Building a Wah - help

[flügenheimer]

I've been trying to build a clon of the Vox 847:
http://www.schematicheaven.com/effects/vox_847_1980_LAH.pdf

But I just can't get it to work. I only get a volume pedal, that has "no signal" when the pedal is pressed forwards. It makes no difference if the pedal has the battery switched on or off.

http://i222.photobucket.com/albums/dd16/flugenheimer/100_3981-1.jpg
http://i222.photobucket.com/albums/dd16/flugenheimer/100_3982.jpg

I have tried to change the 4uf capacitor and tried with a electrolytic capacitor with no change. I am a newbie in building pedal, and i don't know how to figure out what's wrong.
What could be wrong?

I hope you can help me.

Here are the transistor voltages:

Q1
C: 0,295 V
B: 0,003 V
E: 0 V

Q2:
C: 0,2 V
B: 0,016 V
E: 0 V

[Paul Marossy]

Are you sure that your inductor is actually in the circuit or not defeated somehow? That sounds like symptoms of a non-functional inductor.

[Gus]

please read
http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

Transistor and other node voltages will help people to help you.

[flügenheimer]

Are you sure that your inductor is actually in the circuit or not defeated somehow? That sounds like symptoms of a non-functional inductor.

How do I check this?

[panterafanatic]

ohm meter, simple lightbulb test, anything to show electricity is flowing through

[Gus]

Lightbulb test is not a good idea for a fine wire inductor.

For voltages go to www.geofex.com and read "The Technology of the Wha Petal"

Q1 collector voltage should be closer to 4.5VDC Q2 is biased from Q1

Check the resistor values and connections around Q1

[flügenheimer]

A little update on my project:
I solved the problem with the wrong voltages - the power supply connection was wrong and I changed a dead resistor.

Now I have a new problem:
I get the wah sound (a bit thin for my taste - but that can be changed later) but when the pedal is pushed all the way forward, the sound disappears.

Q1
C: 4,3 V
B: 0,69 V
E: 0,09 V

Q2
C: 8,67 V
B: 3,67 V
E: 3,21 V

[Gus]

Q2s base reading does not look right
Q1s collector is at 4.3VDC
Q2s base is at 3.67VDC

4.3VDC - 3.67VDC = .63VDC

Q1 C is biasing Q2 by the 470K, Q1 C to Q2 B.  .63VDC/470K =1.34ma.  That is high base current for this circuit.

Check Q2 emitter resistor value(R10 in the first link did you install a 100 or 1K ohm instead of a 10K?) and the 470K resistor(R4) value to the base of Q2.

[flügenheimer]

Q2s base reading does not look right
Q1s collector is at 4.3VDC
Q2s base is at 3.67VDC

4.3VDC - 3.67VDC = .63VDC

Q1 C is biasing Q2 by the 470K, Q1 C to Q2 B.  .63VDC/470K =1.34ma.  That is high base current for this circuit.

Check Q2 emitter resistor value(R10 in the first link did you install a 100 or 1K ohm instead of a 10K?) and the 470K resistor(R4) value to the base of Q2.

Just checked the resistors mentioned, and they were the right values.

[Gus]

My mistake .63VDC/470K = .0013ma not 1.3ma that looks OK.

What value sweep cap?

With no power on the wha use a meter set to ohms.  Measure wiper to one end of the pot as you move it from end to end then measure the wiper to the other side as you move it from end to end.  This is to check for contact of the wiper to the pots resistive trace maybe the wiper loses contact when all the way forward.

[flügenheimer]

I have now got my wah pedal to work, and it sounds good clean. but the problem is if I have a distortion/overdrive pedal after the wah - then the wah sounds dull and the sweep range is very narrow. I then tried to build a buffer and put it at the end of the wah, but still the same problem. then someone suggested a source follower at the end of the circuit - but still the same problem.

Any ideas?