IOT cooling contamination
Feb 1, 2009 8:27 AM, By Russell Brown
Air cooling
Figure 4
Click to enlarge
Even liquid-cooled transmitters use air to do some of the cooling. RF components like the output stack, solid-state IPAs, input cavity and power supplies are best cooled by air. All transmitters have air filters at the air intake for these fans, which should be cleaned regularly. Some transmitter rooms are sealed off from outside air and just recirculate the conditioned air within the room; these are the cleanest rooms without outside dust and dirt getting into the transmitter. But still, the air filters must be checked and cleaned on the transmitter.
Figure 5
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A good example of why the air filters should be cleaned happen when an IOT that had been working for several years suddenly developed a high-voltage problem where it would not hold up anymore. As soon as high voltage was applied, it would crowbar. It was thought that the tube had failed and a new one was on order, but when the old one had its input cavity removed, the problem was revealed.
It seems some type of contaminate had worked its way between the silicon boots that surround the ceramic gun insulator found at the bottom of the input cavity. This contaminate allowed an arc path to develop between the grid ring (~34kV) and the IOT’s pole piece (grounded). (See Figures 3, previous page, and 4.)
Figure 6
Click to enlarge
Once the first arc occurred, the carbon created on the silicon boot made it that much easier for the next arc to occur. It looked much worse than it was. Once the silicon was cleaned and the arc pitting was sanded down, they were as good as new. (See Figure 5.) Both the input cavity and IOT were “high-potted,” tested with high voltage, and they both passed. (See Figure 6.) The input cavity was not suspected because the quality of input cavities has improved greatly over the years. In the early days of IOTs for television, there was a great deal of trouble with the input cavities — but that is in the past. In this case, the culprit was some type of contaminate that got past the air filters.
As in all high-voltage systems, cleanliness is the best way to staying on the air. By keeping both the cooling liquid and air clean, transmitters will last longer and have fewer problems.
The next “Transition to Digital” will continue with transmitter coolant contamination issues.
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