dr quack mystery cap

[peterg]

http://www.muzique.com/schem/quack.gif

I have bread boarded dr quack and am not getting the classic 70s funk sound. There is no output unless the .05 cap bewteen the 47k res and the 2.2 res is bypassed in the sens pot to the range pot section.

Any ideas of what is going on?  I've checked all components and they are as per the scheme.

[Mark Hammer]

The envelope follower section - which is what you are describing - ends up providing a DC voltage (or close to it) that is proportional to the amplitude of the incoming signal.  In order to do that accurately, any incoming DC accompanying the input audio signal needs to be eliminated, so that the eventual envelope DC voltage is 0v when there is no audio.  The .047 (.05uf) cap is intended to block the DC, as well as infrasonic beats and other stuff in the audio range that will only confuse the envelope follower.

If there is no stray DC coming from whatever you are feeding the Quack, be it pedal or guitar, then it will still work, even with the .05uf cap bridged.  However, it might not work as well, because of all the other audio crap that cap is designed to keep away.  So, it is always better to have it than to NOT have it.

The bigger question is why you needed to bridge it to get the thing to work, and that would suggest either a bad cap or one or both solder joints connecting the cap in the circuit to be bad joints, or possibly an unseen crack in the board.  Since you're using a breadboard, that would tend to eliminate both solder joints or PCB cracks, making it likely a bad cap.

[peterg]

Mark - thanks for the detailed explanation. I ended up redoing it and got it going.

[teemuk]

Since you're using a breadboard, that would tend to eliminate both solder joints or PCB cracks, making it likely a bad cap.

Or simply a bad breadboard . 

The connections in those aren't usually great examples of mechanical sturdiness or electrical conductivity.

[Mark Hammer]

It's funny, you know.  You never see much written/posted about how to maintain breadboards so that the contacts remain good.  It's as if they are somehow immune from  anything and everything.  They AREN'T, but we tend to act and talk like they are.

[tubegeek]

It's funny, you know.  You never see much written/posted about how to maintain breadboards so that the contacts remain good.  It's as if they are somehow immune from  anything and everything.  They AREN'T, but we tend to act and talk like they are.

Mark:

do you happen to know any ways to keep a typical "solderless breadboard" minty fresh? I'm not very experienced with them and I'd love to know.

[Mark Hammer]

So would I,my friend, so would I.