need help debugging tube screamer

[sigmundfreud]

First posting here, pretty new to electronics. Built 5 working stompboxes to date and some little amps.

1.What does it do, not do, and sound like?
   OK but not great TS distortion and sustain at max guitar volume and PCB pots at max. When the level of the signal drops to about 1/4 of full, it goes fizzy, noisy, sounds terrible, cuts in and out.  Sine wave signal is not clipped like it should be at output.
2.Name of the circuit =
   ITS8 (Ibanez Tube Screamer 808TM Replica)
3.Source of the circuit (URL of schematic or project) =
   General Guitar Gadgets http://www.generalguitargadgets.com/pdf/ggg_its8_sc.pdf
        Printed and etched PCB from GGG.
4.Any modifications to the circuit?
   Y - "Expensive Boutique Modifications" http://www.generalguitargadgets.com/pdf/ggg_its8_mods.pdf
5.Any parts substitutions? If yes, list them.
   R7=10k. R6=1k. C3=220 nF. Added C3 bypass switch.  LM833 IC. Transistors are 2N3904. Had no 1uF film caps so used 2, 2.2uF electrolytic back-to-back (neg to neg) to make a bipolar 1.1uF cap.
6.Positive ground to negative ground conversion?    
   No
7. out of circuit battery voltage? =>
   9.06
Voltage at the circuit board end of the red battery lead = 9.06
Voltage at the circuit board end of the black battery lead = -.04

Q1 2N3904 - it looks like the biasing is wrong; insufficient drop between B&E
C =9.06
B = 3.7  [- General guitar gadgets stock TubeScreamer = 3.0]
E = 3.4 [- General guitar gadgets stock TubeScreamer = 2.5]

Q2 2N3904 - it looks like the biasing is wrong; insufficient drop between B&E
C= 9.06
B= 3.76  [- General guitar gadgets stock TubeScreamer = 3.0]
E= 3.4  [- General guitar gadgets stock TubeScreamer = 2.5]

IC1 LM833 - these voltages are about the same as Gen Guit. Gadg.
P1 = 9.06  4.3
P2 = 4.3
P3 = 4.6
p4 = 0
p5 = 4.23
p6 = 4.23
p7 = 4.23
p8 = 9.06

D1
A  = 4.27v
K  = 3.9

D2
A = 4.22
K = 4.3

D3 (red LED)
A = 4.25
K = 4.2

D4 (red LED)
A = 4.2
K = 4.25

D5
A = 9.21
K = 0

Have verified the solder joints (inspection, re-melted all just in case).
Verified each component is correct.
Verified orientation of polarized components.
Checked for and found no obvious short circuits (e.g between PCB tracks).
My best guess is that the transistors are improperly biased and are cutting out before they should but I have no idea why.

[Slowpoke101]

Welcome to the madhouse forum.

9V on pin 1 of the IC is odd. Look for a short circuit (piece of solder, stray wire) between the two points shown in the picture.

[antonis]

My best guess is that the transistors are improperly biased and are cutting out before they should but I have no idea why.

2N3904s exhibit about double current gain of 2N4401s at 340μA working current..
You may have to alter base bias resistors - but this isn't your major issue for the time being.. 

[sigmundfreud]

Thanks -
Slowpoke - made a typo - pin 1 on the IC is actually 4.3 like the other pins, where it should be.

[Slowpoke101]

Typos happen quite often - not to worry.

Don't be too concerned about transistor biasing yet. Just make sure that the transistors are installed correctly. Sometimes manufactures do change the pin-outs for whatever reasons.

Go over your modifications thoroughly. If you are uncertain about something, just ask and someone will usually reply.

But something to try that shouldn't take too long - assuming that you used a socket for the IC. Change the IC to a different type - the RC4558 is usually used in this circuit and plays well with it. Other ICs sometimes do not - they work but can sound unpleasant.

If the effect is still not working correctly can you upload some clear pictures of both sides of the circuit board and your wiring. Everything helps.

[sigmundfreud]

Solved, mostly.
The major problem was that I mixed up two green leads coming off the board, wiring them to the wrong pot/switch respectively.
It now distorts well, and the tone diminishes smoothly without sputtering and noise.

The transistor voltages are still wrong; that is the emitter is still larger than the base. So I guess they are not turning on, and perhaps what I have is a TS without the input and output buffers.

[thermionix]

The transistor voltages are still wrong; that is the emitter is still larger than the base.

3.7>3.4

[antonis]

The transistor voltages are still wrong; that is the emitter is still larger than the base. So I guess they are not turning on, and perhaps what I have is a TS without the input and output buffers.

Hmmm...

400mV V_BE shouldn't be considered as a "healthy" Si junction forward voltage drop but those buffers DO work - otherwise you shouldn't hear anything..
(when a BJT CC (or CE) amp is at cut-off state, no signal comes out of its Emitter (or Collector)..)

As far as your input signal peak is less than 3.4V you shouldn't worry about buffers performance..

[sigmundfreud]

Both transistor voltages are now
C =9.06
B = 2.7  [- General guitar gadgets stock TubeScreamer = 3.0]
E = 3.3 [- General guitar gadgets stock TubeScreamer = 2.5]

B<E rather than the other way around.

[thermionix]

So that's changed since you posted voltages above?  I wonder if you're seeing meter loading.  But yeah, what antonis said.

[sigmundfreud]

Perhaps that's the explanation.
I used a different DMM (last readings were with an o-scope) and got B=3.6 and E=3.2 this time.
Never heard of meter loading, thanks for the tip.
With that settled, I'll box it.
Cheers and thanks for the help, all.

[antonis]

Never heard of meter loading,

Until you used a meter of input impedance lower than some TΩ.. 

In case of a low input impedance DMM (1M say..) just imagine 1M resistors placed in parallel with R2+R10 and R12+R10 and calculate "new" Q1 & Q2 base voltages..