Issues biasing silicon Fuzz Face

touringbubble · Post by **touringbubble** » 03 Apr 2016, 23:47

So I breadboarded a standard fuzz face a while back and recently moved it to a circuit board. I'm not getting any audio output.

I used an audio probe and sound reached the base of Q1 but doesn't go past there. The collector is silent, as is the gain pot. Voltages at Q1 are correct per what I found online.

However, Q2 collector Voltages were wrong. I had the wrong resistors there. Apparently my breadboard was messing with them. Not sure why.

I had 69k of resistance on the board in R5 instead of the 8.2k that's on the schematic. I read it wrong. But, on the biard it was biased at 4.7v with that 69k value.

What I found when troubleshooting is that I am getting 9.17v at the collector of Q2. I got that with the 69k and now with the 10k trim pot set to 8.2k. The voltage just doesn't drop across the resistors.

Is this a common issue with a specific component failure? And ideas?

Thanks!

touringbubble · Post by **touringbubble** » 04 Apr 2016, 04:58

Did some more testing. I have output, but it's super low and clean. Little fuzz, if any. Signal volume plummets comparing output of Q2 base to Q2 collector.

These are my measurements:

Q1
C 1.04v
B 0.58V
E 0v

Q2
C 9.14v
B 1.03v
E 0.62v

The voltage to Q2 C only drops by 0.02v even with a verified 8.7k ohm resistance. I have no idea why the voltage is so high. Any help is greatly appreciated!

mictester · Post by **mictester** » 04 Apr 2016, 06:31

You have a wiring error! The collector of Q1 should be about 1.4V and Q2 collector should be in the region of 5V.

touringbubble · Post by **touringbubble** » 04 Apr 2016, 13:25

mictester wrote:You have a wiring error! The collector of Q1 should be about 1.4V and Q2 collector should be in the region of 5V.

Yeah, I know. But I don't know where. I've checked everything I can think of. Any ideas?

Electric Warrior · Post by **Electric Warrior** » 04 Apr 2016, 13:40

Maybe it's just a short between Q2C and the battery?

touringbubble · Post by **touringbubble** » 04 Apr 2016, 15:51

Electric Warrior wrote:Maybe it's just a short between Q2C and the battery?

One of my first thoughts. There is proper resistance between + and Q2C though. I'll look deeper.

Any other thoughts?

induction · Post by **induction** » 04 Apr 2016, 16:14

There is proper resistance between + and Q2C though. I'll look deeper.

Sorry to be pedantic, but I assume you mean there is a proper resistor there. The question, though, is whether that resistor is shorted (ie. there is proper resistance there). If you get the same voltage reading with a 68k fixed resistor or the 8.2k setting on a trimpot, then a short is indeed likely.

You mentioned breadboarding the circuit before moving to the pcb. I can only assume the circuit worked correctly on the breadboard (otherwise you wouldn't have moved it to the pcb). Am I correct?

What transistors are you using and what is their hfe?

Post high resolution pictures of your build. Maybe we'll spot an error.

touringbubble · Post by **touringbubble** » 04 Apr 2016, 16:47

induction wrote:
There is proper resistance between + and Q2C though. I'll look deeper.
Sorry to be pedantic, but I assume you mean there is a proper resistor there. The question, though, is whether that resistor is shorted (ie. there is proper resistance there). If you get the same voltage reading with a 68k fixed resistor or the 8.2k setting on a trimpot, then a short is indeed likely.

You mentioned breadboarding the circuit before moving to the pcb. I can only assume the circuit worked correctly on the breadboard (otherwise you wouldn't have moved it to the pcb). Am I correct?

What transistors are you using and what is their hfe?

Post high resolution pictures of your build. Maybe we'll spot an error.

With the input jack not grounded and no battery I'm able to check the actual resistance. With probes on the 9v bus and the Q2C, I get around 8.8k resistance. The rheostat in place is set to 8.4 and there is the 330 ohm resistor as well, so the resistance seems correct.

With a short there would be much less resistance, right?

The build is a little rough, but I'll get some pics and try to trace over it as a guide.

Thanks for the input!

touringbubble · Post by **touringbubble** » 04 Apr 2016, 16:48

And yeah, it worked on the breadboard. And strangely, the Q2 biased at 4.7v on the breadboard with the 69k resistance I had originally.

induction · Post by **induction** » 04 Apr 2016, 16:52

touringbubble wrote: With the input jack not grounded and no battery I'm able to check the actual resistance. With probes on the 9v bus and the Q2C, I get around 8.8k resistance. The rheostat in place is set to 8.4 and there is the 330 ohm resistor as well, so the resistance seems correct.

With a short there would be much less resistance, right?

That sounds right to me.

And yeah, it worked on the breadboard. And strangely, the Q2 biased at 4.7v on the breadboard with the 69k resistance I had originally.

That suggests some kind of build error, possibly in the pcb, but it's hard to say anything for sure at this point. Pictures should help.

touringbubble · Post by **touringbubble** » 04 Apr 2016, 17:27

Reading another forum I see that without a clear path to ground, the resistors won't properly lower voltage. I've checked the resistance across my ground points, but honestly don't fully understand how grounds affect the circuit past the point where they connect to the first component.

The volume pot is working, which tells me that the ground there is good. The fuzz pot isn't, which makes me think that there might be an issue there, either with the pot or the ground. I know the cap is oriented correctly. Or, maybe the Q2 transistor is bad, preventing the circuit on the C side from grounding.

touringbubble · Post by **touringbubble** » 04 Apr 2016, 17:49

So, here is the schematic I was working from. I noted voltages at each point where I could get a reading. Not sure what voltages should be at R3, but that's something to look at too. Working on pics of board now.

touringbubble · Post by **touringbubble** » 04 Apr 2016, 18:21

Board images. I've flipped the bottom to match the top for easier tracing.

touringbubble · Post by **touringbubble** » 04 Apr 2016, 18:25

One more, in case it helps.

induction · Post by **induction** » 04 Apr 2016, 19:30

Is the red wire that curls around the center of the board your input wire? If so, it's attached to the wrong side of C1. This gives a DC path through your pickups, and could conceivably be causing your problem.

touringbubble · Post by **touringbubble** » 04 Apr 2016, 20:50

Are you flipping kidding!? That is the input. I'll double check.

touringbubble · Post by **touringbubble** » 04 Apr 2016, 20:55

induction wrote:Is the red wire that curls around the center of the board your input wire? If so, it's attached to the wrong side of C1. This gives a DC path through your pickups, and could conceivably be causing your problem.

You nailed it! It is wired backward. I didn't check it because I was getting a signal to Q1. I had a signal because I'd effectively bypassed the tone cap.

I'll move the input and give it a shot. Thanks so much for taking the time to look everything over!

induction · Post by **induction** » 04 Apr 2016, 21:18

Cool. Let us know if that sorts out your biasing problem.

touringbubble · Post by **touringbubble** » 05 Apr 2016, 01:33

induction wrote:Cool. Let us know if that sorts out your biasing problem.

Sadly, the same issue exists with the input corrected.

induction · Post by **induction** » 05 Apr 2016, 15:11

Damn. I must admit I am flummoxed. The only suggestions I have left are:

1. Double-check continuity on all of your jumpers (especially the blue wires, looking for a possible break inside the insulation)
2. Replace Q2