Gaussmarkov Blues Driver not working

[duck0872]

Hey everyone, i need some help debugging a gaussmarkov blues driver. 

The source for the project with layout, schematic and diagram are located here http://gaussmarkov.net/layouts/bd2/bd2-project.pdf

I built the version with no vref op amp.  I followed the parts list exactly with the exception of C12 and C13 which were substituted with .0068 instead of .0056 due to parts availability.  The IC is a TL071, the nfets are J201, the PNP's are 2N3906, and the NPN is a 2N3904.

There is absolutely no sound coming out when the pedal is on.  I have verified the wiring is correct, as are the orientation of all transistors, the IC, the diodes, and the electrolytic caps.  I also could not find any solder bridges.

My voltage readings are as follows:   

voltage into the board- 9.37

IC1- TL071     

p1- 9.34 
p2- 8.86   
p3- 8.86   
p4- 9.34   
p5- 9.34
p6- 8.80
p7- 9.37
p8- 0

q1 (201)
D- 9.36 
S- 9.36 
G- 7.65

q2 (201)
d- 9.37 
s- 9.36 
g- 7.24

q3 (3906)
e- 9.37
b- 9.36
c- 9.33

q4 (201)
d- 9.36
s- 9.36 
g- 4.64

q5 (201)
d- 9.37
s- 9.36
g- 7.22

q6 (3906)
e- 9.37
b- 9.36
c- 9.33

q7 (3904)
e- 9.25 
b- 6.34 
c- 9.37

[antonis]

No transistor (FET or BJT) is in working condition..
(that indicates to me a major lack of GND - but what's puzzling me are the Gate & Base voltages..)

First of all, ensure that you have the correct Vref at junction of R25/R26/R21/+C22 and then for the same voltage on FETs Gates

Also, I think that your IC1 needs a 180 degrees rotation (upside down)

P.S.
Maybe you didn't take in mind the following Notes paragraph:

Note that the transistors can be socketed with in-line sockets. Q2, Q3, and Q5 require 4-hole sockets so
that one lead can jump over a trace. We have included pads and holes for the extra (unused) socket. Do
not use socket connected to the square pad. If you are not using sockets, then do not use the square pad.

[duck0872]

So I went back and found some possible bad solder joints and resoldered.  This is what I'm getting now

IC1

1- .14
2- 4.68
3- 3.17
4- 0
5- .14
6- 4.68
7- 9.35
8- 0

q1-   8.68 4.84  3.81 (d s g)
q2- 9.35  4.84  4.86  (d s g)
q3- 9.36  8.67  4.87  (e b c)
q4- 6.61  5.88  6.60  (d s g)
q5- 9.35  5.88  0       (d s g)
q6- 9.35  6.61  0       (E b c)
q7-3.19  2.76  9.35    (E b c)

I'm guessing those two 0's on the q's have something to do with it, so I'm checking those paths right now.  I did skip over the traces as directed in the instructions, and everything is oriented as shown in the diagrams.  Now when it is plugged in I can hear static but no guitar signal is going through.  i can definitely hear the volume and the tone pots working.  I checked the vref at the intersection r26 etc. and there is 4.66 volts.

[duck0872]

Well I've looked and looked and still have no idea what's wrong.  There's good ground continuity to the board, the caps are all the correct sizes except c14 and 15 which are 15nf and not 18nf, and all resistors are the correct values.  I'm out of ideas.  Debugging is not my strong suit.   I have an audio probe, but I'm having trouble figuring out the exact audio path through the schematic.

[PRR]

> q6- 9.35  6.61  0       (E b c)

Q6 Base either not really soldered, or Q6 is "dead" (actually burned, wrong pinout, backward, or wrong polarity type). Less likely: R13 not really connected.

> q2- 9.35  4.84  4.86  (d s g)

It seems very unlikely Q2 should be "almost zero" voltage gate to source. Part-volt or more is most likely. This could be short G-S.

[antonis]

I have an audio probe, but I'm having trouble figuring out the exact audio path through the schematic.

You roughly can test on:
IN -> Q1 drain -> Q3 Collector -> D1/D4 junction -> Q4 drain -> Q6 Collector -> Level pot lug 2 -> Op-Amp pin3 -> Op-Amp pin 6 -> OUT.

But you first have to settle all DC voltages to a working condition.. 

[duck0872]

Thanks for your help so far guys.  I replaced Q6 and made some progress.  The circuit is now working, but the output is very low.  I can definitely hear the gain, tone, and level pots working, there's just a large volume drop when the circuit is turned on.  Here are the new voltage readings.

IC1
1- .14
2- 4.67
3- 3.16
4- 0
5- .14
6- 4.67
7- 9.35
8- 0

Q1  (dsg) 8.67  4.84  3.82
Q2 (dsg)  9.35 4.84 4.86
q3 (ebc) 9.35 8.67 4.87
q4 (dsg) 8.67 4.84 2.32
q5 (dsg) 9.35 4.84 4.85
q6 (ebc) 9.35 8.67 4.86
q8 (ebc) 3.18 2.77 9.35

Using the audio probe I found good signal all the way up to q6 collector, but very little at the level pot lug 2 or beyond
-

[antonis]

Using the audio probe I found good signal all the way up to q6 collector, but very little at the level pot lug 2 or beyond

I suppose that you checked it with pot all the way up..

If so, you have to check R18 (value & continuity), C13 (value & short), C14 (value & continuity)
(you can skip C12 & C15 for the moment..)

[duck0872]

That was it, c14 didn't have continuity to the board.  Resoldered and everything is working correctly now.  Thanks for all the help everyone.  Maybe one day I'll be better at debugging, every time I do it though I learn a little bit more.  Thanks again!