Audio probe build questions

[Garydean]

I'm looking to build an audio probe, looks easy, hardly anything to it. Basically the probe is a capacitor in series with a 1/4" plug into an amp with the sleeve grounded to the chassis you are working on.

What type and size cap is used here?

I have an old heathkit signal tracer used to troubleshoot radios. Is this the same thing?

The signal that's inserted into the amp being fixed, can that be from a signal generator at 1,000hz sine wave or is that too high?

Am I just tracking the signal path in the preamp and thru the PI, and listening for changes in the signal?

Thank you!!

[Enzo]

Yes, the Heathkit is exactly the same thing.

To make one, the size of the cap is not important. The voltage must be ready for anything in the system under test. So I'd use a 600v film cap. I have a lot of 0.047uf caps, is that is what I'd use. But we are not going for fidelity here, all we want is to hear that signal is present or not and if it is grossly distorted or not. SO a 0.022 or 0.01 or whatever will be fine.

The probe tip is one leg of the cap, and the other leg goes to the input of the tracer. I would add a high value resistor from that point to ground - the tracer input. That will provide a charging path for the cap.

You can inject any signal you like for testing. I tend to use 100Hz instead of 1000Hz, because I can listen to 100 a lot longer than I can 1000. 1000 is piercing. If I need 1000 or higher, I can always adjust my signal jenny. I also have a music feed from the shop stereo handy at all times.

And one signal I use all the time for go/no-go tests is simple hum. Touch the input of something with a screwdriver or even my finger and it injects a hum. if all I want is to see if signal passes, that is good enough.

[Garydean]

Thank you Enzo for the education.

Gary

[gbono]

Probe for tracing noise

Any ideas on what to use (i.e. probe design) to trace down noise in various stages of an audio amplifier? O-scope not too handy here (unless you have very good resolution). I was thinking of using an isolted pre-amplifier with some type of output limiting and a pair of head-phones. THX

[Enzo]

You can use phones, or a speaker. Google "Heathkit signal tracer" for a lot of hits on it. There are a bunch for sale cheap on eBay. Competitor Eico made them as well, so did Knightkit, and probably a lot of other people.

Use the probe design as described here above, it is very simple. In fact, just make the probe, and use a little practice amp or something for the rest of it. The little cheap amps they include with starter strat-packs are perfect for it.