How to build & use audio probe?

[markr04]

I keep seeing 'use an audio probe to debug'. Okay. I went to Geofex and looked at it. There's only one thing: a drawing. And the drawing is pretty sketchy at best. This is how I understand it:

Put .5uF cap in series between signal lead and + tab of plug.

Take ground wire from ground tab of plug and ground it back to the amp??

Plug the 1/4" jack into the amp.

And this is a guess: Use the signal lead to touch around on the board?

Can anyone elaborate? Or post a link to a better picture or diagram. Thanks so much!

[petemoore]

. To connect ground from the amp to the circtuit, connect circuit ground to the sleeve of the cable going to the amp.
 To finish the probe, use four alligator clips connected [one at each end] to two wires ends. You now have two Test clip wires...
 Connect one test wire to the tip of the cable coming from the amp. Connect another test wire to the other end of the capacitor. Done
 signal wire to the amp needs an inline DC Blocking capacitor, to keep DC out of the amp, and ground must be made to the circuit from the amp.
 Use your schematic to trace the signal path through the circuit, and test where it's getting through/not getting through at different points in the circuit.
 I debugged a HMP today, just using a screwdriver connected to my thumb, working < way from the output toward input following signal path, and found I was losing it at the second Opamp's input [this is where with the gain turned up the 'thumbuzz' got real quiet, [thumbuzz...that sound when you plug your guitar in and grab the end of the cable]. I like thumbuzz debugging cause setting it up includes finding a 3'' or longer conductive shaft or wire, and it seems to do just great finding problem area.
 Anyway I found [with the DMM] that the second opamp signal path voltages were off, [ they should be about 1/2 of supply voltage] one was sitting at 1.02v of 14v supply...then switched to 'beep' mode and found the connection from one of the pots was wrong...kwik change of that fixed it right up. I keep a test clip at the end of the - lead of the DMM and like sometimes to clip on the offboard wires and check where they're actually going :oops: .
 Applying the DMM and audio probe can find the problem quickly, often the solution is also quick...finding 'that' short [should you ever get one] can be a laborious visual inspection though.
 There are many ways to apply a DMM, I used to look through the manual and figure new ways to use it. Check your math against the DMM and some resistor equations for hours of fun as a last resort...I've found bad resistors and wrong resistances this way...in circuit...calculating all the possible paths, lifting one end of a resistor that won't read what it should in the circuit because of the alternate path.
 DMM Don't measure around through caps so good er something ... I haven't been able to verify.
 You can put it in beep mode and check connections that are supposed to be there and see if they're made....I leave component lead long enough on top of the board for the DMM's probe to reach them...usually...otherwise I've found myself on top and bottom of the board trying to 'verify'...lol...I don't like trying to be two places at once.

[Greg Moss]

With regards to constrtuction - Some people like to have the cap soldered to the tip lug of a 1/4" jack, and an alligator cilp soldered to the ground lug of the jack via a length of wire.  Then you can plug a guitar cable  the probe jack and your amp, clip the alligator clip to the ground of the effect, and probe with the remaining lead of the cap.

Is this the illustration you saw?
http://www.geofex.com/FX_images/audioprb.gif

I just made a new probe but instead of leaving the second cap lead bare, I soldered a second wire with a second alligator clip.  This allows me to clip the probe to be able to test certain part of a circuit "hands free" in case I am adjusting a pot or something.  If I need something more probe-like, I just put a piece of stiff wire in the probe clip, and poke around with that.

Hope this helps - My writing is somethimes confusing!

Greg

[markr04]

Thank you both for your help! Although I poked around with the probe, I'm not finding the problem with Dr. Quack. I'll post the question in the forum.

[Nasse]

I have used lately just a car radio amplifier, a leftover mobile radio cased 5 watts speaker (I have a 12 volts power supply in my bag), and there is some odd potentiometer and dc blocking cap. But some buffer and ac mV  or VU meter would be nice. They sell nice Fluke crocodile clips and all kinds of nice grippers but they are not cheap... On Friday I put banana sockets to the amp´s power leads, so it is more professional looking and faster now. Maybe I could put a 386 in and some 9 volt battery...no need for power supply, and I could play guitar trough it...

[Paul Marossy]

Here is a drawing I made for the audio tester, based on the one at GEOFEX.com:

http://www.diyguitarist.com/PDF_Files/DIY-AudioTester.pdf

[Greg Moss]

Here is a drawing I made for the audio tester, based on the one at GEOFEX.com:

http://www.diyguitarist.com/PDF_Files/DIY-AudioTester.pdf

That's actually the diagram I was originally going to cite, but I forgot the link.

I often wish I had a volume pedal to use with the probe since you can get a really wide range of signal strength while poking around a circuit.

I am going to be traveling with a band in a couple of months, and I'm thinking of bringing along a smokey type amp to use as a monitor for the probe.  Maybe I should just throw together a lm386 type project.  Do those work well as headphone amps?

[David]

Here is a drawing I made for the audio tester, based on the one at GEOFEX.com:

http://www.diyguitarist.com/PDF_Files/DIY-AudioTester.pdf

That's actually the diagram I was originally going to cite, but I forgot the link.

I often wish I had a volume pedal to use with the probe since you can get a really wide range of signal strength while poking around a circuit.

I am going to be traveling with a band in a couple of months, and I'm thinking of bringing along a smokey type amp to use as a monitor for the probe.  Maybe I should just throw together a lm386 type project.  Do those work well as headphone amps?

Better believe it!

There's a thread here from last December discussing the HeadWize article by, IIRC, Alan Campbell.  I get EXCELLENT results with that headphone amp.

[Paul Marossy]

You know, even though you don't have a volume pedal, you can get a little plastic RadioShack project enclosure and put a volume control in it. I made one, and it looks like this: http://www.diyguitarist.com/Images/ExtVolControl.jpg
I made this as a pedalboard mounted overall output level control to my amp, but you could also use it with an audio probe, too.  

[D Wagner]

This is what I am using as an all-in-one tester.  http://www.talkingelectronics.com/Projects/MiniBenchAmp/MiniBenchAmp.html

Scroll down to near the bottom of the page, and you can use the artwork to generate a PCB for your personal use.  For reference, it is 4" x 1" if I remember correctly.

I soldered a pin to the tip, and have two alligator clips on 24" pieces of wire.  I didn't get fancy with the battery holder like they did.  I just taped it onto the back of the PCB.  I also have a 1/8" stereo jack wired as "dual mono" to use my stereo 'phones.

Derek

[Paul Marossy]

That's a cool little gizmo...