wolfy]

[GGBB]

5.Any parts substitutions? If yes, list them. 2n2222a instead of bc108c, 22uf in place of 20uf and 820k instead of 8.2k (to bias q2c)

I'm not an expert by any measure - I read these threads for the sake of learning as I build my own fuzz face.  But ... although your voltages look withing range, the fact that you had to use an 820K in place of an 8.2K for the Q2C bias sounds really suspect.  Are the transistors okay and oriented correctly?

[trjones1]

5.Any parts substitutions? If yes, list them. 2n2222a instead of bc108c, 22uf in place of 20uf and 820k instead of 8.2k (to bias q2c)

I'm not an expert by any measure - I read these threads for the sake of learning as I build my own fuzz face.  But ... although your voltages look withing range, the fact that you had to use an 820K in place of an 8.2K for the Q2C bias sounds really suspect.  Are the transistors okay and oriented correctly?

Yeah, that part substitution is no good.  Just slap a 10k in for the 8.2k.  You do have 10k resistors lying around, don't you?

[LucifersTrip]

+1 to the above two comments....I stopped reading at #5 and did a WTF...

Anyway, you should be able to hit ~4.5V with a resistor in ~ the 4K to 15K range (I shoot for 5K - 11K), so a common setup is a 5K cutoff with a 10K pot. If you need to go too far out of that range, it's doubtful you will get the "right" sound.

Try a 20K pot and if you can't hit ~4.5V, doublecheck the transistor and resistors and connections around that area

[wolfy]

Yeah I knew that big ass resistor wasn't the answer. I was getting 9.3 odd volts with the 8.2k.

It has gotta be something else. Perhaps my ground setup - I will continue to read.

[wolfy]

Rebuilt this circuit three times now and am still getting the same results. q2 collector's voltage is around the the 8-9v point, it takes a big resistor to get it down. Driving me nuts.

[ogandou]

Somehow I had to do the contrary and reduce the 8.2k resistor to 560 ohms... I don't get it... Maybe bad transistor? It seems to work fine like this...

[LucifersTrip]

Rebuilt this circuit three times now and am still getting the same results. q2 collector's voltage is around the the 8-9v point, it takes a big resistor to get it down. Driving me nuts.

since you're probably just repeating the same, simple mistake, how bout posting a big, clear closeup of your build/breadboard, etc and we'll try to spot the error.

a common rookie mistake I've seen made numerous times is connecting the 8.2K to (+).  Even though on the schematic it may look like it's the top rail (+), only one side of the 33K & 330 are connected there (+)

[wolfy]

Ok will do. I'm resoldering new leads on my pots and jacks just to eliminate that potential problem.
Looking forward to getting this baby going. Too many days of failure!

[wolfy]

This was the best I could do photowise.

GND to 9v+ - 8.68v
Raw Batt power- 8.82v
Q1 is about 188 hfe, Q2 is around 200 hfe.

Q1:
C-1.27v
B-0.64v
E-0v

Q2:
C-8.59v
B-1.27v
E-0.77v

Checked all resistors ohms - all good.






Help much appreciated. I wanna play guitar.

[wolfy]

All parts except the 20uf c2 are used. I used 22uf for c2.

[wolfy]

Trannies are 2n3904s.

[wolfy]

Tried replacing 1k pot with 1k resistor. Got same readings so it's not the 1k fuzz pot.

[wolfy]

I plugged it in and I get some fuzz - have to turn the amp up loud to hear it though.
Volume pot works but fuzz pot seems to have no effect when turned.
I will replace the cap near the fuzz pot perhaps?

[wolfy]

I still can't find out what's wrong. I'm thinking the trannies hfes are too high (around 200).
What I can see there isn't a big enough difference between q2 b and e. Around .4v . Tried to change R1 to a lower value but it made q2's e and b voltage rise together evenly, so I was left with the same difference.
I'm glad I've had so much trouble as I'm learning.

[GGBB]

The picture's not the clearest, so I can't make out the transistor pinouts and cap polarities.  Otherwise, it looks like the circuit is laid out correctly.

[wolfy]

I'm sure the pinouts and polarities are correct. I have checked so many times.

[deadastronaut]

heres how you should have it.....i can't tell by your pics..


iirc i stuck a 10k pot where the 8.2k is...and tweaked..


out cap is 100nf...and thats a 330r resistor ok...

[wolfy]

Thanks for the picture. I'll check it with mine.
I think the transistors hfe is too high for the circuit. I think they will work in http://fuzzcentral.ssguitar.com/schematics/axisfuzzschem.gif.
I'm going to try that because this circuit is breaking my heart. I'll try it when I get the right trannies.