Build This Custom Switch Controller
     

Chief Engineer Greg Manfroi shares a useful tip with Workbench readers.

His backup transmitter at WUIS is in need of a rebuild. That could be a fun project; but Greg has to find the time, and he only has one well-used 4CX15,000A for it. A local commercial broadcaster will donate one of their well-maintained former backup transmitters; but Greg needs funds to transport it, and he’ll have to cut a new door into his transmitter building. Projects never end!

At present Greg is operating in HD with separate antennas. He’s using a Broadcast Electronics FXi 250 exciter/FMi 201 combination to feed the HD antenna. This exciter can be programmed to go into FM+HD mode when a closure is held on programmable pins on the remote connector.

Greg feels it would dangerous to put the coaxial antenna switch for the main and backup transmitters in the hands of present-day operators (many years have passed since operators were required to take FCC exams). There are interlocks in the coaxial switch but he prefers not to tempt fate.

Greg built a simple comparator circuit that looks at the forward power sample of his main BE FM30A transmitter. He selected the comparator voltage divider resistors to cause the comparator to operate when the forward power of the main transmitter dropped below 20 percent.

When this occurs, the comparator output turns on a transistor that energizes a relay coil. The relay contacts are connected to a programmed remote pin on the HD transmitter/FXi 250 exciter. The exciter analog settings are set to match the FXi 60 exciter feeding the FM30A main transmitter (analog).

If the main transmitter goes down, the HD exciter goes into FM+HD mode immediately. The result is that the station is only off the air for a half-second after the failure. Of course it comes back at reduced power, but it is still on, with no intervention by the air staff. This allows time for Greg to get to the site, put on a higher-power backup transmitter and repair the problem.

Greg housed the components of the schematic shown in Fig. 1 in an old black aluminum chassis, a controller saved from the NPR SOSS system. Greg also had an old LED illuminated push button (Push ON/Push OFF) switch that he used for the “enable/disable” switch/indicator.

Fig. 1: This automatic ‘power-sensing’ RF switch controller can be used to control FM+IBOC switching.

He knows his remote control can be programmed to do something similar; but all of the relays on the remote are spoken for, and with funds at a premium, the home-brew solution seemed best.

Greg installed this about four years ago and has had a few transmitter failures since; the circuit worked as planned, keeping the station on the air.

* * *

RFI stories can keep engineers entertained (or frustrated) for hours. Educational Media Foundation Transmitter Engineer Dr. Ross du Clair is no exception.

Ross recalls that as the CE and DOE of a flame-thrower AM station in Sacramento, Calif., he once took a call from a person who had recently moved in and apparently was receiving the station’s entire broadcast day on every appliance in her home.

Ross drove to the address, which was familiar. As he got out of his truck, the new homeowner met him in the driveway, understandably upset with the interference.

When she finished explaining, Ross asked if she knew anything about those two 662-foot Franklin antennas across the street.

“Oh, yes. The real-estate agent said not to worry about those things, as they are cell phone towers.”

Not quite.

Ross worked on her phone (adding filters), television (adding more filters) and a radio (which he couldn’t hope to clean up). The station purchased a Bose for her.

About a week later Ross took another call from a pottery shop owner in the town next to the AM station location, the same flame-throwing powerhouse. The shop owner complained that her new kilns would not maintain temperature; she felt sure the AM station was affecting the thermocouples.

As an engineer, Ross believes anything is possible. Arriving at the pottery shop, it took him less than five seconds to grasp the situation.

Both kilns were wrapped in a thin layer of stainless steel sheet. They looked pretty, but they were not grounded.

Ross left a 10-foot ground rod and a roll of 2-inch copper strap for her electrician. He explained that his company’s rules prevented him from doing anything further but if the electrician would follow his detailed instructions, the thermal controllers would work just fine. The electrician did the work and the controllers worked as advertised.

* * *

One of my favorite RFI tales goes back to early in my career.

We’d just installed a modified DAP and a Gates Solid Statesman limiter at our 5 kW AM. The signal was loud! We received a call from a nursing home located down the street and in the major lobe of the station.

The staff had been confused when the elderly residents started saying “turn down that music” or “stop screaming.” It took some time for the staff to realize their residents weren’t hallucinating but hearing our jocks hollering call letters and playing “hot” top 40 hits via their overloaded hearing aids.

Care to share an RFI anecdote? E-mail it to johnpbisset@gmail.com.

Contribute to Workbench! You’ll help your fellow engineers, and qualify for SBE recertification credit. Send Workbench tips to johnpbisset@gmail.com. Fax to (603) 472-4944.

Author John Bisset has spent 43 years in the broadcasting industry, and is still learning! He works for Tieline Technology, is SBE Certified and is a past recipient of the SBE’s Educator of the Year Award.


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If the comparator's input pins are close in voltage for any time the output could get noisy, leading to relay chatter and unpleasant behavior. The solution is to add some positive feedback with a resistor from pin 1 to pin 3 of the LM2902. About 30mV should work fine, or 15K to 18K for the circuit and values shown. Also, dding ~470 ohms in series with the sense line on either side of the clamp diodes also makes for a much more rugged input stage.
By Robert LaJeunesse on 8/1/2011

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