PICTURE: Fuzz Face Debugging - / w Voltage Readings

ammalato · July 23, 2014, 12:16:09 AM

Hey Gang:

I have two, near identical Fuzz Faces clones here. PNP Germaniums. One pedal is working well, the other is not giving me any sound in the "on" position. I checked under the board for broken traces or bad solder joints, but couldn't find anything. I took readings of both circuits and put them side by side below for comparison. I can't make heads or tails of it right now, but there are definitely significant inconsistencies as you'll see below. Any suggestions on what might be wrong with the non-functioning pedal? The pedal DOES bypass fine. The only difference to note is the Chicklet Caps. The working one is a .018uf while the non-working is a .01uf...which shouldn't account for much of a change. Also, the reading on the + side of the 2.2uf cap in both cases provides a steadily "decreasing reading" in voltage when measured to ground.

LucifersTrip · July 23, 2014, 01:13:21 AM

You might not see a bridged or broken trace. Try comparing resistances between random points...and see if the in circuit resistances of the resistors are equal between the two.

Did you remove and test the two transistors?

duck_arse · July 23, 2014, 08:56:43 AM

we can't see your fuzz pot and wiring. all is tikkety-boo there? good to-ground pot?

Govmnt_Lacky · July 23, 2014, 09:31:33 AM

The Collector of Q1 should have the same voltage as the Base of Q2.

In your non-working case... it looks like Q1 Collector is -.004 and Q2 Base is at -1.298

Check the connection between the 33K resistor and the transistors

My guess is that there is a break in the trace leading to Q1.

ammalato · July 23, 2014, 04:24:13 PM

Quote from: duck_arse on July 23, 2014, 08:56:43 AM
we can't see your fuzz pot and wiring. all is tikkety-boo there? good to-ground pot?

i'll post pics later tonight / triple check the wiring, but everything there appears consistent with the working model.

Quote from: Govmnt_Lacky on July 23, 2014, 09:31:33 AM
Check the connection between the 33K resistor and the transistors My guess is that there is a break in the trace leading to Q1.

Thanks, Govmnt_Lacky - You were a tremendous help with my Phase 100, so I'm crossing my fingers on this bit of advice as well. Will check tonight and report back. A bad trace on the board WOULD make a lot of sense since all of the wiring seems identical to the other unit.

italianguy63 · July 23, 2014, 04:30:16 PM

Greg is right. The Collector Q1 and Base Q2 are on the same trace. They should match.

LucifersTrip · July 23, 2014, 06:49:46 PM

ammalato, I guess you didn't understand my suggestion to measure the in circuit resistances and compare.

for instance, if one side of the 33K is disconnected it will measure 33K, but in the good circuit, it will measure lower.

btw, even if there is a break in the trace leading to Q1, that doesn't explain why Q1C votlage does not equal Q2B. The trace between Q1C & Q2B is the one to check...which again, you'd be able to pinpoint by taking resistances.

ammalato · July 23, 2014, 10:24:43 PM

Lucifer - Roger all of that.

I'm happy to say, you were all correct and it turned out to be the tiniest fracture in the trace between the two. I joined it with some solder and Eureka, the pedal sprung to life. It's almost there though. I'm seeing about 6.5vdc on the collector of Q2 now. I know that's too high for proper FF operation and it probably should be closer to the 4.5-5vdc range. Since I don't have a bias pot in this unit, I'm assuming I'd want to replace the 8.2k resistor (or possibly solder another on the underside of the board) to get that tranny in range?

LucifersTrip · July 23, 2014, 11:14:54 PM

Quote from: ammalato on July 23, 2014, 10:24:43 PM
I'm seeing about 6.5vdc on the collector of Q2 now. I know that's too high for proper FF operation and it probably should be closer to the 4.5-5vdc range.

there are many that like it in the 6V+ range...you never know

Quote
Since I don't have a bias pot in this unit, I'm assuming I'd want to replace the 8.2k resistor (or possibly solder another on the underside of the board) to get that tranny in range?

you can do that if you don't have to alter it much, maybe 2-3K each way, otherwise the tone will change. many builders will alter the 33K instead

Govmnt_Lacky · July 24, 2014, 07:24:05 AM

Glad you got it up and working!

italianguy63 · July 24, 2014, 07:59:52 AM

Quote from: ammalato on July 23, 2014, 10:24:43 PM
I know that's too high for proper FF operation and it probably should be closer to the 4.5-5vdc range. Since I don't have a bias pot in this unit, I'm assuming I'd want to replace the 8.2k resistor (or possibly solder another on the underside of the board) to get that tranny in range?

This is my technique. Not the only way, nor do I claim it to be the right way. But it works well for me. I will leave it at that.

I shoot for 50% of the base voltage. So 4.25V to 5V generally.

I either put in a trimmer to adjust, then remove it and replace it (the 8.2K) with a fixed value carbon comp. Or, with some skilled wire bending, you can take a 10K cermet pot. Cut off the front leg-- and replace the 8.2K with a nice trimmer! A better option if you swap the trannies out (socketed).

Use the fixed value if you are trying to keep that "vintage" FF look.

MC

Electric Warrior · July 25, 2014, 09:24:26 AM

Quote from: ammalato on July 23, 2014, 10:24:43 PM
Lucifer - Roger all of that.

I'm happy to say, you were all correct and it turned out to be the tiniest fracture in the trace between the two. I joined it with some solder and Eureka, the pedal sprung to life. It's almost there though. I'm seeing about 6.5vdc on the collector of Q2 now. I know that's too high for proper FF operation and it probably should be closer to the 4.5-5vdc range. Since I don't have a bias pot in this unit, I'm assuming I'd want to replace the 8.2k resistor (or possibly solder another on the underside of the board) to get that tranny in range?

What does it sound like?

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PICTURE: Fuzz Face Debugging - / w Voltage Readings