Fuzz face voltage help

Started by mills, April 20, 2008, 11:48:07 PM

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mills

Well, I'm back asking for help, but this time with the voltages on a pretty standard PNP positive ground fuzz face.  I sat down the other night to throw this together on perfboard with a little help from the GGG layout, and tacked a variable cap from the easy face schematic onto the front, and a bigger output cap on the end.  Tried playing through it, and got rediculously gatey, useless sounds.  I figured it wasn't a problem since I had used a trimpot on Q2 and I could adjust it to the right value, but I'm getting the same (wrong) voltage on Q2's collector regardless of the trimpots setting.

So,

Q1
C  -.580V
B   -.129V
E   -0V

Q2
C  -8.84V
B  -.575V
C  -.560V

I've played with the trimpot and it doesn't seem to change the voltage on Q2's collector at all... so I assumed I'd left some kind of solder bridge to the -9V connection to the board.  I can't find one, and measuring from the DC jack to the leg of the transistor gives a resistance in about the range it should be in.  I've checked the wiring a few times, and can't see any connections where there shouldn't be, and nothing seems inappropriately disconected.

I used matched Ge transistors (2n1305) from smallbear, and have used all the values recomended with one small exception.  The documentation suggested 10K ohms between Q2's collector and where the output comes off, and then 680 ohms from there to the -9V.  I didn't have a 680 ohm resistor, so I used an 820, but I thought that I could set the trimpot 140 ohms lower and it should give the same result.  I don't think that thats the problem, but thats the only mod or difference that's really anywhere in the circuit. (there's a 4.7K ohm resister in series with the 10K trimmer just to be sure I could get the values I needed)

I haven't found a datasheet with a pinout to check, but in the metal can style a metal tab usually marks the emitter?   Thats how I have it set up right now, but my googleing for something definite isn't working.

I dunno, I'm at a loss and hope that someone reads through my ramblings and can help.  Otherwise, thanks for all the patients with my plethora of "noob" question threads lately.


smallbearelec

The sound is farty because Q2 isn't conducting. The Base should be at least .1 volt more negative than the emitterfor that to happen. Are you sure that the feedback resistor is actually connected? Use your meter to to check continuity of every point to every other point. The problem could be an open rather than a short...

SD

mills

#2
I just ran through and tested everything I could think of to test for continuity, and either side of the feedback resistor seems to be connected to everything that it should be.  The resistor looks like its marked 120k ohm (unless its 1.3k, but I've never ordered that value)...  Checked it with my meter and its reading the 120K ohms with the transistors out of the circuit, and about 8.4K ohms with them in.  I think that should mean that when the transistors are in place, all the parallel resistors.etc in the circuit alter the reading, so the feedback resistor is connected to something.

Otherwise, I'd thought that the worst value on the transistors was the full 8.84V of the power supply being at Q2's collector, or will that correct itself once I manage to get the rest of the connections in the circuit set up? 

Finally, only semi-related (to the feedback resistor at least) if the color code is brown-red-black-orange-brown, how does one tell which side it starts on and which side it ends on?

smallbearelec

Quote from: mills on April 21, 2008, 12:37:29 AM
I'd thought that the worst value on the transistors was the full 8.84V of the power supply being at Q2's collector, or will that correct itself once I manage to get the rest of the connections in the circuit set up? 

Yes. Once the Base of Q2 is biased, the voltage at the Collector will be pulled down. Something isn't connected, but I can't tell you what from a distance.

mills

Well, that narrows it down for me at least a bit.  I can at least try and focus on the right area now.

Thanks for the help!