Signal Generator

[ricach]

I think its time I get me a signal generator. More for verification that my o-scope is working properly. I found this kit for $8, is this good enough for general use?

Features
signal output: sine wave, triangle, square wave and integrator (selectable through jumper)
signal frequency: about 1kHz (fixed)
output level: adjustable from 0 to 100mVrms (LEVEL potentiometer)

Or should I look for something with an adjustable frequency?

[Chuck H]

Adjustable frequency is nice because you can see what a circuit is doing at other frequencies. And a higher output is also nice because it allows you to inject signals at other points in the circuit and also compensate for loading when you want to test transformers out of the circuit. if you only plan to work on guitar amps you don't really need anything but a sine wave. Square and triangle waves would be for testing the linear accuracy of an amp. I don't see the need with guitar amps myself. Though it can be revealing. I have yet to find a tube guitar amp that recreates a square wave with any accuracy.

Chuck

[jrfrond]

I have yet to find a tube guitar amp that recreates a square wave with any accuracy.

Chuck

And you won't. Not only are the rise times not there, but you don't want that kind of inherent accuracy in a guitar amp. Solid-state amps can do it easily, and well, you know how many of THEM can sound.

Square wave analysis is not only good for showing linearity, but it's great for displaying frequency response as well. Tone control effects can be readily-seen with a 1kHz square wave.

[ricach]

....And a higher output is also nice because it allows you to inject signals at other points in the circuit and also compensate for loading when you want to test transformers out of the circuit. ....
Chuck

I can see the usefulness of a higher output, but I'm not sure I want to spend multiples more just to get it. Any suggestions on something low cost? I'm not finding anything.

Another side question, I need a new scope probe and I'm not sure what the frequency rating on the probe tells me. For troubleshooting music electronics, what features do I look for?

[J M Fahey]

Anyway it's better than nothing, and far more repeatable and stable than plucking a string , all of it for $8. Go for it and in the future get something more complete.
Do you have a link for it, o can post the schematic?
Thanks.

[ricach]

Anyway it's better than nothing, and far more repeatable and stable than plucking a string , all of it for $8. Go for it and in the future get something more complete.
Do you have a link for it, o can post the schematic?
Thanks.

Its a kit -> Velleman Signal Generator Kit : MK105 Several electronics places sell it, MCM.com and Altex.com have it for $6 but shipping more than doubles that. Fortunately I have MCM local to me.

[J M Fahey]

When you have it, post the schematic; I'm sure you can at least switch add some other frequency to it, maybe even add a sweep pot.

[olddawg]

Most decent scopes (unless yours is from the stone age) have a calibration stud to adjust your probes that puts out a fixed frequency and amplitude. You can also use your computer as a signal generator with either a cheap program or an online utility. I have a big old tube HP signal generator on my bench. I love the feel of the giant old aluminum dial.

[ricach]

Another side question, I need a new scope probe and I'm not sure what the frequency rating on the probe tells me. For troubleshooting music electronics, what features do I look for?

Any suggestions? I've got a lot of unknowns happening right now so I want to pick up a new probe, and signal generator so I can verify my o-scope is functioning properly. The calibration stub (it's an older HP scope) does not seem to be putting out a signal. Or is it? thus all the unknowns.

[olddawg]

Any suggestions? I've got a lot of unknowns happening right now so I want to pick up a new probe, and signal generator so I can verify my o-scope is functioning properly. The calibration stub (it's an older HP scope) does not seem to be putting out a signal. Or is it? thus all the unknowns.

For audio just about any cheap probe will do. I recommend one with a 10:1 switch or a set of two which includes a direct probe and a 10:1 probe. To test the calibration stud just hook any clip lead on it and clip the other end on any (small) resistor lead. Then push that lead into the center conductor of your scope jack. That should give you a direct connection with a common ground. If you can't see the signal, then there's something wrong with the scope, the test oscillator, or you don't know how to trigger the scope. For a lot of test you can use jack adapters and plug rca plugs, etc. into the scope inputs directly. It's audio. You can rig your own probes if you like with shielded cable and a needle. The spectrum isn't that challenging. Just don't fry yourself on HV.

[J M Fahey]

*IF* 100 mV is enough for you, strictly input jack fodder, an even cheaper solution is just to download a 1KHz (or whatever frequency you need) MP3 and play it on your trusty MP3 player.
I always carry everywhere 440Hz, 1kHz, pink noise and a funny Rhode and Schwartz supplied "multi frequency signal", excellent for live PA testing.
These usually last between 1/2 to 2 minutes, but the "repeat one" setting keeps them on for as long as you want.
A couple homemade fiery guitar licks are useful too.(or at least some AC DC stuff, good for softening speaker surrounds).

[oc disorder]

While you're in the mood...

"When you have it, post the schematic; I'm sure you can at least switch add some other frequency to it, maybe even add a sweep pot. "
They look well made boards.

[J M Fahey]

Unfortunately I had just checked it and got *very* dissapointed; no sinewaves at all to be found in there.
Don't waste time on it, that's why I suggested those MP3's.
As a free sample:
Download Audio Tone Files
Selected sinewaves from 100Hz to 10 KHz.
Very useful.
Remember to set your MP3 Player to loop or "repeat one" or just keep pushing the Play button, they last around one minute each.

[oc disorder]

Just put "Free PC Signal Generator" into google.
Few options there.
An old laptop or desktop for that matter loaded with various free software
could be a useful addition ..
But for portability and ease of use J M Fahey's suggestion seems the way to go.

[Enzo]

In terms of audio, guitar amps are about the LEAST demanding circuits there are. They are not hifi, they do not have extended freq ranges.

Obviously you are starting out, at least in terms of building up a shop. You COULD spend a lot of cash on a fancy unit with tons of features you wouldn;t understand. But really, start with something simple and cheap, and as you gain experience, you will discover on your own the shortcomings of the simple thing. For example you might discover the output level range doesn;t cover enough situations, or as pointed out above, the lack of variable frequency. It won;t take long at all to learn from your own work what features matter and what don't.

I have some nice generators in my shop. I have an old HP200 somewhere in the warehouse, a nice Leader on the bench, and a few old Heath units as well. But what I reach for MOST of the time is a line out from my shop stereo. I have a signal cord plugged into the "record out" jack on the rear of a cheap little Panasonic receiver that I listen to in the shop. Sure it is hotter than a guitar signal, I don;t care. I can turn the volume down if I have to. I tune it to a local music station.

Playing music through the amp I get lows, I get highs, and I get mids. More than enough to determine if the tone stack is working, and it puts a real world signal onto a speaker.

Where the generator is very useful is wringing out buzzes. I can use a geneerator and swep the freqs to zero in on a particular resonant peak that might be buzzing a speaker or a cab. The stereo signal won;t do that.

A lot of times I am looking for a lost signal. Signal goes in but it won;t come out the other end. Often as not, in this go/no-go search, I just use hum as a test signal. A small screwdriver touched to a tube grid usualy gives more than enough "test signal" level for go/no-go.

One of those cheap little Yamaha or Casio keyboards works too. The smallest cheapest one that has a line out jack on the back. You can easily find a tone patch that produces a useful sound, and it is easy to wedge a key down for a continuous tone. And many of those have a DEMO patch, which plays a little tune over and over.

SO those simple techniques will net you a test signal at no cost at all.


Your little inexpensive kit would work, and if you look up the 555 timer IC they based it on, you will see how you could make the frequency variable by fiddling with R1, R7, and C3. But it is utterly basic.

And for your scope probe, again, this is not demanding work. ANy of the basic cheap probes work fine. I use them in my commercial shop. When your guitar amp has a freq range that falls short of 10kHz, wondering whether a 50MHz probe or a 100MHz probe is better becomes pointless.

yes, make sure to get a x10 probe or a switchable X1/X10 probe. MCM sells cheap probes. You don;t need all the fancy accessory tips, don;t pay extra for them. Just grabber hook is all you need. And even those pop off to reveal a plain point.