Tips, tricks, hints and more on finding a cable break
Jan 1, 2009 12:00 PM, By John Landry, CSRE
Give me a break
Sometimes a difficult task has an easy solution. Recently a colleague spent hours working on what he thought was a software issue, only to find that the data cable was faulty. In this installation, the cable was quite long and had been pulled through the ceiling for about 70' through a few twists and turns. It was late and we were tired. And since it was only serial data, we opted to find the break, make a good splice and be done with it. The question was how to find the break.
A simple way to find a cable break is to use the regular telephone tone generator and the inductive amplifier (sniffer). By sliding the sniffer along the cable, the tones should be loudest where the break is. There are some drawbacks to this method: Sometimes the cable in question is in a conduit. Or it is one of thousands in a cable tray. Still other times an adjacent cable will have some signal that overpowers the tone generator.
Some time ago, a fellow engineer at ABC Radio and I were trying to locate an intercom cable and were running into the same problem. We got around this problem by plugging the tone generator into a Crown D-75 amplifier, and connecting the bad cable to the speaker terminals. We tried this trick again with an amplifier that was on hand and sure enough we were able to find the break in this data cable. Fifteen minutes later (with the addition of two DB-9 connectors) everything was working.
Yet another cable search and repair technique is to replace the audio tone generator with an RF signal generator. Then a portable AM radio (FM will not work very well for this use) allows you to find the break. This method works especially well for long data and logic circuits, such as alarm wiring or the command wiring at an AM transmitter site (as long as the transmitter is off).
High tech cable testers are readily available. Many use time domain reflectometry and can measure overall cable length, velocity factor, give impedance and reactance values, as well as locate kinks, splices and breaks. For some applications, such as finding a break in the transmission line on a tower or underground, the TDR-based tester is the only option.
Another simple tester that is very valuable on the work bench is the Electronic Series Resistance (ESR) tester. Many failures in equipment made in the past 10 years can be traced to the failure of electrolytic capacitors. Typically as they age, the electronic series resistance of a large electrolytic capacitor will change, leading to heating and eventual failure (sometimes catastrophic).
ESR testers used to be very expensive. The Sencore Z-meter, one of the first, cost several hundred dollars, but it is still one of the best tools for evaluating capacitors. Sencore has a nice PDF on their website showing its capabilities: www.sencore.com/uploads/files/LC103gold.pdf.
However, a handy and affordable tester has been introduced by Anatek. The Anatek Blue will test a wide variety of capacitors and read out the ESR on a display. An ESR vs. capacitance graph is on the front panel for handy reference. www.anatekcorp.com/blueesr.htm.
Landry is an audio maintenance engineer at CBS Radio/Westwood One, New York.
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