/* Filed under Audio, Hardware, How-To, Music */
This article is a great troubleshooting tip based on the â€œtool of a thousand usesâ€, the good old tone generator, tone detector. Everyone has (or should have) one of these in the tool bag. They’re typically used to locate cables you forgot to label, labeled incorrectly, or got cut somewhere during house construction.
Next time you have a dead audio path, you can use the toner to work from the audio source component to the final amp, or use the signal generator from the amp and work backwards to the source.
Since we need to make an adaptor for the alligator clips on the end of the tone generator leads to an RCA stye plug, itâ€™s a easy step to add a simple filter/amplitude reducer to make the tone generator signal more amp/preamp friendly.
Better known as the cable signal generator/detector (or â€œtonerâ€), these two devices are, in fact, the perfect audio (and even video) test gear.Â Â And theyâ€™re cheap.Â Â
Photo 1. Typical tone generator (with alligator clip leads) and tone detector.
The tone detector is a very high gain, high impedance audio amplifier that can be used to locate sources of both audio and NTSC video.Â Â To find out whether an audio component is putting out a line-level audio signal, simply stick the probe end of the detector into the RCA jack (see photo 2).Â Â
Youâ€™ll definitely hear the audio.Â Â You can also touch the tip to the end of a line-level audio cable to hear the music.Â This trick also works to detect NTSC video except that what you hear will sound like a nasty noise (the horizontal/vertical sync pulses).Â Â But thatâ€™s not all.Â Â Want to know whether audio is getting to a speaker?Â Simply hold the tip of the detector on the speaker cable or touch into the speaker terminals.
Most signal generators output a pair or alternating tones, something like 500 and 700 cycles.Â Â Unfortunately, upon closer examination of these tones on a scope, they are anywhere from 6 to 10 volts peak-to-peak and a nasty square wave with high amplitude spikes.Â Â You could inject this signal into an amp, but it might cause either amp or speaker damage if the amp was cranked up. Typical line level audio is about 1-3 volt peak.
Since we need to make an adaptor for the alligator clips on the end of the tone generator leads to an RCA stye plug, itâ€™s a easy step to add a simple filter/amplitude reducer to make the tone generator signal more amp/preamp friendly.Â Figure 1 shows the schematic of the two resistors and a capacitor needed to do this.Â
Figure 1. Schematic of simple tone generator adaptor.Â
Two resistors and a capacitor.Â Values are not critical.
All the parts are easily available at Radio Shack.Â This filters out the high frequency components (spikes) of the toner signal and lowers itâ€™s amplitude to less than 1 volt peak.Â Â Photos 3 and 4 show the adaptor I built hooked to the tone generator, and a close-up of the component hook-up.Â Simple.Â I tested this with three different toners, two from Fluke and one from Ideal (seen in photo).Â All work fine. Keep it in the tool bag or the bag that holds the toner/detector.Â
Photo 3. Ideal tone generator hooked to the audio adaptor with the alligator clips.Â
I built it with two RCA plugs for quick stereo testing.
Photo 4.Â Close-up of the adaptor components.Â The RCA cable center conductors connect to the junction of the two resistors, grounds go to the end on the left.
Next time you have a dead audio path, you can use the toner to work from the audio source component to the final amp, or use the signal generator from the amp and work backwards to the source.Â
Written By:Â Grayson Evans, Training Reels