MXR BlueBox clone intermittent noise

[il_mix]

Hi, everyone.

I've closed up my BlueBox clone. Everything seems fine. BUT, when plugged in an amp, when the guitar is not playing (but the volume knob is turned on) an intermittent noise came out.
It looks like the pedal is amplifying a feedback when the guitar is near the amp. It disappears when the guitar is more the 1.5mt from the amp. Also, the intermittent frequency is lower when the guitar is further; the volume is instead wuite the same.
The sound that came out is similar to the one you get when there is a cell phone on the amp (but processed by the BlueBox).

Now, I try to explain my thought.

I have a fuzz based on a 4558 OpAmp (like the BlueBox), that is a sort of "clipper". It has a lot of background noise since it amplify very small signals in input.
The BlueBox has a lot of fuzz, too, but it seems to have some sort of "gate", so that when you are not playing, no sound came out.
Question 1: how is this "gate" implemented?
I suppose that Q1, Q2 (see the schematic) are the ones that implement the gate function. Like that if the base-emitter voltage drop is too low, there is no current flow and no sound will came out. Am I right?
NOTE: the noise can be heard turning off the octave, so the fuzz stage is the one that generate the problem.

Now, my idea is that there is a feedback from the amp to the guitar (due to ground problems or whatever), so an intermittent noise is generated from the guitar. This noise can be hardly heard, but when the BlueBox is turned on, it is high enough to go beyond the "gate level", so it will be amplified, distorted and send out, and becomes well audible.
Question 2: any idea about what can be changed to avoid the problem?

I've used the TonePad schematic, that I'll attach for fast reference.


Thanks!
MIX

[duck_arse]

I played with this on the breadboard once, and it just ocillated, all the time, sig or not.

IC1A gooses the signal enough to drive the rectifier at D3//D2, which provides some collector volts for Q2 and Q3 to play with. no signal means nothing from these transistors. IC1B gives more gain to signal, and drives Q1 base via voltage divider R7//R9. the collector load goes to 9V, so this transistor provides logic level pulses for IC2B and drives Q2 base to provide the "dry" signal. IC2B divides the clocking by 2, and feed IC2A, which divides by 2, which feeds Q3 base with "dry divided by four". the mix pot chooses which of the dry//4 sig goes to the vol pot.

I couldn't get it to work (there is a thread somewhere on the process, you could search). I don't know if this is any help.

[il_mix]

Hi duck_arse,

thanks for the answer.
Your post helped to better understand what is going on (well, I have to read it a little more carefully), if not how to let it work better.
Mine didn't "always oscillate". As said, it just generate this intermittent (oscillating) noise when some feedback get in to the guitar from the amp. Maybe you had a less stable behavior due to the fact that you have a breadboard circuit? (mine is enclosed in an aluminium box)
I'll search for the post you mentioned (didn't find it before, but I try again).

[duck_arse]

here is the thread I was thinking:

http://www.diystompboxes.com/smfforum/index.php?topic=105930.0

and one I'd fergotten:

http://www.diystompboxes.com/smfforum/index.php?topic=105991.0

[il_mix]

I've already found the second one and I'm carefully reading it. I will look to the first one, too.
Thanks!