(click thumbnail)Fig. 1: Surges can cause catastrophic failures.
(click thumbnail)Fig. 2: Use winter months to schedule tree or landscape help. Keep your towers clear. Joe Stack with Modulation Sciences is an occasional contributor to Workbench. In discussing poorly engineering sites, Joe says he has run across two scary uses for Belden 8451 audio cable. Both should make Belden's Steve Lampen cringe but won't surprise him.
Once Joe discovered the common audio cable being used to light up small, socketed 4 watt 115 VAC “night lights” above an audio patch bay. The installation was unique in that the lights were turned on and off via a lever switch that was depressed by the patch bay front-panel door opening and closing. After its discovery, this set up was replaced with low-voltage lamps.
Also, sometime in the mid-1980s, Joe's boss discovered a control line from a Gates BC-1T transmitter that was wired directly to a tie block with 8451. A punch-down tool was raked across these tie block terminals (while punching down an adjacent circuit) and the sparks flew.
The wiring melted inside the conduit all the way back to the BC-1T. Joe and his boss pulled new wiring and installed a 24 volt relay interface inside the transmitter, so that only 24 volts appeared on the tie block. No one was hurt and no other equipment was damaged.
Joe's nugget of advice is it's always a good thing to beware of the “sleeping serpent,” electricity. This is especially true for older installations. Keep in mind that not everyone may use a connector or wire type as it was intended.
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So did you get a digital camera for Valentine's Day? Tim Guentz, a regional engineer with NRG Media, won't leave home without his. We dubbed the digital camera perhaps the most useful tool for today's broadcast engineer, eclipsing the Simpson 260 VOM of a quarter century ago.
Tim writes that it's hard to believe he's worked without one for so long. He uses the camera for visually reaching into areas that cannot be seen, as well as documenting sites. How about viewing the rear of tube sockets, or just spotting details that the naked eye cannot see in a dark transmitter cabinet?
Price is certainly no longer a factor, and you can always wait for the next post-holiday sale. Added bonus: You can use your camera to contribute Workbench tips. RW Editor Paul McLane has a list of handy tips to help you avoid common photography problems when shooting such images. Drop me a note if you'd like a copy.
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Speaking of making an engineer's life easier, Hank Landsberg's Henry Engineering always seems to come up with a new trick for the trades. It's said that Henry builds for today's engineer what old-timers used to build themselves. The difference is that back then, we handled one, maybe two (and AM and FM) stations. Today's engineer can certainly build customized boxes, but who has the time? The company has filled a niche with a variety of useful products, which can be seen at www.henryeng.com.
Henry Engineering also distributes an effective surge suppressor or TVSS (transient voltage surge suppressor). The series was developed by Sine Control International for military installations in the early 1980s. Henry is the exclusive marketing agent for PowerClamp TVSS units.
These devices will help broadcasters solve AC spike and surge problems, and avoid the kind of damage seen in Fig. 1.
What makes these TVSS units different? Many such suppressors rely solely on metal oxide varistors to clamp voltages. The problem is that the MOV threshold must be well above the nominal AC voltage to prevent MOV overheating.
This margin can be 200 percent of the line voltage, which means that on a 120V line, a surge of 350 volts could pass before any attenuation occurs from the MOV. Henry's PowerClamp is a hybrid device that couples a rise-time sensitive circuit to the MOVs. The result is a 2 volt clamping level above the normal AC line voltage.
PowerClamp's attenuation circuitry responds to both voltage level and waveform rise-time. Hank's got a theory of operation paper, some case histories and a data sheet on the PowerClamp Series 8 at his Web site.
As I mention at transmitter workshops and BE/Workbench SBE programs, check with your station's insurance carrier about their policy on TVSS devices. Some insurance companies, recognizing the reduction of loss claims, will pay for all or part of the cost of these devices.
At the least, when these are installed, your premium should be reduced — just as when adding a theft deterrent to your car will reduce car insurance premiums. Obtaining a free TVSS device, protecting the station equipment investment or lowering premium rates are steps that should get you noticed by your management.
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Winter months are a good time to line up landscape or tree removal assistance. Spring is right around the corner. Don't get caught like the station pictured in Fig. 2.
Keep tower field growth under control.