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Old 10th April 2012, 03:38 PM   #11
shanx is offline shanx  Canada
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As suggested, it is likely the transistors bias should be checked. According to the site you should measure a collector voltage at Q2 of about 4.5 to 5 VDC.
They suggest installing a 10K trimpot in place of the fixed resistors for adjustment. I would not do that without keeping some fixed resistance to avoid possible direct short when trimpot is at minimum. I would use a 5K fixed and add a 5 or 10 K trimpot in series to adjust it.
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Old 11th April 2012, 01:06 AM   #12
jjman is offline jjman  United States
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My DIY kit has a 10k pot instead of the 8.2k resistor.

I had major frustration for months with (seemingly) intermitted times of low output. Eventually I noticed that placing any "buffered" type pedal between the guitar and the fuzz avoided the low-output problem. The input to the fuzz loads down the guitar pickups more than they prefer. Different setups will have different sensitivity to it. Try placing any Boss brand pedal (in "off" mode) between the guitar and the fuzz to rule out the loading problem.
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Old 11th April 2012, 02:08 PM   #13
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by jjman View Post
My DIY kit has a 10k pot instead of the 8.2k resistor.

I had major frustration for months with (seemingly) intermitted times of low output. Eventually I noticed that placing any "buffered" type pedal between the guitar and the fuzz avoided the low-output problem. The input to the fuzz loads down the guitar pickups more than they prefer. Different setups will have different sensitivity to it. Try placing any Boss brand pedal (in "off" mode) between the guitar and the fuzz to rule out the loading problem.


No. The low input impedance of a fuzz-face, and the loading of the guitar it causes is an essential part of its sound.

It is not a problem; without it, ie driving the fuzzface from something with low output impedance, will give you a horribly harsh sound.

I include a series pot in the input as an extra control, in case it's to be used after another effect, and I believe others do likewise - don't worry about it for now though, get your unit working first.

This is not the cause of your problem, many thousands of fuzzfaces have been built and used straight after the guitar.

Measure the voltages referred to by previous posters.
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Old 11th April 2012, 04:21 PM   #14
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Hi everybody, thanks again for all the feedback.
Sorry for the delayed reply, got a bit hectic going into the holidays, but now I've measured the voltages at each transistor and they are as follows:
Q1: b- 0,475 e- 0,003 c- 1,95
Q2: b- 1,95 e- 1,28 c- 1,3
Now I'm sure the values of Q2 are off (not even close to the 4,5 shanx suggested), but no idea what's wrong, or how to remedy it.
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Old 11th April 2012, 06:51 PM   #15
Simon B is offline Simon B  United Kingdom
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Attachment 276601

Ok, getting there. What's the voltage at the other end of the 8k2 resistor connected to the collector of Q2? Where it meets the 330R resistor and the 0u01 capacitor? How much voltage is being dropped across the 330R resistor? Suggest you check the value and connection of that 330R resistor - is it 330R? Not 330k for instance?!

The collector voltage on Q2 is too low - the base-collector junction is forward biased - fits totally with what you've described sound wise.
Attached Images
File Type: gif fuzzfacenpnschematic.gif (5.2 KB, 48 views)

Last edited by Simon B; 11th April 2012 at 07:00 PM.
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Old 11th April 2012, 07:02 PM   #16
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Are you sure you are using a 1K pot for the Fuzz control and the Q2 collector voltage stays stable as you rotate the pot? Reducing the 8.2K resistor (experiment with a trim pot) should get the collector voltage in range.
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Old 11th April 2012, 07:17 PM   #17
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by Loudthud View Post
Are you sure you are using a 1K pot for the Fuzz control and the Q2 collector voltage stays stable as you rotate the pot? Reducing the 8.2K resistor (experiment with a trim pot) should get the collector voltage in range.

If the Fuzz pot is correctly wired and the capacitor on it is good, rotating it will not affect the bias of either transistor.

The voltage at the emitter of Q2 looks about right to me, feeding through the 100k resistor it gives about 8uA bias current into the base of Q1, which should be bumping along the bottom rather. Negative input half cycles get clipped first.

Q2 has reasonable forward bias across the base-emitter junction, 0v68, given the apparent lack of current flowing through it. I'd be looking at the 330R and 8k2 first of all.
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Old 11th April 2012, 07:17 PM   #18
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Q1 is off for whatever reason thats why Q2 is hard on .

The base of Q1 should be approx 0.63V and you only have 0.475V.

Check Q1 and its connections .
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Old 11th April 2012, 07:20 PM   #19
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by epicyclic View Post
The base of Q1 should be approx 0.63V


Why should it be 0.63v? It gets fed a tiny bias current. 0.63v might be what you'd expect in a "normal" circuit. This just isn't one of those circuits. Anyway, it isn't "off", it's got 8uA flowing into its base.

Last edited by Simon B; 11th April 2012 at 07:28 PM.
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Old 11th April 2012, 07:39 PM   #20
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Sorry guys, when i went back to check the value of the resistor i discovered i had actually connected one leads on the voltmeter to the wrong terminal (I didn't even know that would show results) rechecked the values and they are as follows:

Q1: b- 0,64 e- 0 c- 2,12
Q2 b, 2,12 e- 1,45 c- 1,45

sorry for the mistake

resistor is correct 330 (no K) and voltage between resistors is 8,68
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