DIY WH10 wah - Buffered bypass problem

[QuentinBzt]

Hello all,

After searching for long hours all over the internet, I come to this place as my last hope to mae my build work.

I built a WH10 V1 wah from scratch: drawn the schematic on Kicad, then the PCB, had the PCB built and put all the components in.

Problem: the wah works fine when engaged, but there is litteraly no sound coming out of it in bypass mode. Since it is not true bypass, there's something wrong in the buffered bypass circuit but I can't figure out what. I want to keep the buffered bypass, to make it as close to my original as possible.

Something tells me it has to to with the switching system, which I never really understood on the schematic:



Looks like the signal is going nowhere when the switch is disengaged. Can anyone help with this?

Thanks a lot!

[GibsonGM]

Welcome to the forum, Quentin!  We'll get you up and running.

Could the switch be wired incorrectly? Please post the entire schematic.

Have you built/know how to use an audio probe?  It's one of the first steps in debugging...

[QuentinBzt]

Thanks for the reply! I had actually never heard of this. I've looked into it and it seems fairly simple. I've only got some 68 nF capacitor left, will it do the trick?

EDIT: I've done a quick test, and when the probe touches the output jack with the switch OFF I've got signal through the amp. The tricky part is I don't know which components are supposed to let the signal come through with the switch OFF.

Here is a picture of the complete diagram (link to the downloadable version: https://www.tonehome.de/ibanez/power-series-10/wh10-wah-pedal/)



The switch in the original one is an SPDT switch, and only two wires connected to it. I've tried several wiring configurations, this is the only way I'm able to get the LED to light up and the wah effect when engaged.

A picture of the original circuit:



A picture of my "homemade" PCB:



Don't mind the yellow cloth wire (I forgot a track when I designed the PCB on Kicad) and the jumpers on the input and output jacks (I had to bridge them to get a signal, otherwise it wouldnt work with the switched jacks I'm using).

[antonis]

Timeo JFEts et dona ferentes..

P.S. Welcome..

[QuentinBzt]

OK, I think I got the reference to the Trojan horse  Can a JFET cause that kind of problem? knowing that the wah pedal works perfectly fine when the switch is engaged?

[antonis]

Can a JFET cause that kind of problem?

Personally speaking, I'd trust a burned out BJT more than a brand new JFET..

[QuentinBzt]

When using the audio probe to test the JFETS, is sound supposed to be heard on all three lugs? Sound is only coming from 2 of them (drain and source which seems to make sense, but I'm a rookie with this stuff...)

Also for R13, R14 and R15 there is no sound on one side of them.

Q6 (2SC1815) has no sound on all three lugs, wether the switch is on of off.

I don't really know what to do with that information, so if anyone has ever worked on a similar type of circuit, I'm all ears 

[PRR]

...Sound is only coming from 2 of them (drain and source which seems to make sense....

If it is a SWITCH, yes, just two "sound" pins.

A wall light-switch is also "3 pin". Power in, power out, and the 'control' (finger lever) which does NOT have power on it. (As you say, makes sense.)

In a FET used as a switch, S and D pass signal, G does not. It only goes a few Volts up or down to do the change-over. Just once, so at most it is a 'click'.

[PRR]

Replace C14. (Or measure stuff both ways, but I guess that's not happening.)

[ElectricDruid]

The gist of the switching circuit is that there are two paths:

1) Through the effect. From the input buffer Q01, to IC01 one half, then the other half, then to Q02, the FET. And then out to the output buffer Q04.

2) Bypassed. From the input buffer Q01, straight across the top to Q05, then to Q03, another FET, and then to the output buffer Q04 again.

The two FETs Q02 and Q03 turn on either one path or the other, but not both. Those paths are controlled by the voltage on their gates, which is controlled by the Effect On/Off switch directly for Q03, but which is inverted by Q06 for the other FET Q02. Basically, when the switch is open, R19 pulls the voltage high for Q03. That also turns off the LED. But high voltage on the base of Q06 turns it on, so it pulls the voltage *low* for Q02. When the switch is closed, you have the rverse situation. Switch is on, transistor is off, LED is on.

Hope that helps you get a handle on it.

Tom

[QuentinBzt]

Replace C14. (Or measure stuff both ways, but I guess that's not happening.)

What makes you think C14 is faulty?

@ElectricDruid, wow that is SO helpful, thank you. I understand things way better, that's gonna be very, very helpful for my troubleshooting. So Q06 is not supposed to give me sound with the probe, since it's not really involved in the "sound circuit path", right?

I guess the next step for me is getting a multimeter.

[ElectricDruid]

So Q06 is not supposed to give me sound with the probe, since it's not really involved in the "sound circuit path", right?

Exactly! It's only part of that control circuit that switches the FETs on or off.

I guess the next step for me is getting a multimeter.

It's a useful tool, for sure!

[PRR]

...What makes you think C14 is faulty?...

If C14 is short, Q03 will never turn-on and pass dry signal.