Audio Probe.. Not Anal Probe

This is not a recap of the book Communion: A True Story (1987). This is some notes on audio probing and troubleshooting your circuit  These are not my notes since I am trying to grasp how to accomplish this technique. I have just found notes from various places and I am archiving them here.

guitarmageddon Wrote:

OK, my world was turned upsidedown by the building of an Audio Probe. Can’t say enough good things about using one. Put simply, it lets you evesdrop on what’s happening at any given point in your circuit.

You need your guitar plugged in to the pedal, the pedal powered up and on and the amp on. The guitar needs to be producing sound. If you’ve got one, record something into a looper so you’ve got your hand(s) free.
The probe sheild is connected to the pedal earth (case or out put sleeve) and the tip goes to the amp’s input.
Start by strumming the guitar and touching the probe tip to the input jack tip. You should have signal as if in bypass.
Try the switch next, lug 4 (of the BYOC type bypass) should also have the input signal.
Now lug 5, the effect input, should still hear input signal.
After this it’ll get more interesting.

Check your board’s input, then, followig the schematic, trace the audio path as it flows through the curcuit. listen to both sides of each componant. (Anything connected to ground will be silent on the earth side)
You should be able to hear changes, capacitors affecting the tone, resistors dampening the signal strength, etc, Transistors should amplify the signal from what it sounds like at the base, to what it sounds like at the collector. Don’t forget to listen to the pot lugs either.
Assuming your pedal produces no output, what you’re looking for is where the audio signal stops.

If you’ve got some idea of where the problem might be, go straight to there and listen up and downstream.

Also, if things go dead inexplicably at some point, check that there isn’t a short to ground by using a DMM in continuity setting. Connect one probe to ground (screw hole of enclosure) and probe the signal path. Unless there’s a short, there shouldn’t be any continuity.

duhvoodooman Said:

When I first started using it, I found it much easier to understand the hook-up & function of a signal probe by thinking of it merely as a “movable output jack”. Plug your output cable into the jack of the signal tester, hook the black wire clip to ground (the solder tab for the sleeve of the input jack is always a good choice), and then touch the red probe to wherever in the circuit path you want to check.

more to come…