I Should Have Known This

I was teaching the CEDIA Advanced Boot Camp class a few months ago where part of the class team exercise is to hook-up a rack of A/V gear for a home theatre. DVD player audio/video out through preamp, through 5-ch. EQ, to 5-channel amp., to surround speakers. About 30 cables or so. After hookup, visual checkout, and power-up, no audio to speakers. Hmmm. Everything looks good. Proper inputs selected, modes set correctly, still no audio. We had lots of hand tools but nothing to use as test equipment. No iPod, no scope, nothing useful. So the only choice was to start “shotguning” it by removing cables and rerouting signals, first from the DVD player directly to the amp, and so on. Unfortunately, the interconnect cables were already dressed nicely, coiled, cable-tied, etc. It took about an hour to finally figure out the EQ wasn’t working. You have all had a similar experience sometime, usually with equipment in a rack where access to the cabling was difficult.

After that exercise, I was thinking about what we could have used to troubleshoot this problem quicker. If only we had a simple audio signal detector. That night it came to me over a beer. Of course, there was one sitting right in the lab only a few yards away! I can’t believe I didn’t think of it!

Audio Signal Generator and Signal Detector

Better known as the cable signal generator/detector (or “toner”), these two devices are, in fact, the perfect audio (and even video) test gear. And they’re cheap.

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. 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).

Photo 2. To test for audio, just stick the tip of the probe into the jack.

You’ll definitely hear the audio. You can also touch the tip to the end of a line-level audio cable to hear the music. This trick also works to detect NTSC video except that what you hear will sound like a nasty noise (the horizontal/vertical sync pulses). But that’s not all. Want to know whether audio is getting to a speaker? 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. 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. 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, it’s a easy step to add a simple filter/amplitude reducer to make the tone generator signal more amp/preamp friendly. Figure 1 shows the schematic of the two resistors and a capacitor needed to do this.

Figure 1. Schematic of simple tone generator adaptor.
Two resistors and a capacitor. Values are not critical.

All the parts are easily available at Radio Shack. This filters out the high frequency components (spikes) of the toner signal and lowers it’s amplitude to less than 1 volt peak. Photos 3 and 4 show the adaptor I built hooked to the tone generator, and a close-up of the component hook-up. Simple. I tested this with three different toners, two from Fluke and one from Ideal (seen in photo). All work fine. Keep it in the tool bag or the bag that holds the toner/detector.

Photo 3. Ideal tone generator hooked to the audio adaptor with the alligator clips.
I built it with two RCA plugs for quick stereo testing.

Photo 4. Close-up of the adaptor components. 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.

If you have any questions about anything in this article, drop me an e-mail at grayson@trainingdept.com I will actually answer it! Visit our web site SUPPORT page for additional TIP and articles at www.trainingdept.com