GGG ITS8 Problem - very little gain!

[djaveh]

Hey,

I have recently attempted to make a general guitar gadgets ITS8 (the tube screamer clone) not from the kit and I am having trouble debugging my circuit.

I have followed the schematic and the layout from this: http://www.generalguitargadgets.com/pdf/ggg_its8_lo.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a
with the exception that I use a linear 25k pot for the tone (R20).

The problem I am having is that I seem to have made a distortion pedal that doesnt distort! The signal passes almost completely unaffected by the circuit. I have an oscilloscope and I am passing a 1V p-p sine wave (is this a sensible level?) through the circuit and measuring the output. I am also using a 9V powersupply

A basically only ever get a sinewave out, no matter what configuration the pot or diode switch. The level pot changes the amplitude from 0 - 1.6V p-p which I assume is much too small - I thought the output should be up to 4.5V.

The drive pot only has a small effect on the amplitude from 1.2 - 1.6V with max level. Switching the diode gives a slightly different output - either a normal sinewave or a slightly lower amplitude ever so slightly "bent" sinewave.

The pin voltages are:
IC1:
1:4.59
2:4.71
3:4.59
4:0
5:4.59
6:4.59
7:4.51
8:9.23

Q1
C:9.23
B:4.04
E:3.61

Q2
C:9.23
B:4.04
E:3.62

Compared to the values given by GGG
http://www.generalguitargadgets.com/pdf/ggg_its8_instruct.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a my base and emitter voltages are slightly higher than would be expected.

I would be greatful for any help you could offer me.

Cheers!

[Kipper4]

i know nothing much but have you checked your transistors orientation against the schematic?
I'm sure someone knowledgable will be along shortly and put you on the right path.
I have built this and it was a devil to debugg for a relative newcomer.
I ended up prefering the sound of and making the Son of Screamer.

[djaveh]

Hey, thanks for you your reply!

I will double check the transistors. I know what you mean, my electronics is not so great and this sort of half working kinda problem is beyond by skills of interpreting the schematic.
Cheers

[djaveh]

I checked the transistors and they are fine.

The funny thing I found was that if I shorted the pin 1 on the tone pot and pin 3 or the level pot - the circuit worked! Therefore there must be a problem with the tone bit of my circuit. Probably a stupid mistake with the circuit?

[oskar]

Hello!

What is the voltage on the point marked Vr?
I'd start by checking the R6/C3 net. If the connection to ground is open you would end up with unity gain.
You would also end up with unity gain if the negative feedback of IC1a is shorted. This could happen by damaged
components like D1/D2 being fried.