Collector Voltage Trouble

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Ueleoj
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Post by Ueleoj »

Just put together the Axis Face fuzz from fuzzcentral, http://fuzzcentral.ssguitar.com/axisface.php and I'm having trouble adjusting the Q2 collector voltage to 4.5 VDC. It just sits just below 9 volts, adjusting the trim pot seems to do nothing. I've gone through all my connections and grounds and everything looks like it should. Obviously I've mucked something up, but for the life of me I keep missing it! Here are the voltages I'm reading on the two transistors with the trim pot set to 8.2K: Q1 E 0, B 0.67, C 1.47 / Q2 E 1.0, B 1.47, C 8.87

Thanks in advance for any help you guys might be able to provide!

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DrNomis
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Post by DrNomis »

Ueleoj wrote:Just put together the Axis Face fuzz from fuzzcentral, http://fuzzcentral.ssguitar.com/axisface.php and I'm having trouble adjusting the Q2 collector voltage to 4.5 VDC. It just sits just below 9 volts, adjusting the trim pot seems to do nothing. I've gone through all my connections and grounds and everything looks like it should. Obviously I've mucked something up, but for the life of me I keep missing it! Here are the voltages I'm reading on the two transistors with the trim pot set to 8.2K: Q1 E 0, B 0.67, C 1.47 / Q2 E 1.0, B 1.47, C 8.87

Thanks in advance for any help you guys might be able to provide!

It looks like Q2 isn't getting turned on enough, check to see if it is orientated correctly.... :hmmm:


It could also be that the trimpot is faulty.... :hmmm:
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.

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Ueleoj
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Post by Ueleoj »

DrNomis, I think your right about Q2 not turning on. I was hoping the 0.47 difference between Q2's base and emitter would be enough. So I replaced Q1's 33K collector resistor with a trim pot so I could fiddle with it's collector voltage (Q2's base voltage). It seems no matter what I set Q1's collector at the difference between Q2's base and emitter remains just shy of the minimum 0.5 it needs! I double checked Q1 and Q2's data sheets and they're orientated correct, trim pots both test good. Any ideas?

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DrNomis
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my favorite amplifier: Self-Built Valve Amp Head :)
Completed builds: Dallas Arbiter Fuzz Face,Tone Bender Professional Mk 3,Tone Bender 3-Knob,Baja BK Butler Tube Driver,Baja Real Tube Overdrive,Roger Mayer Octavia,EH Soul Preacher,Tech 21 XXL Distortion,MFOS Weird Sound Generator.
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Post by DrNomis »

Ueleoj wrote:DrNomis, I think your right about Q2 not turning on. I was hoping the 0.47 difference between Q2's base and emitter would be enough. So I replaced Q1's 33K collector resistor with a trim pot so I could fiddle with it's collector voltage (Q2's base voltage). It seems no matter what I set Q1's collector at the difference between Q2's base and emitter remains just shy of the minimum 0.5 it needs! I double checked Q1 and Q2's data sheets and they're orientated correct, trim pots both test good. Any ideas?

Yeah, Si transistors need from .6 to .7 volts across the B-E junction to turn on, if the voltage on Q2's base isn't getting any higher then that suggests that Q1 might have gone C-E short-circuit, so try replacing Q1 with a known good transistor, also try replacing Q2 as well because it could have gone B-E short-circuit.... :hmmm:
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.

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