Failures From Wire-Wound Resistors

by John Bisset

Fig. 1: Fling one of these to your promotions director.

One of the best things you can do to foster teamwork among fellow department heads is to pass on useful information. For programmers, it may be a new feature that your hard drive automation system will perform. For the sales manager, maybe it's the new strip mall that you saw on the way to the transmitter site - ripe pickin's for new spot sales.

For the promotion manager, it's the Fling! This is a reasonably priced giveaway, great for remotes and station promotions.

Ideal for a game of catch, the flexible Fling springs from its case and is thrown like a Frisbee or similar flying disc. More information can be obtained by heading to the Web site www.flingprom.com Download the info and pass it on to your promotions folks. You may be pleased at the reaction you get.

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Don't leave grounding for critical facilities to chance. If you're in charge of planning and constructing new studio or transmitter facilities, Lyncole Industries is hosting another two-day grounding course, this time in St. Louis, May 11-12.

The Lyncole Grounding Course covers all aspects of electrical protection, grounding, lightning protection, surge suppression and ground system testing. Tuition is usually $795, but if you mention that you read about the course in Workbench, a reduced fee of $695 will be charged. This discount only applies to the St. Louis course.

The company has 20 years of experience in grounding everything from computer clean rooms to communications towers. Even if you can't attend the course, head to the Web site for interesting white papers and test instrumentation.

For assistance regarding the course curriculum, or to register, call John Totten at (800) 962-2610, extension 183. More information can also be obtained at www.lyncole.com/contact.asp

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Fig. 2: Broken ceramic on wire-wound resistors usually indicates failure.

Robin Cross, the chief at the University of Missouri-Kansas City's KCUR, writes that the computer tech at the station, Chris Prewitt, has established an e-mail reflector for computer-related problems for broadcast. Chris' intent is to field specific problems related to broadcasting, and not general computer problems like fixing the sales staff's computers.

Examples would be live streaming in any of its forms, podcasting, archived programs or these type of server questions. As things seem to be changing faster than the speed of radio waves, having such a resource should be helpful to broadcast engineers straddling the broadcast/IT divide.

The e-mail address is IT4Broadcast@yahoogroups.com. The site is http://groups.yahoo.com/group/IT4Broadcast.

Thanks, Robin, for helping to get the word out.

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David Wigfield, assistant technical supervisor at KCBS/KLLC in San Francisco, read our mention about occasionally checking transmitter bleeder resistors - or for that matter, any high-power wire-wound resistor, as seen in Fig. 2.

He was reminded of a similar problem experienced at KLLC a couple of years ago. In David's case, the problem wasn't with a bleeder, but rather a wire-wound step start resistor. Occasionally, the main breaker on their BE FM20B would pop whenever there was a quick power failure.

David's first thought was that the breaker was getting old, and needed to be replaced. Before replacing the breaker, however, they looked at the step/start resistors.

Visually they looked fine. A test with an ohmmeter, however, revealed that two were open. Closer inspection after the resistors were replaced found a fine hairline crack that was not visible when they were mounted in the transmitter.

With the new resistors in place, no more circuit breakers blew. David now makes it a practice to measure them with an ohmmeter whenever he is in the transmitter.

Good advice. It only takes a couple of minutes, and in the case of the bleeders, discovering an open resistor could save your life.

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Fig. 3: Look hard enough and you'll find the cause of the failure.

Laverne Siemens of Canada's Golden West Radio has been following our discussion and pictures of the solenoid breaker reset schemes with interest.

Laverne had an issue with a Continental 816R-2C a few years ago, where he could have used one of those devices. However, the reset device wouldn't have solved his problem. Read on ...

His site was located a full hour's drive away from the studios, and one summer the Continental's plate breaker started tripping. There was no backup transmitter in place, so after a couple of breakneck trips across the back roads to get the station back on the air, Laverne and his staff knew they could no longer write the breaker trips off to random power blips.

The engineering staff suspected the problem was heat-related and that it likely had something to do with the power control circuitry. To further investigate, they took a hair dryer to the unit.

If you stand on a step ladder at the back of the transmitter, you can direct the hot air through the top-panel ventilation grid work. There's no need to place hands inside the transmitter; from this angle, it's easy to hit the power control SCRs with the hot air. This procedure is performed while the transmitter is on the air. After just a few minutes of applying the heat, they could get the breaker to trip.

A few tries later and the problem was narrowed down to the exact SCR. Laverne and his staff concurred that the SCR was breaking down and that the excessive heat was causing it to misfire, which in turn caused the excessive plate current draw.

The SCR was replaced and the breaker hasn't tripped since.

Laverne Siemens is director of engineering for Golden West Radio.

Laverne's diligent searching eventually paid off in disclosing why the transmitter was shutting down. Look closely to diagnose the failure. The burned rectifier stack in Fig. 3 was obvious when the front cover of the HV supply was removed, but it might have been the smell or the smoke marks on the top that led the engineer to this problem.

John Bisset has worked as a chief engineer and contract engineer for more than 30 years. He is the northeast regional sales manager for Broadcast Electronics. Reach him at (571) 217-9386, or jbisset@bdcast.com. Faxed submissions can be sent to (603) 472-4944. Submissions for this column are encouraged, and qualify for SBE recertification credit.