Tones to Troubleshoot AM Xmitters - Radio World

Tones to Troubleshoot AM Xmitters

Engineers are perhaps the most resourceful members at a radio station - quick to save the station money, slow on getting kudos from management. It's like it's expected.
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(click thumbnail)Fig. 1: Wiring at WJNV is worthy of the phone company.Engineers are perhaps the most resourceful members at a radio station - quick to save the station money, slow on getting kudos from management. It's like it's expected.

Take telephones. You may consider refurbishing your existing telephone system, especially if it's the on-air key system, rather than seek replacement. If you're planning a telco project, you'll want to use Lamar Owen's recommendation: Mike Sandman at www.sandman.com.

If it has to do with repair or refurbishing telephone systems, Mike has it. Sandman.com sells a cleaning kit that is just incredible, Lamar writes. There's also a cabling installation video that will make you bald or balder as you keep slapping your head telling yourself, "So that's how it's done."

If it deals with phones, this is the site. Thanks to Lamar Owen of WGCR(AM) in Brevard, N.C., for sharing the information.


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Speaking of "how it's done," I had to chuckle when I visited with Lewis Moore at WJNV(FM), Jonesville, Va.
(click thumbnail)Fig. 2: With the phone mounted near a door, you can step outside and talk in a quieter environment.

Lewis works for the phone company, so you can expect the wiring will be done properly. Take a look at the lace job in Figure 1. I didn't even know you could still buy that waxed lacing twine!

Jonesville isn't the biggest town, which has its advantages. Consider the phone book. Because it doesn't weigh a lot, Lewis was able to mount it on a cord by the telephone. Handy when you need it for reference.

The phone also is near the door, making it easy to step outside if the transmitter blower is competing with your voice on a phone call. See Figure 2.

Once, while checking over a site where I had taken over as chief, I noticed a full complement of telephone directories in an old wooden desk drawer. I was impressed at the thoroughness of my predecessor.

However, when I pulled the books out to use them, I saw that they had become mouse-nesting material. The mice had crawled into the top of the closed drawer and chewed through a good portion of the books. Fortunately, the tech manuals were in a metal file cabinet.

Lewis Moore had another piece of equipment that will make many readers of this column smile. Figure 3 shows a 3M Wollensak reel-to-reel. It still works!

I remember these from seventh-grade Spanish class: "Hola Isabel, como estas?" But I also ran across them at many radio stations. Weren't they the original ENG recorder for news reporters? Seems I remember a back-pack option.


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The radio-tech page of Broadcast.net recently had an interesting discussion on troubleshooting older plate-modulated transmitters.

An engineer was having trouble getting decent modulation and suspected the audio processor. Not so fast, the brain trust warned; the processor may be fine.

(click thumbnail)Fig. 3: This 3M Wollensak reel-to-reel is still in operation.
To isolate the processor, Tom Osenkowsky, an engineering consultant and RW contributor, suggested feeding a sine-wave tone into the transmitter directly, and looking at the modulation envelope with a scope.

Speaking of resourceful, Tom suggests using the test tone from an Otari MX5050 reel-to-reel if you lack a tone oscillator. Be careful about tones from other sources; they may not be pure sine waves.

Looking at the modulated waveform will yield useful troubleshooting information. For example, is the distortion on both the positive and negative peaks? Is the RF drive level normal? Are tube grid voltages normal? What about clipping on the modulator tube grids?

If the modulator tubes have gone soft, before replacing them consider the drivers. It may be prudent to replace the tubes in the entire audio section, not just the modulators.

Once you've verified that the transmitter is OK, you can investigate the audio processor. Use the 'scope to verify excessive clipping or flat-topping on the output of the processor.


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Another engineer complained about the loss of low end on an AM Optimod that was installed after sitting idle for a long time.

Tom McGinley, RW technical advisor and DOE for the Infinity Seattle market, writes to say he suspects the electrolytic capacitors, especially those used as coupling capacitors in the audio path, which take out the low end when they go bad.

When electrolytics sit for a long time without power, they dry up and don't form the proper capacitance when charged up again. The result is these capacitors behave like caps with a fraction of their original value.

The solution? You could buy a $49 capacitor tester, but if one cap has failed, others are probably waiting. Tom suggests you save your time and "shotgun" the box, replacing all the electrolytics, power supply included. PC mount coupling capacitors can be found at inexpensive prices from Digikey or Mouser Electronics.

Rob Meuser adds a caution. When you "shotgun replace" electrolytics, keep your replacements only to the electrolytic capacitors. Processors use precision polypropylene capacitors that are critical to proper filter function. Aggressive replacement of all capacitors (including the precision caps) will leave you with a non-functioning processor.

That said, if you don't have a complete manual to identify the electrolytics, download one from the Orban site before you start.

Submissions for this column are encouraged, and qualify for SBE recertification credit. Fax your submission to (703) 323-8044, or send e-mail to jbisset@harris.com.

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