Pedal debugging using a Function or Audio Generator, How do I hook up to input?

[brokenbones]

Hello,

I'm trying to use the conventional Audio Probe debugging method to isolate problems with a recent pedal build. Instead of just using the guitar or computer to generate my tone/wave, I have an old audio generator. My question is how exactly can I hook up the input audio signal from the generator to the pedal? Do I just hook up the pos. and neg. leads from the audio generator on to the input jack of the pedal via alligator clip? Or should I use the clips and attach those to a 1/4" jack? Or do I need another type of device?

TIA,
-Eric

[lukeferg]

Not sure about the audio generator. What kind of outputs are they? RCA? If they're RCA then you'll only use one channel and the tip will be tip and the sleeve will be sleeve. You could use either red or black.
If it's speaker outputs then don't hook that up.

The other way is to download a tuner on your phone. I've got one that plays a constant note that I plug into the pedal.

[brokenbones]

The generator is a Heathkit IG-72 and the outputs are your typical banana plug type you'd see on a DMM.

I'm trying to avoid using my devices or computer. Using an online audio generator I managed to fry my mac's sound card. Thankfully I was still under Apple warranty.

[lukeferg]

So those types of plugs are invariably speaker outputs, not line level or mic level audio outputs.
I would steer clear of those definitely unless someone more knowledgable than me tells you differently.

Another option would be to get an old CD player with a headphone output and use that. It won't be a single note unless you have a rather boring cd collection but it's a start.

[anotherjim]

Just use the range attenuator to keep the signal small (and that load switch always on). Not higher than 1V.
I would get some coax (old TV aerial flex will do) and fit banana plugs one end and a 1/4" jack the other. It might be a little extra safety to put a 10k resistor in series the jack tip for some protection.

I find a 1/4" plug is the easiest way to hook up to a completed pedal circuit. I find croc clips fall off when you aren't looking and if the the fault you're chasing is "no sound" -  well, it will be a while before you notice. 
For breadboard use, I just have a 1/4" socket with single core wires fitted to stick in the breadboard contacts.

[lukeferg]

Like I said, someone more knowledgeable than me.
Jim is right. Keep the voltage low and you'll be ok.

[blackieNYC]

It's a fine idea. You want to be as accurate as possible. Here's what I think is essential. You need to see how flat your setup is.
1-generator or iPhone, the output impedance won't be quite right. Put a buffer or a clean adjustable boost perhaps, after the generator and before your pedal. The buffer might see a mismatch but your pedal won't. (If this pedal is to go right after the guitar, you should build a pickup sim)
2-connect generator to buffer to your pedal. Put an Ac voltmeter across the input to your pedal. Set to 400 hz or 1k at like 100mv or -16dB. Guitar- like level.
3- at this same test point, measure a few frequencies from 75 Hz to 12k. Whatever - 75,200,1k,4k,12k. You'll probably find the response isn't flat but thats ok - if 12k is down 4dB or a dozen mv or whatever, you'll know what your pedal is seeing. If you see a frequency loss at the OUTPUT of your pedal, compare it to the input.
Otherwise you'll be thinking your pedal is losing highs, or lows maybe. If you are looking for a midrange notch out of your pedal, or a lowpass, you can more accurately measure that. I like my iPhone generator a lot, but it is down 5dB at 12k.  Once I discovered that, I was ok.

[PRR]

> a Heathkit IG-72

Lived with one for many years.

What Blackie says. Keep level down around 0.1V, make connection any way you can. Half an old guitar cord is convenient, but I used gator-clips a lot.

I think you can use the "0.2V" position and keep the level trimmer so the meter reads half (making 0.1V). 0.2V won't blow any pedal, 0.1V leaves a little headroom. Fuzzes should break-up at much lower level. "Clean" pedals should take 0.5V or more, but work up to it.

Yes, the output of that machine is "wrong" unless you keep the Load on (or are feeding true-600 systems, which you aren't).

> invariably speaker outputs

They were all-purpose UN-shielded connectors. There is a stacking double banana plug for them, and Pomoma used to sell adaptors to ANY thing.

In Hi-Fi they have survived only as "speaker", true. In this case, because the signal generator is low impedance and we are only looking for signal, not doing fine recordings, unshielded is acceptable. In critical debugging we might want shield; that's where the half a g-cable may be preferable. But since he's got the pedal case wide open, he's not at that level of low-low interference yet.

[Tubebass]

I got one of those Heath generators off Ebay a few years ago. Works great and is presumably harder to blow up if you connect it to the wrong place in a tube amp. The previous owner had fitted it with a BNC connector.