So today I made an audio probe

[Willybomb]

I've got a Kentauride sitting here, and for whatever reason, it wasn't passing signal.  The TS-10 side was working fine but the Klon was no go. Once I realised that my dpdt footswitches were momentary, I switched that out and tried again. Still nothing. Everything looked ok, so I made an audio probe from some spare bits and off I went.

Using my laptop playing MP3s through my Focusrite 18i8's headphone output as a source, I found that it was passing signal fine but as soon as I plugged a guitar in..... nothing.

After some messing about, it turns out that I have to run the headphone amp pretty hard to get anywhere so the guitar just doesn't make enough volts to be heard.  Everything on this looks ok as far as bridges, solder, and cap orientation so I'm just going to have to go through and measure every part and see what I've done wrong. 

[Rockhorst]

I use a looper pedal to send signal through at guitar level. What you are hearing when using the headphone out is probably just cross talk. The switch terminals are little antennas that send signals to each other. Same goes for PCB traces. With a signal that has been boosted enough it becomes audible. It has tricked me in the past into thinking "well, it's passing signal" but actually it's not. You've just created a transmission line. Something to keep in mind.