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The Case of the Missing 13,000 Watts

The Case of the Missing 13,000 Watts

Early in my career, I did consultant work for small radio stations. It is not uncommon for a radio station to lose a screwdriver or a soldering iron. But when a station loses 13KW, it needs a detective. Since it could not find a detective, it called me.

The radio station used a Gates FM-10 transmitter with an output of just under 10KW. Because of the antenna gain, the station was licensed for 50KW ERP. When I arrived at the transmitter, the screen voltage had been reduced to nearly zero volts. Plate voltage was slightly high at about 13KV, and the ammeter indicated plate current was normal at just over an ampere. The RF power output meter showed 0KW, and listeners could not receive the station. The transmitter did not tune properly, either.

I was puzzled about where the power was going, especially if it was not going to the antenna. It did not seem like a bad tube, because bad tubes do not behave that way. Needless to say, replacing a 4CX 10,000 is not an easy job.

I opened the back of the transmitter, shorted out the 13KV supply for safety, and opened the power amplifier output cavity. I expected to see melted components, but to my surprise, the tube was dead -- stone cold.

Without any further action, I closed the cavity and removed the front panel of the transmitter. I disconnected the plate ammeter and replaced it with a Simpson 260. I was very uncomfortable because the meter was on the high side of the 13KV supply. I turned on the transmitter and increased the screen voltage until the plate current was one ampere. After slight retuning, the transmitter worked perfectly. The shunt inside the ammeter had been eliminated by unknown forces. The 13KW was never really missing; it was a metering error.

This entry was submitted by Frank Karkota and edited by Rob Spiegel.

Frank Karkota worked with power transmitters in the range of less than 1MHz to 5GHz. He designed and built equipment for radio stations and eventually started a company that made commercial and consumer receivers that covered 500kHz to almost 1GHz.

Tell us your experience in solving a knotty engineering problem. Send examples to Rob Spiegel for Sherlock Ohms.

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