Ugly Face problem, oscillator bleeding through

[soggybag]

I just noticed some oscillator bleed through in my ugly face. Normally it's very quite. When the Threshold is set to self oscillate, I can faintly hear the oscillator, in bypass. I checked it with the scope and I can see a wave form at the power supply. As I was testing I noticed that I could see wave form in the scope by just moving the probe over the 555! It seems the volume of the noise is tied to the Volume pot also. I noticed that if I turn down the volume pot, the volume of the noise gets lower, remember this is bypass mode.

Has anyone else noticed this? If so did you find any solution to removing the noise?

I'm open to suggestions. I need a few good ideas.

I was thinking of using a larger power supply bypass cap. The current cap is 22uf. Maybe add an extra smaller cap in parallel.

I was also thinking that shutting off the oscillator when in bypass mode would fix the problem. I had two ideas.

1) Put the third pole of the stompswitch between the power supply and pin 8 of the 555. The LED would have to be in series with this.
2) Since the threshold turns on and off the oscillator, the third pole of the stomp switch could be connected between 9v and the 22K resistor going into the threshold pot. This assumes the threshold needs to be closer to +9v to oscillate.

I don't quite get what's happening with pin 4 on the 555. This is labeled reset. Why is voltage on reset causing the 555 to oscillate or not in the first place?

[slacker]

The way the uglyface works is that the 555 is an oscillator with the speed controlled by the frequency pot and the LDR. The oscillator is turned on and off by the voltage on the reset pin (pin4) when the voltage on this pin is less than about 0.7volts the oscillator is off and when it's above this it's on.
When you turn the threshold pot all the way down the voltage on this pin is about 0.27volts which means the oscillator is off. The output from the 386 is also attached to pin 4 and feeds it a loud guitar signal, I think it's about 2volts if you hit an open chord. Whilst the guitar signal plus the voltage set by the threshold pot is above 0.7volts the oscillator is turned on, so what you get is the sound of the oscillator modulated by the guitar signal.
When you turn the threshold up so the voltage on pin4 goes above 0.7volts the oscillator will be on all the time. When the negative swing of the guitar signal pulls the voltage below 0.7volts the oscillator will turn off, that's how you can still play guitar on top of the self oscillating sound.

The easiest way to stop it oscillating when bypassed is to wire the stomp switch so that when the effect is bypassed it connects the wiper of the threshold pot to ground. That will stop it oscillating no matter where the pot is set.

[soggybag]

Thanks for the explanation Slacker. That made a lot of sense.

I'm using the standard blue 3PDT switch. Which leaves me with 1 pole for the LED. I was thinking I could put the switch and the LED where the Threshold pot connects to 9V through the 22K. I second thought, this method would also set the brightness of the LED. Better would be +9v > Switch here the signal splits to the LED and Threshold. When the effect is disengaged the Threshold is also disconnected from +9v. This would make a voltage divider so the value of the 22K would need to change.

Not sure if that would work. I need to turn off the oscillator and I have one switch to ground, that also needs to connect to the LED.

[slacker]

If you're using a 3PDT then if you do the simplest form of bypass using 2 parts of the switch like shown in the bottom example in this picture http://www.diystompboxes.com/pedals/images/DPDT3PDT.jpg that leaves you one section of the switch free.

Then to use the method I suggested wire the centre lug to ground, connect your LED and current limiting resistor to the top lug and connect the wiper of the threshold pot to the bottom lug.