A high-power tv transmitter will not be a really thrilling factor to take a look at, only a huge steel field (6-feet tall, 12-feet broad, 4-feet deep) full of electronics however principally featureless on the skin. Present fashions are solid-state, full of a whole bunch or 1000’s of high-power transistors, working at round 50 volts. They turned the most typical sorts within the mid-2010s.
The earlier era of UHF transmitters (beginning at 470 MHz, channel 14), have been simply as plain to take a look at, however could possibly be far more fascinating. They used vacuum tubes generally known as inductive output tubes (IOTs)—successors of klystrons. The tube is about 32 inches tall and weighs 64 kilos. It sits in a “circuit meeting” containing tuned enter and output cavities and weighs virtually 350 kilos. Every IOT amplifier, of which there have been normally two, has an influence provide producing 35,000 volts DC and attracts as much as two amps, producing round 25,000 watts of common RF output energy. Sure, the effectivity is low, however that’s a results of plenty of elements, together with the necessity to function at school AB mode.
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What made it fascinating? For one factor, the 35 kilovolts on the IOTs, which can actually hold a transmitter engineer centered. As a colleague as soon as warned me, “Watch out round that prime voltage, Bob, it will possibly kill you for a very long time.” The transmitter has many interlocks, each digital and bodily, to forestall simply that from taking place.
For an additional, whereas the IOT tube was very rugged and will, with correct care and feeding, run repeatedly for greater than 60,000 hours (round seven years), it may be destroyed right away if not correctly protected. Actually, a really excessive proportion of circuitry in an IOT amplifier is devoted simply to defending the tube.
One of many methods it does that is to watch numerous parameters and take away the high-voltage from the tube right away, in case any parameters are out of vary. It does this by firing a crowbar, normally a thyratron tube, just like an SCR, shorting out the excessive voltage which bypasses the excessive voltage provide’s saved power away from the tube till the facility provide’s circuit breaker opens.
A crowbar occasion is an thrilling factor to be round. By thrilling, I imply horrifying. Relays click on, contactors bang, actually huge circuit breakers journey, and generally the lights within the constructing flicker. Then the transmitter resets itself and begins up once more. If an actual fault stays, versus a transient overload, this occurs as much as thrice in a ten-second interval earlier than the transmitter offers up and stays off.
And that’s the place this story begins. One amplifier in our transmitter started intermittently crowbarring. Typically it could occur two or thrice a day, different instances solely as soon as each few days. Not one of the many fault LEDs confirmed any clue to the supply of the issue.
The intermittency and randomness of the crowbar journeys had the texture of a mechanical downside, what with all of the cooling followers, high-pressure blowers and liquid cooling pumps vibrating every part. So, my assistant and I beat the residing daylights out of the transmitter, whacking away at every part we might to attempt to provoke it into tripping. This was very fulfilling. Sadly, it didn’t assist as each time we thought we’d discovered the best spot to hit, it turned out to be a false alarm.
As I discussed, an terrible lot of issues need to be good for the transmitter to be glad and permit the amplifier to be energized. There are circuits repeatedly monitoring every of these items. Throughout operation, if any of those parameters falls out of vary, even for a microsecond, the amplifier will shut down and recycle. Every of those circuits sends a TTL-level good/unhealthy sign to the amplifier’s management board, the place they’re summed by an AND gate. If the output of the AND gate is excessive, the amplifier is allowed to function, and a front-panel LED referred to as “Amplifier Prepared” is lit. Ought to any of the inputs drop out, the LED and amplifier flip off. Our downside was that certainly one of these alerts was dropping out momentarily, and so quick we couldn’t even see the LED flicker.
Because it occurred, we had a logic analyzer, not a typical piece of apparatus for a tv station. As a way to catch the fault in motion, we hooked it as much as monitor the entire AND gate’s inputs and set it to set off when the gate’s output went low. Then we might uncover which safety circuit had fired. Naturally the transmitter was sensible to what we have been as much as, so it behaved itself for the following three days. Lastly, we caught it within the act, and located the “Magnet Present in Vary” sign was the perpetrator.
My assistant began tracing the wiring again to the sampling circuitry. The wires main as much as the management board have been connected through a spring-loaded terminal strip on the backside of the cupboard. He counted off the wires along with his fingers, and when he touched the proper one, it lifted proper up. It turned out the assembler on the manufacturing facility had not opened the spring contact all the best way and the wire was simply touching the contact, held there solely by gravity and the wire’s stiffness. Over time the standard of {the electrical} contact degraded till it lastly started failing. Fifteen years it sat there, taking part in possum, earlier than it lastly awoke.
So, ultimately it turned out we have been proper – it was mechanical. We simply didn’t beat up the best a part of the transmitter.
Robert Yankowitz lately retired as Chief Engineer at a tv station in Boston, Massachusetts, the place he had labored for 23 years. Previous to that, he labored for 15 years at a station in Windfall, Rhode Island.
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