Signal generators, Oscilliscopes and testing....

[mmaatt25]

Hi,

I'm currently trying to debug a PT-80 (seperate post) and have run out of ideas.

I have a Seeedstudio Nano oscilliscope and I've just built a (working) RG's quick and dirty audio test oscillator. I've searched around here and the internet, but cant find any guides/help of how to use these to debug circuits.

I know this is probably very basic stuff and is probably covered somewhere, I just cant find it.

Any advice/help appreciated as always.

Thanks

Matt 

[GibsonGM]

Hi Matt,

PT-80 delay, right?  I would hook up the signal generator to the input jack tip.  Use an audio probe to listen to the signal as it goes along.  You should be able to trace out the path the signal takes by using some deduction on what the ins/outs of the opamps are.    Be sure to listen to the oscillator before you hook it up to make sure it works, lol.    I'd set it for about .7 to 1V RMS with your DMM before connecting, to mimic a typical guitar signal. 
Audio probe is probably the best bet for finding a dead or crispy part of the circuit - be sure it is capacitively coupled with a fairly large (1uF or larger) cap before your listening amp, so DC doesn't fry the amp or scare you by going "POP"!    There are a few ways to do this....inject signal in the middle of the circuit and listen to the output, inject it at the input and listen to the middle....it's about breaking it into smaller chunks, where you know the parts before are working ok.   Good experience to get in there and give a listen.  When you find an opamp...transistor....that isn't working, you get out the meter and check voltages and so on.   

I didn't catch your other post, so I don't know what symptoms you're having.  Posting a link to a schematic would help, too! 
Good luck,

~Mike

[Auke Haarsma]

In addition to what Mike says:

Since you have a scope, hook up the signal generator at the input and follow the signal with your scope. Use the schematic to see how the signal runs through the components.

Audio probe and scope are great debug tools. Audioprobe lets you hear the signal at any place in the circuit. The scope lets you see the signal. Both together can really help understand what is going on!

Btw: I'd say a typical single coil guitar signal is about 100-200mV (0.1 - 0.2V). Humbuckers a tad higher.

[Paul Marossy]

Audio probe and scope are great debug tools. Audioprobe lets you hear the signal at any place in the circuit. The scope lets you see the signal. Both together can really help understand what is going on!

+1

[KazooMan]

Hi Matt:

I continue to follow your efforts to repair yout PT-80, but I have no new ideas at the moment.  Did you get the new 2399?

[mmaatt25]

Hi,

Thanks for all the replies.

I'll sit down with the schematic and work through it. I'm still new to all this, so still learning. I'm getting a little confused going through the circuit.
Am I right in saying that a pedals have two elements to them, the power and the signal, I know this is extremely basic, but is it fundementally correct?

KazooMan, still waitng for that PT2399 to arrive in the post!!!

Thanks

Matt

[GibsonGM]

Yes, you're correct in assuming you'll see power, signal, and of course ground.

The steady-state, average output of most pickups is as Auke said...my figures were the peak voltages, which I use to see if something is distorting.    ;o)     .5V is a good starting point....

[soggybag]

The audio probe is the most excellent debugging tool. I get a lot of mileage out of it.

I never thought to inject a signal into part of a circuit. I'll have to build an signal generator. I had been thinking of trying to use a radio as a test signal. The things you learning hanging out here.

[tiges_ tendres]

The audio probe is the most excellent debugging tool. I get a lot of mileage out of it.

I never thought to inject a signal into part of a circuit. I'll have to build an signal generator. I had been thinking of trying to use a radio as a test signal. The things you learning hanging out here.

I use an mp3 player, or If I have space, My guitar and an Ebow.  That way I dont have to keep strumming and poking!

[served]

I built today a cable with brobe, and connected it to my computers soundcard. I use Visual Analyser 2010 NE-XT as a generator and oscilliscope. At the moment I am found of the whole idea, but we ll see how it goes.

[GibsonGM]

Visual Analyser is great. I used it to make a .wav of 220Hz, 440Hz, and 1kHz tones, each lasting about 5 mins. Then I burned them to a CD, and play them back into a circuit as a function generator!   Just measure the CD player output for like .5 to 1V, and you're good to go!    Lets you see if anything 'changes' with increasing frequency.  I tend to use 1kHz the most; it's right in the middle, so to speak. 

[dthurstan]

I use an audio probe myself, best thing I ever made (cheers GibsonGM  ) and an mp3 player.

Visual Analyser looks the business. I take it it's it free?

I built today a cable with brobe, and connected it to my computers soundcard. I use Visual Analyser 2010 NE-XT as a generator and oscilliscope. At the moment I am found of the whole idea, but we ll see how it goes.

How did you make up your cable and probe?

I do have 1 question. Is the output impedance of your source important?
I pluged my mp3 player into the input of a working silcon fuzzface clone I've built, and the output to an amp.
The output is not the same as when I plug my guitar into it, i.e. not fuzzed up like you'd expect!

Any thoughts on this.

[served]

My probe looks like this.

I connect it straight to my Laptops MIC and start probing. And I also monitor everything with and amp. So that I could hear, if anything is wrong or right. So I use two probes fer now.

[GibsonGM]

The source output impedance COULD be important...you might not get the SOUND you're looking for this way, but you should find trouble easy enough (dead/loose connections, non- or intermittent-functioning parts...).  For actual tone stack testing and whatnot, I'd get a friend to play guitar or suffer thru yourself while adjusting.  If your source has a high output impedance, it can't source enough current and therefore the sound you hear might not be "punchy" enough.   You COULD run it thru a buffer 1st....

I hope that probe has a nice big cap in it, right, Served?  You'll need to keep that DC out of your computer.   Also, if you exceed a certain voltage (depends on your sound card), you can damage your computer with the signal you're monitoring.  I just use an old, cruddy SS amp I keep around for home practice and this purpose.  In a properly functioning pedal, I doubt you'll see more than 5 volts (of signal) from a 9V circuit. 

Important point is to know if where you're going to probe will zap the sound card (!!!!).    If so, you can voltage divide it down (real O-scopes use a "10x probe", which divides the signal by 10).   Another (good) option is to set up a zener diode to clip if you exceed a certain voltage.   Might wanna look at your sound card's specs to make sure.... 

[Paul Marossy]

The audio probe is the most excellent debugging tool. I get a lot of mileage out of it.

I never thought to inject a signal into part of a circuit. I'll have to build an signal generator. I had been thinking of trying to use a radio as a test signal. The things you learning hanging out here.

Although THIS works fine, I find that this version I made works even better.




I can just use a 1X scope probe to trace the signal. The tip of the probe is a little more well suited for probing things and you can clip the probe to ground and forget about it. I find that this version is just easier to use.

It's essentially the same thing as linked above but in a small plastic RadioShack project enclosure and uses a BNC connector for the scope probe and a regular 1/4" mono jack for the output to the amplifier.

I built this because I am doing more and more repairs to pedals, and this is a very reliable, durable and painless method of audio probing.

Also, I built RG's "Quick and Dirty Audio Osillator" to inject a signal into the D.U.T. (Device Under Test). Been using it for years now. 

[GibsonGM]

....and if you use the Q n D oscillator, you won't have a source impedance issue!!  )

[served]

Well yes, You are right. My sound card lives on the edge when I am doing it, and I am using a notebook, so if the soundcard brakes down, I will have to get a new motherboard.
All the soundcards should keep up with max 48V (phantom power) First I didnot connect the cap then I upgraded it with the cap. Also I should put a buffer there. This way its a lot safer.
But from my point of view it's a huge step towards getting better in the electronics and stuff. I could never buy myself a real scope, and this thing lets me see the signal and how it changes. Its deffenetly not as good as the Oscopes in my school, but I still have a chance to build and see and experiment things.

[dthurstan]

served looks good. Mine is a little more slap dash. I have a probe from a cheapo DDM which I wanted to use but Im not sure I can solder a wire to it. I like the look of Visual Analyser but I don't wanna break my Tascam audio I/O so diode is a good idea.

Cheers for clearing up the impedance thing, was just curious, but like you say it doesn't matter for testing. I was thinking about audio testing pedals. By this I mean for example input so audio (from mp3) into a stock DS-1 at various settings then into the exact same audio into modded DS-1 with same settings and compare.