An IOT’s lifeblood is its cooling system, so keep it healthy.
The power amplifier cabinet fan cools the top of the IOT’s input cavity.
The primary and secondary output cavities of the IOT are air-cooled and should be kept clean to keep the high-voltage arc detector working.
Perhaps the most expensive single component in a TV broadcast station is the transmitter's inductive-output tube (IOT). Therefore, it is imperative that the station's engineers operate and care for this tube properly. Aside from improper tuning and mishandling, the biggest threat to the IOT's health is poor cooling. It can be a subtle but real cause of early failure in an IOT or, for that matter, any vacuum tube. Most, if not all, IOTs employ both air and water cooling systems that require regular care.
Air cooling
The air cooling system filters and blows air past the outer parts of the IOT assembly to remove heat. The IOT assembly includes the input cavity, where most of the tube connections are located, and the primary and secondary output cavities. Dirt and dust should not get into the cavities for any reason. Check the blower fan's intake air filter regularly; clean or replace it as necessary. It is a good policy to keep the floor surrounding the transmitter clean either with a wet mop or a shop vacuum cleaner. Sweeping the area with a broom will only transfer the dust from places you can see to places you can't; the best place to use a broom is outside the building. You'll be surprised at how long you can keep the intake air filter clean just by keeping the building floor free of dust.
To make sure that the air cooling system blows sufficient air through the IOT assembly when the IOT is operating, regularly verify that the air blower interlock operates correctly. Check the interlock by shutting off the blower's circuit breaker and verifying that it shuts off the transmitter power amplifier (or puts it in standby mode). Of course, you don't want to unnecessarily stress the IOT during this test, so it's best to turn off the high-voltage (beam voltage) at the same time. Otherwise, with the blowers off for a minute or so, the tube may become dangerously hot.
Inspect cooling fans daily, or every time an engineer is on site, to make sure they are operating properly and not making any unusual noise. A noisy muffin fan, for example, probably has damaged bearings and blows less air than it should. Replace it as soon as possible. It's good policy to keep a spare fan in stock, just in case. The next time you inspect the fans, find out their model numbers and availability.
Poor air filtering can become a serious problem, especially in an urban area where air pollution can cause the IOT's high-voltage arc-protection circuitry to malfunction. The pollution can cause a buildup of soot in the internal output cavities (both primary and secondary), darkening the cavities and preventing the arc detector from functioning properly. This is a serious problem that can only be addressed by dismantling the IOT assembly, cleaning the inside of the cavities and, thereafter, permanently preventing polluted air from entering the air cooling system.
With the removal of the pump module’s side panels, the coolant’s reservoir tank is ready for visual inspection.
This IOT shows corrosion on the collector. The compressed air tank in the background is for drying the collector body after it has been cleaned with water and light brushing with a copper brush.
Water cooling
Most water cooling systems for transmitters employ a mixture of ethylene glycol and water, typically in equal parts. In its warranty requirements, the IOT manufacturer should specify the proper mixture. The major reasons for using ethylene glycol are to keep the solution from freezing in cold weather and to prevent corrosion on the copper piping and IOT collector. If the station uses the solution as an electrical part of the transmitter dummy load (i.e., impedance), monitor and maintain it.
To keep the IOT collector at the proper temperature, make sure to maintain the proper coolant flow. Monitor the level of the coolant inside the make-up tank or reservoir and add some coolant if the level falls near the minimum. Refer to the IOT manufacturer's data sheet to verify the minimum coolant flow — usually expressed in gallons per minute (gpm) — and the maximum inlet temperature. Take note that most IOT manufacturers advise an increase of 20 percent in minimum gpm if you use anything other than 100 percent demineralized water as a coolant. For example, using coolant composed of equal parts ethylene glycol and water with a 64kW-rated IOT (sync output power) requires a minimum collector coolant flow of 12 gpm instead of the 10 gpm specified for water-only coolant.
Also, the freezing point of the coolant depends upon the concentration of ethylene glycol. For example, a 50 percent concentration by weight of Dowtherm's SR-1 has a freezing point of -28.9° Fahrenheit. Increasing the concentration up to 80 percent by weight decreases the freezing point down to -52.2°F. Work with the transmitter manufacturer to determine what concentration you should use depending on the climate and winter weather in your station's particular location.
Flow regulators maintain the correct coolant flow into the IOT. The red LEDs indicate that the flow is above the minimum allowed.
Pumps can be used alternately on a monthly basis. Valves and connections should be inspected for leaks.
This photo shows the heat exchanger with the pump module at the back. Air is blown upwards by the fan, and the radiator is underneath the enclosure at the air intake. The AC power switch for the fans is at the upper left corner.
The coolant pump pressure — monitored at the transmitter in pounds per square inch (psi) — determines the coolant flow. This meter usually is located in the pump module near the main and standby pumps. Monitor this pressure meter and log the measurements in the transmitter's operations log so that you can detect any gradual drop in pressure. Inspect the pumps and repair them as needed. Valves and connections should be inspected for leaks. The main and standby pumps should take turns operating on a monthly basis so that, if one fails it will be detected on the next change cycle and not be discovered only in an emergency.
Like your car's cooling system, the IOT's heat exchanger has a fan and a radiator. The radiator has a coiled coolant path with fins that radiate the coolant's heat to the outside environment. These fins need to stay clean and free of dust and oil deposits to allow efficient thermal transfer of heat from the coolant. Clean them regularly with an industrial-grade solvent. Clean them more often during the summer and autumn months when lawn clippings and dead leaves are likely to clog the fins. Industrial-grade solvents can contain corrosive chemicals like sodium hydroxide, so handle them with care. Wear the appropriate safety goggles, and chemical-resistant gloves and clothing. Make sure to read and comply with the material safety data sheet (MSDS) that accompanies the cleaner. Mixing the proper concentration of cleaner into a portable pressure sprayer can make the job easier. Approach the cleaning procedure with care. Before attempting to clean the heat exchanger, before you even remove the heat exchanger's guard screen, turn off the power to the fan. And beware! The fact that the fan blades aren't turning doesn't necessarily mean that the power is off; it may just mean that the thermostat hasn't kicked in. Verify that the fan's AC power switch is off.
Regularly check for coolant leaks. Any short or arcing on the focus coils can result in costly replacements. Ethylene glycol, like Dowtherm SR-1 and the older Ucartherm, is available with fluorescent color to aid in detecting leaks. Inspect the pump module for coolant leaks, especially at the output of the pumps and the drain valves. Also check the IOT collector coolant connections. Leaks on the collector near the magnet frame can disturb transmitter operation by showing up as increased body current or can trip the circuit's breaker.
IOT manufacturers recommend flushing the system and replacing the coolant after each winter season. But you can choose the less expensive option of closely monitoring the system's health and using the coolant for more than one winter. If you choose the latter option, use a pH tester to check the coolant for its acidity. The pH should be somewhere between 8 and 10. If it drops to less than 8, then the coolant is starting to become acidic and it will start to corrode the copper piping and the collector of the IOT. You can mitigate this problem to some extent by adding heat-transfer fluid (HTF) inhibitors. Perform the pH test monthly and send a sample of the coolant to a laboratory for the standard corrosion testing at least once a year.
A proactive program of maintaining the cooling system for the IOT power amplifier requires a fair amount of effort. But, in the long run, it will save time and effort and avoid the perils that a damaged IOT can bring to the engineering department.
Rolin Lintag is an RF engineer with the Victory Television Network in Little Rock, AR.
