My Fuzzface requires a signal stronger than my guitar...

[joakim]

I have built a Fuzzface effect pedal, using some old transistors I found at the University. Pretty sure these are germanium PNP transistors, but they are only marked "AC125 CS9". Here's a photo, and I think this is the datasheet. From research, I've understood that the red dot indicates both PNP, and which leg is the collector.


Here's the schematic I've followed (Fuzzface modified for positive power source), and my Eagle version as well as the finished circuit board.


The situation is this: If I connect a guitar, I can hear (using an "audio probe" connected to an amp) that the input signal is "warped" into a fuzzed signal. However, no audible signal is found on the output (i.e., if I connect the circuit output to the amp, I hear silence regardless of how much I turn up the guitar, and turn up the volume pot on the pedal). Still, I'm pretty sure the cicuit does work, since, if I connect a "stronger" input signal (e.g. an iPad), I get a nice fuzz version of the input on the output.

So, my conclusion (= guess) is that the circuit can't "handle" the weaker guitar signal, which is obviously a problem since this is the whole purpose of the circuit. Perhaps this is due to specifications of my randomly chosen transistors, and perhaps I need to use different resistors than what's drawn in the original schematic to compensate?

Hope someone can help :-)

[joakim]

PS! The blue lines superposed on the PCB photo illustrate the copper tracings on the back side.

[italianguy63]

Schematic looks good.  But, I have seen it before, and I believe there is an error in it.  The .1uF cap by the volume should actually be .01uF.  It won't cause a big problem, it just effects the tone.  Maybe your transistors are suspect?  MC

[Seljer]

Measure the voltages on the terminals of both transistors and get back to us

[joakim]

Measure the voltages on the terminals of both transistors and get back to us

Sorry for being such an amateur, but how, specifically, do you mean? 
Should I measure voltage on each of the three legs relative to ground, or relative to each other (B-C, B-E etc)?

[Kipper4]

relative to ground me thinks

[joakim]

Thanks
And, maybe it's obvious, but there shouldn't be any input signal present while measuring, right?

[joakim]

Ok, here goes:

Q₂
C: 0.67V
B: 8.61V
E: 8.63V

Q₁
C: 8.60V
B: 8.64V
E: 8.72V = which is also VCC (using a standard 9V battery)

Btw: Please do explain what this information might tell you about the circuit and the problem!  

[deadastronaut]

22uf shouldn't be connected to q1...that should be off fuzz pot lug 2./q2 emitter

haven't you got any electrolytic caps?..

(going by the eagle schematic posted.) 2.2 uf  electrolytic cap not 22uf on input.

[Seljer]

If you compare the voltages to a known working fuzzface circuit, you can see where things are awry
I noticed you built a PNP circuit with a negative ground, so the voltages you measured are the inverse of what people usually post for their measurements (i.e. subtract them from the VCC to get 'normal' measurements to compare). Generally, they sound alright when Q2's collector voltage is around 5V. If it's not at that voltage the circuit is misbiased and it sounds very choppy and fizzy. The layman's approach is "mess around with the 8.2k resistor" until that voltage gets in range (many people put a 10k or 25k trim-pot there to dial it in just right). Theres hundreds of "fuzz face biasing" threads all over this forum and the rest of the internet which expand into both an art and a science The fuzzface article on geofex.com is required reading (and describes how to test your transistors to see if they are too leaky to work well in the circuit).

Also, if the photo you posted is of your actual circuit board I'd wager that the base lugs of your transistors are lacking in solder! (unless your board if double sided or your photo is from before you completed soldering everything)
edit: ah, saw the other photo, so I assume this is not an issue

[joakim]

22uf shouldn't be connected to q1...that should be off fuzz pot lug 2./q2 emitter

haven't you got any electrolytic caps?..

(going by the eagle schematic posted.) 2.2 uf  electrolytic cap not 22uf on input.

Thanks for replying!

Whether the cap should be connected to Q1 or somewhere else, I can't say. But, as seen in the photo, the cap C1 on input is indeed 2.2uF -- not 22uF (there's an error in my Eagle file).
Regarding electrolytic caps: C3 = 20uF is electrolytic, isn't it (this means it's polarized, right)? Again, this is a simplification in my Eagle file. Sorry for those!

But please help me reroute C1, if there is an error in the original schematic
(still, I'll emphasize that the circuit does appear to work if the input signal is "strong enough"...)

[italianguy63]

C1 is OK.  But it should be 2.2uF as you indicated.  Somebody will chime in, but it appears the voltages are off.  Q2's collector should be around 4.5V to 5V.  As astro said-- look for a bad connection.  I'm still guessing a faulty tranny.

MC

[joakim]

If you compare the voltages to a known working fuzzface circuit, you can see where things are awry
I noticed you built a PNP circuit with a negative ground, so the voltages you measured are the inverse of what people usually post for their measurements (i.e. subtract them from the VCC to get 'normal' measurements to compare). Generally, they sound alright when Q2's collector voltage is around 5V. If it's not at that voltage the circuit is misbiased and it sounds very choppy and fizzy. The layman's approach is "mess around with the 8.2k resistor" until that voltage gets in range (many people put a 10k or 25k trim-pot there to dial it in just right). Theres hundreds of "fuzz face biasing" threads all over this forum and the rest of the internet which expand into both an art and a science The fuzzface article on geofex.com is required reading (and describes how to test your transistors to see if they are too leaky to work well in the circuit).

Also, if the photo you posted is of your actual circuit board I'd wager that the base lugs of your transistors are lacking in solder! (unless your board if double sided or your photo is from before you completed soldering everything)
edit: ah, saw the other photo, so I assume this is not an issue

Thank you, Seljer! I'll try to modify R2 (8.2k), maybe replace it with a small trim-pot. Interesting stuff!

[LucifersTrip]

Don't worry about the 8.2K yet...wait till you're in the ballpark.

The voltages you noted above are so far off that there's definitely an error (soldering bridge, misplaced component, wrong component, dead component, etc...)

For example, you have Q2 E & B almost identical. That means they're almost (or are) shorted!

On the other forum, someone was cool enough to save all the voltages of good sounding ones that builders reported and then averaged them:

q1e: 0
q1b: 0.1    
q1c: 0.578  

q2e: 0.447
q2b: 0.578  
q2c: 4.51

here's some explanations behind the voltages that you asked for earlier
http://www.geofex.com/fxdebug/bias_prob.htm

[dwmorrin]

Ok, here goes:

Q₂
C: 0.67V
B: 8.61V
E: 8.63V

Q₁
C: 8.60V
B: 8.64V
E: 8.72V = which is also VCC (using a standard 9V battery)

Btw: Please do explain what this information might tell you about the circuit and the problem!  

Emitter to base voltage should be 1 diode drop if the transistor is on.
Ge diode drops are usually 100-300mV.
Your Q1 shows a Vbe of 80mV, which is low but could be right.
Collector to emitter voltage tells you what the transistor is doing.
Q1 Vce is 120mV, which indicates saturation, or fully on.

Q2 has a 20mV Vbe, and an 8V Vce, which means it is near cutoff.
Q2 is barely on.

Just an explanation of what those voltages mean to me.
I would test the transistors out of circuit.

[duck_arse]

your eagle file suggests you have a 100k (100,000 Ohms) pot connected as "P1". the more usual value here is 1k, or 1,000 Ohms. this will choke your Q1 very efficiently, and may just be the cause of your problem.

you can simply clip/tack solder/hold a 1k 1/4 watt resistor across your current P1 at points "3:P1A" and "5:P1A" to see if this is the problem. back-off your volume control before you try this, and let us know what happens.

[edit:] oh, and seeing as no-one else has said it, hello and welcome to the forum. and good luck with the fixxe.

[joakim]

your eagle file suggests you have a 100k (100,000 Ohms) pot connected as "P1". the more usual value here is 1k, or 1,000 Ohms. this will choke your Q1 very efficiently, and may just be the cause of your problem.

you can simply clip/tack solder/hold a 1k 1/4 watt resistor across your current P1 at points "3:P1A" and "5:P1A" to see if this is the problem. back-off your volume control before you try this, and let us know what happens.

[edit:] oh, and seeing as no-one else has said it, hello and welcome to the forum. and good luck with the fixxe.

Unfortunately, this is yet another error in the Eagle file... embarassing
The actual pot in the stompbox is indeed 1k.

The plan is to desolder the transistors (especially Q1) and test them out of circuit. The voltages I measured to seem to indicate something wrong with Q1, or (if I'm lucky) a short circuit somewhere.

Thanks for the welcome!  Everyone's made me feel real welcome already; great forum!

[pinkjimiphoton]

it looks to me like your transistors are in backwards.

your voltages look to me to be close enough for rocknroll on some of the faces i've built... but backwards.

especially since you said you need to put a strong signal thru, then it works.

reverse beta. turn the transistors around 180 degrees and i bet it works, and is nice, loud and dynamic.

[joakim]

it looks to me like your transistors are in backwards.

your voltages look to me to be close enough for rocknroll on some of the faces i've built... but backwards.

especially since you said you need to put a strong signal thru, then it works.

reverse beta. turn the transistors around 180 degrees and i bet it works, and is nice, loud and dynamic.

I was wondering if I might have mis-identified the legs on those transistors, yes... Not very easy to interpret that old school datasheet, and not even sure I found the right sheet.
Still, several sites/forums say that red dots indicates the collector on PNP transistors. But maybe these sites discuss specificly those "black glass" type transistors?


Anyway, I've concluded that I need to desolder and remove the transistors anyway, to test them. I should be able to test the transistors (to see that they work), and also identify the legs in the process, right? I've got a standard multimeter, with a transistor test thingie. But how careful do I have to be when de/soldering? I've heard that germanium transistors die easily??  

[LucifersTrip]

more careful than with silicon...try not to use higher than a 30 watt iron, do it quickly and use a heat sync.  red is c. the dmm will probably let you determine lugs, but not give u accurate hfe since leakage is not considered.