原创 Bringing a transmitter back to life

Back in the latter years of the 70s, there was a television programme called WKRP in Cincinnati. The station manager was the son of the station owner. He mismanaged the station and could not be fired, because, after all, "What mother would fire her own son?"

I worked for such a station as a contract engineer. It was smaller, and the manager was younger and less competent than the guy in charge at WKRP. The station transmitter was located in a riverbed, which provided an excellent ground and no structures within nearly a thousand feet of the tower. The building was made of concrete and contained a Gates BC-1T one-kilowatt transmitter, along with some audio processing and monitoring equipment. The single tower was next to the building, and the matching network was just inside the building.

Because of the concern of flooding, there was a large sump pump and a plank forming a tall door jam at the bottom of the only door. Just as long as the power did not fail, the pump could easily remove any water that seeped through the floor or concrete walls.

One day, I got a call from the owner's son that the station was off the air. I rushed out to the transmitter. When I arrived, I discovered that what had been an open field only days ago was now a large lake with a tower in the middle.

I asked him what happened. "I went out to see if the transmitter was OK, and when I opened the door, the transmitter turned off," he said.

He had some waist-high boots for me. I yanked them on, and we went out to the transmitter shack. It was scary walking through two feet of strong river current to the transmitter several hundred feet from the shore. By this time, the water level was below the plank, and we went into the building. The sump pump had removed most of the water. I opened the transmitter and made an assessment. Every transformer was damaged except the modulation transformer, because it was well sealed. The transmitter was dead.

Every minute that a station is off the air means lost revenue, and this particular station was so poorly managed that it was losing money even when it was on the air. I had to work quickly. I decided that I needed to replace the main power transformer (which was 220-6,000V), the intermediate power (IPA) transformer (which was 220-1,000V), and the modulation choke.

I contacted a friend who lived nearby and asked if he had any transformers lying around. He said he had a few television power transformers. I said I'd take them. He said that there was a big transformer lying around at an FM station some 40 miles away, but it weighed a ton. I was sure that his statement was hyperbole, but I did not want to take any chances. I got every man who worked at the station into my little Toyota, and we went to the distant station. By the time we arrived, it was dark.

The distant FM station had a graveyard of old transmitter parts in the basement. I dug through the parts until I found a big transformer. The trip to the distant station was not bad, but after we loaded the transformer into my trunk, the Toyota did not handle too well. When we got back to the station, the river had acquired a layer of ice.

We all worked to get the transformer from my car to the transmitter shack and placed it next to the transmitter. Everybody, except me, went home. I watched them cross the semi-frozen river in the darkness, and then they disappeared at the shore of this strange new lake. I stared into the darkness and heard the eerie cracking of the ice as the water receded beneath it. I suddenly realised that I was alone. I was all alone. I felt like the last living cell in a dying corpse. I closed the door to preserve the heat. When I turned around, I saw directly in front of me a dead transmitter. I wondered if it was really possible to take old discarded parts to reanimate a dead transmitter. But I knew that I had until sunrise to find out.

I wired the two-television transformer in series to get about 800V centre tapped, more or less. I ran some long wires from where the power transformer was originally mounted to the temporary transformer. The next problem was the modulation choke. The modulator used a push-pull amplifier consisting of two 833A power triodes. The balance prevented saturation of the transformer. Running the RF amplifier plate current through the transformer would have saturated the core and reduced the fidelity, so the designers bypassed the RF plate current through a choke. Since the choke was unusable, I ran the RF plate current through the secondary of the modulation transformer. I decided not to run the transmitter at full power until I could replace the modulation choke.

After many hours of work, I stood back and looked at the transmitter. Would it come back to life? Or would it explode? With great trepidation, I reached for the plate switch. I wondered if a giant spark would jump out of the transmitter and zap me. With all the courage that I could muster, I pressed the plate switch. I looked at the meters, and they had proper readings.

I screamed, "It's alive! It's aliiive!"

I knew that the villagers would have their little pitchforks ready to eat bacon and eggs with their morning news and music as the sun rose over the receding river. I had not disappointed them.

Before going home, I brought the damaged power transformer and the modulation choke to a local motor repair shop. Then I ordered a replacement IPA transformer. Finally, I went home and got some sleep. Within a week, the transmitter was completely repaired and back to full power.

This story was submitted by Frank Karkota for Frankenstein's Fix, a design contest hosted by EE Times (US).

Frank Karkota started work in broadcasting in the late 1960s as an engineer and subsequently worked as a contract/consultant broadcast engineer. From 1968 to 1970, he worked with a team that maintained a tactical troposcatter system in Vietnam. He later operated a small company, ComPol Inc., that manufactured SCA receivers.