A discussion developed at one of my NAB transmitter workshops about cleaning these specialized devices. A participant complained about the high cost of cans of pressurized air for "dusting" transmitters, computers and other gear. A similar discussion showed up on Dave Biondi's firstname.lastname@example.org listserv with some interesting comments.
Brian Force of Metro Networks points out that MCM Electronics offers a small "electric duster" for around $50. It's a half-horsepower blower with a 12-foot cord that weighs about 2 pounds. The air stream is not as precise as canned air but works well. Reach the company at (800) 543-4330.
Sid Schweiger, MIS Manager for Entercom Boston, recommends a soft, bristled brush and a vacuum cleaner with the edge attachment. The advantage is that dust is sucked away, not blown all over.
Care must be used when blowing dust and dirt inside a transmitter. I've seen engineers successfully couple compressed air and a vacuum to catch what the compressed air dislodged.
If you use the compressed air, be sure to wear safety goggles and a mask; there's no telling what kind of crud you'll be stirring up.
Reggie Sadler of WGGC(FM) in Glasgow, Ky., uses a small pancake air compressor. You can purchase these at TSC, Lowes and other hardware-type stores.
However, Mike Patton of Michael Patton and Associates warns that if you use any kind of compressor, keep in mind that they tend to generate wet air due to compression and the nature of water vapor in the air. This is an issue particularly in the humid deep South.
Oily air is another potential problem. Mike's solution is to install a water/oil separator in the airline, close to where the air leaves the tank. Remember to purge the water occasionally. Adding this kind of device gets the moisture in the compressed air down to less than a condensing level.
Several engineers eliminate the problem of wet air by using a scuba tank. It's dry air; if you are a diver, it's easy to come by.
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Looking to redo the tower lighting feed to your AM tower? Consider the neat, efficient method that Mike Gilbert of Multicultural Broadcasting in Seattle used.
Using a standard bowl feed-through, Mike mated a copper pipe "T" fitting on the end of the hollow feed-through bolt. Copper tubing soldered to a right-angle fitting and connected to the upper end of the "T" provides the RF feed. The AC wiring off the lighting choke feeds through the open bottom end of the "T," and out to the tower.
Yes, that's cedar planking used for the ACU building. They don't fool around in Washington State.
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Joe Stack always has a good engineering nugget to contribute. His suggestion about filament voltage is no exception.
Most tube transmitters have a calibration pot or rheostat for adjusting the filament meter reading. This is true particularly if the filaments are read on a multimeter select switch.
When measuring and adjusting the tube filament voltage, remember to make this measurement at the tube socket (with the tube inserted, the transmitter filaments "on," but with no high voltage). In all but a few cases, the filament voltage "meter" is an arbitrary reading; it can be adjusted. So unless the transmitter is equipped with a true RMS meter at the tube socket, check the value yourself.
Be sure to use a true RMS voltmeter meter and that it is accurate. An inaccuracy of even 0.1V will make a difference in tube life. You don't want to guess.
Measure the voltage at the tube socket, adjust it to the manufacturer's recommendation and calibrate the transmitter filament voltmeter to this value. Many transmitters use an internal or external filament voltage regulator to compensate for AC voltage fluctuations. If your AC wanders a lot, consider investing in this kind of regulator to keep the filament voltage constant.
Joe has another tip for verifying exciter output power: Use an in-line wattmeter.
It's true the transmitter input may not be purely resistive, but this measurement is a good benchmark, and will tell you if the exciter front-panel power meter is accurate. Make this measurement into a dummy load, as well as into the transmitter, and keep these measurement results on file. They may help you diagnose a problem one day or keep you from chasing your tail as you troubleshoot.
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Here's a neat product for the storage of CDs. Go to www.disk-store.com to find a solution to cracked jewel cases and the space problems they create.
Disk-Store.com offers three-ring hole-punched inserts that can be stored in any three-ring binder. Unlike usual disk insert pages, these are heavy-duty. The inserts are manufactured using 7.5-mil, industrial-strength vinyl, designed with a special embossment to make sure the disks don't stick inside.
The openings even have locking straps to keep disk secure, no matter what position the binder is in. Disk-Store.com also has added plenty of room for any inserts or printed material that might accompany the CDs, and several page choices are available. Imagine being able to store 50 CDs in a 2-inch binder rather than 19 inches of shelving.
Disk-Store sells just the vinyl pages, so you purchase the number of insert pages you need, without paying additional money for a binder.
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