AM Site TLC Stops Off-Air Headaches

by John Bisset

Cold winter mornings are not the time to babysit AM contactors. If you maintain a directional AM, chances are these little workhorses are hidden somewhere in your site, switching twice a day. Like all moving parts, they do need periodic maintenance.

First, watch the contactor as it switches. Does the actuator arm move decidedly or does it hesitate during the transition? Hesitation can be caused by a number of things: the solenoid, rusty or binding linkage, or loose or corroded wiring connections.

Fig. 1: With power off, work each actuator to check for binding.

This device moves; so during a maintenance period, when the transmitter is off, work the actuator manually, as seen in Fig. 1. Turn off power to the phasor control system as well as the RF. Some controllers will interpret your manual movement to mean the contactor is hung up or not in sync with the position of the others, and will apply power - fighting you to switch back to its normal position.

Movement of the linkage should be smooth. Watch the linkage for loose parts, and tighten bolts that have come loose. Check the operation of the contactor after any adjustment. It's possible to misadjust the contactor linkage.

Also important with RF contactors are the interlock microswitches. On newer phasors, each contactor is linked back to a go-no go tally light panel, as seen in Fig. 2. When the lights illuminate, the interlocks of all contactors in the chain are closed, and you can assume all contactors have switched properly. The small interlock switches mounted on the chassis perform this tally function.

Fig. 2: Tally lights help diagnose contactor failure.

Many older phasors used a second set of interlock switches to open the AC feed to the two solenoids. Each solenoid controls one direction of the actuator. When the actuator has switched, the second set of interlock switches break the circuit.

If a contactor is not switching, this is one of the first places to check. If the interlock switch does not open, the voltage is fed to the solenoid continuously, eventually burning it out.

A failure of this small interlock switch can cause this kind of solenoid failure, but so can a misaligned interlock switch. So in addition to watching the actuator linkage switch properly, also make sure that with each actuation, the interlock switches are indeed closing.

Using a lock washer under the microswitch mounting bolts should help lock the switch down, so it won't move with the contactor vibration. A small drop of Glipt brand glue or red fingernail polish will also keep the contactor switching vibrations from loosening these switches.

The last major component you'll inspect visually is the RF connection. Most AM contactors use some type of pin-and-socket or pin-and-spring finger contact assembly to transfer the RF. Ensure that the bolts holding the RF connections are tight.

With a strong light, check to see that the RF contact surface is smooth, not pitted. Pitting occurs when the contactor is switch "hot" - with RF power applied. Depending on the power level, you may only get one or two tries switching hot before the assembly burns up. On higher power sites, say 5 kW and above, the contacts can actually weld. Disassembly for repair is a bear.

With so many parts that can fail and take you off the air, it's a good idea to have spare parts on hand, and better yet, a complete spare contactor that can be put into service while the older one is being repaired. A good source for contactor parts is Kintronic Labs (www.kintronic.com). Kintronic also stocks inductor clips, insulators, an array of inductors and both vacuum and mica capacitors.

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While we're on the subject of things that can take you off the air:

Fig. 3: Visually inspect the transmitter air interlock.

When was the last time you checked your transmitter airflow switch? In dirty transmitter site environments, it's possible for dirt to clog vane/microswitch assemblies, so if the blower fails, the clogged vane won't move. This means the blower interlock won't open, and your transmitter potentially could melt down.

When you change a tube, visually inspect this vane/switch assembly. Be gentle; some of the vanes are very delicate and can be bent or broken easily. In a quiet environment, you can hear the microswitch closing as the vane is deflected by hand. Checking switch continuity, with the power off of course, is a better idea.

Newer transmitters use a pressure or pressure-bellows type switch. See Fig. 3. The little grey plastic plug on the left can be removed and the set-screw beneath this plug adjusted for best sensitivity. Refer to the transmitter manual for the correct procedure. Misadjustment can be disastrous.

Like the contactor parts, an air interlock switch is a good spare part to keep on hand.

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Alan Fisher, engineering assistant at Clear Channel in Richmond, Va., passed on a note about keeping flashlights free from battery corrosion. Nothing's more frustrating than reaching for a flashlight only to have it fail because the batteries are corroded.

To prevent this problem in two-cell flashlights, Alan suggests reversing the first battery (+ to +, or - to -) with respect to the second cell. This will prolong the shelf life of the batteries, and prevent them from corroding inside the flashlight.

The only drawback is that you have to unscrew the flashlight and reverse one of the batteries prior to use. But this is a small price to pay to keep a flashlight working.

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. Send faxed submissions to (603) 472-4944. Submissions for this column are encouraged, and qualify for SBE recertification credit.