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ARRL Repurposes AM Broadcast Transmitter for Ham Radio Use:

from The ARRL Letter on March 8, 2018
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ARRL Repurposes AM Broadcast Transmitter for Ham Radio Use:

Thanks to a joint effort by ARRL and the Vintage Radio and Communications Museum of Connecticut (VRCMCT, a classic Gates BC-1T AM broadcast transmitter will enjoy a second life on the Amateur Radio bands for occasional use under W1AW or under the ARRL Headquarters Operators Club call sign, W1INF.

Tim "Timtron" Smith, WA1HLR, digs into the Gates BC-1T broadcast transmitter.

Spearheaded by broadcast engineer Dan Thomas, NC1J, VRCMCT volunteers restored the1 kW transmitter to operating condition, after obtaining it from the National Capital Radio and Television Museum in Bowie, Maryland. The VRCMCT will retain ownership of the transmitter, while the League houses and maintains it on loan. The transmitter will be located in the ARRL Lab, and Assistant Lab Manager Bob Allison, WB1GCM, said the transmitter could be on the air as W1AW during such operating events as the AM Rally and the Heavy Metal Rally.

ARRL turned to AM guru and veteran broadcast engineer Tim "Timtron" Smith, WA1HLR, of Skowhegan, Maine, to handle shifting the BC-1T from 1,340 kHz to the ham bands. Timtron not only has been an AM mainstay on 75 and 40 meters over the years, he's engineered all manner of AM, FM, and HF broadcast transmitters in his extensive career. This combination of familiarity and experience made him a logical choice to handle the conversion to amateur use of the Gates BC-1T.

Various stipulations added a level of complexity to the endeavor. First, the transmitter had to be modified as little as possible, retaining original components. The 833 final amplifier tubes (left), better suited for broadcast-band use, would be retained as would the inductance-heavy tuning circuits. Another requirement -- this one set by Smith -- ambitiously called for the transmitter to function on 75 as well as on 160 meters.

Each RF stage was converted, starting with the Colpitts oscillator -- which offered two octal tube sockets to hold broadcast crystals, and a selector switch. More complicated was changing out feedback and loading capacitors in the oscillator stage, along with the buffer tank circuit. The driver tank circuit was next. Removing one-half of the windings on the multiple tank, changing some connections, shortening long leads on RF bypass capacitors, and modifying the neutralization circuit were necessary.

Assistant ARRL Lab Manager Bob Allison, WB1GCM, at the helm of W1INF at ARRL Headquarters. The BC-1T is on the right.

The output tuning circuit proved to be the easiest to convert; parallel capacitors that enabled broadcast-band operation were rewired in series to resonate on the amateur bands. A spare inductor, not required for higher frequencies, was repurposed in place as a dc safety shunt. The modulator just needed only minor changes. All was documented

Initial tests at 250 W on February 22 demonstrated the success of the modifications and marked completion of the first phase of a new lease on life for the BC-1T as ARRL's flagship AM amateur band transmitter. "It took many volunteers and their resources to make this project come together," said Allison, who calls the BC-1T "The Ambassador."

"It's an ambassador for the AM mode, reaching out a friendly hand to radio amateurs old and new," he said.

The project began in ARRL Lab on February 18 with the presentation to "Timtron" of an official ARRL Lab coat. As if stepping from the pages of a 1960s ARRL Handbook, he looked the part and was ready to begin the operation. -- Thanks to Clark Burgard, N1BCG, and Bob Allison, WB1GCM


The ARRL Letter

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