Mistakes BITE

Basic maintenance protects your station from equipment failures.
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“A little neglect may breed mischief: for want of a nail the shoe was lost; for want of a shoe the horse was lost; and for want of a horse the rider was lost.” This maxim about nails and horseshoes from Benjamin Franklin contains an important message for broadcasters. In much the same way Franklin's rider was horseless for want of a nail, it's the little, overlooked elements of basic maintenance that can unexpectedly cause a broadcaster to lose his station for want of a properly grounded system or because of a loose bolt on a power transformer.

This article focuses on four maintenance topics — proper grounding, mechanical connections, AC power and staff training. These are simple concepts, but they are often taken for granted until problems occur.

Proper grounding

Electric current seeks the path of least resistance. The key use of a facility grounding system is to route any unwanted electrical current to the ground or serve as a return path to the source.

Proper AC grounding provides protection by serving as a default current path instead of allowing current to flow through a person's body. (See Figure 1.) This protection is usually provided by the third, round pin on equipment plugs. It's important that the third prong of every AC plug be properly connected to a ground system so any unsafe current is diverted to ground.

A common ground system also helps prevent ground loops from developing in low-frequency circuits like audio. Figure 2 illustrates two circuits connected to ground at unrelated points. Figure 3 shows that a noise voltage then develops because of the difference in the voltage potential between the two grounding points, A and B. The noise voltage source (VN) will therefore add to the audio signal, resulting in hum caused by the ground loop.

Hearing a hum on an audio circuit can indicate that a ground loop has developed. Check interconnecting cables and plugs, and you'll likely find the culprit.

Instead of grounding each circuit at its nearest ground point, use a single point ground and tie the ground point of both circuits to the same ground point. This way, no ground loop develops.

Lightning protection

A good understanding of a grounding system is critical to preventing damage to your gear. In Figure 4, note the simplified equivalent circuit of a lightning rod system connected to an earth ground. The lumped parameters jXL and R represent the link's combined inductance and the resistance.

At low voltage DC, most of the impedance to any current going to the earth ground is due to the resistance R. However, for a fast transient high voltage and high current such as in a lightning strike, jXL becomes large, thereby helping to block the lightning current from shunting directly to ground.

Copper wires react to RF and lightning much the same. (See Figure 5.) In other words, high current and high frequencies can get shunted to the ground.

It's important to remember that for the lightning protection to be effective, jXL should be minimized the same way as the resistance R.

Use large cables or copper straps to lower both the resistance and impedance of the ground path. Try to avoid folds and kinks.

Unnecessary turns of a copper strap increase jXL. Inspection and repair of the ground wires and their connection to the earth should be included as a regular maintenance routine because they tend to deteriorate.

This is especially the case with outside connections and on towers. Such maintenance may prevent mysterious equipment failures that seem to happen only during thunderstorms.

Install surge suppressors on individual equipment that you want to protect, and do not rely on a single surge suppressor at the bulkhead panel. Distributing surge suppression throughout the station is a good investment to ensure that voltage spikes do not get through to the equipment.

Poor connections

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.

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.

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.

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.

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.

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.