A Fly Brings Down a Transmitter

Also, a tip for those who have Sage boxes with thermal printers
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In Mark Tomlonson’s nearly 30 years as a radio chief engineer, he has seen transmitters go down due to wind (it happened at AM station WAYT, which is now WJOT, in 1981), fire (at FM station WMUK, 1991) and flood (FM WIDR, 2005). This is the first time he has lost one to a fly.

Insects, especially swarming ones, can give engineers fits. But this was no swarm. A lone housefly recently brought down the box that changes the HD1 and HD2 audio into HD Radio at WMUK. Without that box there is no HD.

The device is a Linux computer, so it has a hard drive. One side of the drive usually is left open for cooling; at WMUK the open cooling faced a side panel. The errant fly crawled between the hard drive and panel and came into contact with the power leads for the disk.

That was the end of life for both fly and hard drive. And without the HD signal, the exciter shut down; with no exciter signal, the HD transmitter shut down.

The chain of events seems simple in retrospect, but Mark could not have guessed that a fly had been the source of the failure until he investigated.

There’s nothing special about the drive; Mark replaced it with one in stock. WMUK is back on the air full strength, with both HD streams.

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Fig. 1: An old plastic phone card saves the Sage printer mechanism. * * *

Mark Voris is the chief engineer for the Spirit Catholic Radio Network in Omaha, Neb. He has a tip for engineers who maintain older Sage Endecs that use thermal printers.

Grab an old phone card or game card and attach it with Velcro brand or similar hook and loop fastener to the front of the Sage Endec. As seen in Fig. 1, the hard edge of the card is ideal for tearing off the EAS printer receipts.

After advancing the paper, use the card edge to tear the paper receipt cleanly. Yanking on the paper, or pulling and tearing the paper strip can damage the printer mechanism. If you can’t get your operators to follow the proper procedure, roll the strip and secure it with a paper clip until someone removes the strips properly.

* * *

Contract and project engineer Mike Patton enjoyed our March 10 tip from Ted Fuller, who figured out why the MW-1 meter kicks upwards at turnoff and told how to fix it.

Mike too has developed some tips for this transmitter. One is to replace the audio transistors on the PA modules with Moto (now On Semi) MJ15024s. The transistors originally were 2N6254 RCAs, more recently Motorola MJ15011s or MJ15015s.

The MJ15024s have higher voltage standoff and current ratings than the 15015s or 15011s and should be much tougher in the field. To Mike’s knowledge, none of his retrofitted PAs has suffered a failure.

Mike notes that current gain on the 15024s is a bit lower; so change them all at once, in all modules. That makes 39 transistors counting the driver, at about $10 each — it’s not cheap. But even with lower gain, there is enough current gain in the audio chain not to load down the driver, and positive peak performance is unaffected.

For folks in the Deep South, Mike’s firm provides support and parts for transmitter models that are no longer supported by the makers or that were made by companies that are no longer in business. In addition to parts and repairs on the Harris MW and SX series, Mike’s firm can retune rigs on-site or in his shop.

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Workbench columns are archived online; go to www.radioworld.com, select “Columns” under the banner, then “Workbench.”

Here’s a tip prompted by an item that appeared in November 2008.

Broadcast engineer Paul Sagi, in Kuala Lumpur, responded to the column titled “What’s Wrong With This Picture?” The article suggested that you check tower grounding after you clear debris and weeds from the tower base.

Paul shares a tip about the type of conductor used for grounding when copper strap is not available.

Rectangular conductors make better grounds for RF and lightning-caused impulses than do round conductors, because the rectangular cross-section configuration results in lower self-inductance (other factors being equal). So if copper strap is not available, go with rectangular conductors.

I mentioned this to Nautel’s lightning and grounding maven John Pinks. “Compared to any other geometric shape,” he told me, “a circle has the smallest circumference that encloses a given area. Conversely, the cross-section of a wide, thin strap has a comparatively long perimeter compared to a circular conductor.

“As RF currents tend to flow on the surface of conductors due to skin effect, the useful area of a strap is much larger than that of a round conductor. This effect is, however, frequency dependent and more significant at the higher end of the lightning’s frequency components, i.e., 5 MHz.”

Thank you, gentlemen, for the tutorial.

John Bisset marked his 40th year in radio in broadcasting recently. He is international sales manager for Europe and Southern Africa for Nautel and a past recipient of the SBE’s Educator of the Year Award. Reach him atjohnbisset@myfairpoint.net. Faxed submissions can be sent to (603) 472-4944.

Submissions for this column are encouraged and qualify for SBE recertification credit.


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