The Case of the Unexpected FM Transmitter Turnoff

After installing a new 10kW FM transmitter for KOST FM in Los Angeles, I ran into an interesting problem. When I switched an RF relay to transfer the auxiliary transmitter from the auxiliary antenna to the dummy load for testing, the main transmitter would turn off, taking the station off the air.

The first two times it happened, I didn't notice the station had gone off the air. The transmitter shut down so quietly I couldn't tell. The blower noise from the auxiliary transmitter covered up the sound of the main transmitter going off the air.

Luckily I happened to walk around to the front of the main transmitter, and I realized the monitor speaker was dead. I wondered, how did that happen? I noticed it was off before the programming department had a chance to call and ask what happened.

I knew it had to be crosstalk in the control wiring, because there was no direct connection between the RF relay and the transmitter. This same control circuit wiring had been working perfectly with an older transmitter that used relays for remote control interfacing.

The problem manifested itself not long after the new solid state transmitter replaced the old tube-type transmitter. The problem turned out to be an OFF command line connected to the remote control system. I proved this when I again switched the RF transfer relay and observed the main transmitter go off the air again. An examination of the remote control interface schematic showed that it was an overly sensitive circuit.

The inductive pulse from the motor-driven RF relay was crosstalking into the remote control command OFF line. I fixed it by installing a 1000 Ohm resistor across the OFF command line at the transmitter. This effectively converted this overly sensitive OFF transmitter input line to a 20 milliamp loop. After this modification the inductive pulse from the RF relay did not contain enough energy to influence the 20 milliamp loop circuit. The problem was solved.

This entry was submitted by Marvin Collins and edited by Rob Spiegel.

Marvin Collins began working in broadcasting in 1954 at KCBH 98.7 in Beverly Hills, Calif. This was back in the days when not many people knew what FM broadcasting was. Later he worked at KPOL AM 1540, in Los Angeles, followed by KRLA 1110 in Pasadena, Calif. In 1976 he moved to KFI AM 640 and KOST FM 103.5 where he became chief engineer in 1980.

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