Mistakes BITE
Nov 1, 2006 12:00 PM, BY ROLIN LINTAG, CSTE
Basic maintenance protects your station from equipment failures.
Poor connections
Figure 5. Equivalent circuit of a wire at RF
Faulty mechanical connections can be another source of catastrophic failure. Worst-case scenario aside, smoke and fire aren't the kind of things you want in the transmitter building.
Figure 6 shows an equivalent circuit of a terminal strip with loose connectors. A resistance R appears between the terminals creating a voltage drop.
If the terminal strip is for a 24VDC relay, you might only experience intermittent failures. However, if the voltage is 25,000VDC, the result will be much different.
With voltages commonly found in power amplifier circuits, heat will quickly develop between the contacts. Depending on the amount of current flowing, heat will generate, and if sufficient, will cause a meltdown. This illustrates why it's so important to monitor temperatures and connections throughout a transmitter.
Figure 6. Equivalent circuit of a terminal with good electrical contact and loose contact
Finger stock and vacuum tube connections should be inspected carefully and often. It's imperative to have the proper amount of pressure from the stock flange and fingers stock to maintain good mechanical and electrical contact with the tube.
It is not uncommon for loose finger contacts to become welded onto the electrodes. When this happens, the spot overheating can cause premature tube failure.
Loose bolt connections on either the primary or secondary terminals of a power transformer can also cause intermittent problems and overheating. Sometimes this causes the loss of one phase of a three-phase circuit.
It is good practice to regularly check such contacts, as well as other bolted connections in transmitters and primary circuits. The goal is to find these kind of issues before they become failures. And remember to observe the proper safety precautions in dealing with possibly lethal circuits.
Figure 7. An example of a cheap UPS that switches to inverter only after the loss of the mains input voltage is sensed.
Click image to enlarge.
The effort required to maintain accurate voltage and clean AC power may depend on how far your site is from the electrical company's power station. The first step in maintaining clean and correct AC voltages is to build AC power conditioning into your facility from the beginning. Cutting corners, such as relying on an inexpensive UPS system, will only cause problems in the long run.
The typical cheap UPS units are not true online units because they only switch to battery and inverter configuration when a total power loss occurs. (See Figure 7.) A momentary dip or fluctuating main can cause the UPS relay to chatter, resulting in a host of problems for today's digital equipment. Trust me, you don't want this kind of UPS feeding your control room.
Figure 8. An example of a true online UPS that protects the output regardless of anomalies on the mains input.
Click image to enlarge.
A better UPS system continually draws power from its batteries, regardless of the mains status. (See Figure 8.) In this case, the load side of the circuit never sees the source voltage. All load voltages are generated from the UPS, regardless of what happens on the source side. Critical equipment should always be connected to a good UPS to ensure reliable operation.
Human error causes equipment failures more often than we may want to admit. For example, by using the wrong tool, an engineer can over-tighten a bolt and nut beyond its capability. It's better for such a bolt to break during the process of tightening than give you a false sense of security, only to later fail. Such errors often happen on large units such as motors and fans and on outside equipment such as antennas and transmission lines.
The mechanical connections inside a beam transformer should be mechanically tight inorder for them to be electrically sound.
Aluminum bolts used in some antennas, for example, are used for their strength and lightweight characteristics. However, never tighten them without a torque wrench. Tower crews sometimes try to hurry an installation by not using a torque wrench. Unfortunately, those installations may fail as soon as a storm stresses the tower and antenna.
Waveguides also require an accurate torque wrench and proper nut tightening sequence. Tightening the bolts on a rectangular waveguide, for example, needs to be done properly to prevent buckling.
The diplexers and Magic Tee combiners above your UHF transmitter often use such waveguides. The proper way to tighten the flange bolts is to start at the middle of one side and tighten them, always moving outwards.
Overgreasing rubber O-rings on coaxial flanges can cause unstable transmission VSWR. Again, this is an error that an inexperienced tower climber might commit, and you won't discover it until a problem emerges.
You can save your station thousands of dollars by checking the controller inside the building before calling in tower climbers to fi x a tower light problem. Chances are good that you can fi x the problem yourself.
Similar mistakes include forgetting to put O-rings in fittings or using a hammer or wrench to muscle a connection together. Hitting and denting anything along a transmission line will eventually cause failure.
Have you ever worked on a transmitter exciter and found you just can't get the parameters right? Some engineers seem to delight in tweaking.
Sometimes, there are so many adjustments made, one overcompensating for another, that it's necessary to realign the entire circuit. A good rule to follow is if you don't know exactly what your tweaking will do, leave it alone until you do know.
Routine maintenance must be done, and often at night or on the weekend. Because the hours fall outside the normal workday, these tasks are often assigned to less-experienced engineers, often without supervision. Two engineers should perform these tasks so that each can check the other's actions.
Don't let your station's future be damaged by a lack of staff training. Good engineers should train less-experienced engineers. Give back to the next generation of broadcasters by teaching them what you've learned. After all, don't you wish someone had prevented you from having to learn the hard way?
Rolin Lintag, Certified Senior Television Engineer, is chief RF engineer for Victory Television Network in Little Rock, AR.
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