Last time, we talked about antenna trouble shooting equipment you should have on hand. This time, we are going to discuss equipment to trouble shoot the radio equipment inside the shack.

When a repeater goes down, there are hundred if not thousands of possible reasons why. Just like your automobile, a repeater is made up of a bunch of different parts that must all work together.

The first major part of the system is the power supply. Most repeaters will have a 12-volt DC power supply that provides a voltage similar to that of a car battery. Some of the most common problems with repeater power supplies are:

• Blown fuses.
• Blown power regulator
• Dried out filter capacitors
• Loose connections

Each of these things can be tested using a simple voltmeter. Voltmeters are available from almost any hardware store or your local Radio Shack. Voltmeters come in two varieties, either analog (has a traditional meter) or digital (has a numeric display).

Digital voltmeters are the norm, but have the drawback that they may not always read properly in a high RF environment (like at a highly populated communications site). In high RF environments, the analog meter is your only option.

Usually, the 12-volt supply leads are easily identifiable on a repeater station. Obviously, if your repeater is made up of two mobile radios, the 12-volt wiring will be exposed and very easily identifiable.

Using your voltmeter in the ‘DC’ mode, check for 12 volts or nearly that coming from the supply while the radios are on. Assuming you do see the 12 volts, make sure it is not fluctuating. Also make sure that you do not see an unusually high voltage reading, possibly indicating a shorted ‘closed’ regulator.

An easy way to check the power supply filter capacitors using only a voltmeter is to switch your voltmeter to the ‘AC’ mode. Check the DC output and the meter should read ‘0’ volts. If you do see some voltage, this means that you have some A/C getting to your radios, which can cause hum or can cause them to not work at all. A pure DC voltage with no ‘ripple’ will show zero volts on a meter measuring A/C voltage.

The best way to check power supply filter capacitors is with a capacitance meter. Some high-end voltmeters can also check capacitance, but seldom do they have the ability to check very big capacitors. I suggest a dedicated capacitance meter for the ability to check these larger capacitors and for the accuracy inherent to their design. One thing you need to remember is to disconnect the power cord from the wall and short the capacitors across them selves before testing! Capacitors can store a charge for a long time, and putting your capacitance meter across a charged cap will result in meter damage, or at the very least a blown fuse.

Moving along, if your power supply is dead, often a blown fuse is the culprit. Fuses do not always go out simply due to an overload. Sometime spurs coming down the power line or loss of vacuum can open a fuse and cause your repeater to quit. A fuse that blows over and over is the sure sign of something bad going on. Time to call the radio shop for help.

Using your voltmeter’s resistance mode, check across the fuses for ‘0’ resistance, or nearly that. Once in a while, the ‘slow blow’ fuses will show a high resistance when blown, but not always show to be fully open. This still means that the fuse is bad and should be replaced.

Here is something else to remember: dirt kills. Keeping the electronic parts and boards free of dirt will go a long way toward keeping your repeater on the air. Enough dirt can de-tune your radios and, when moist, can damage vital electronic parts inside. Use a duster like those sold for computer keyboard dusting at your local computer store or Radio Shack. For those large areas, I prefer to use a terry cloth lightly misted with isopropyl alcohol. This is alcohol with very little water content. Water can lead to corrosion of some of the metal parts and electronics in your radios.

Obviously, troubleshooting the radios themselves is a little beyond the scope of this series and should not be attempted by the average user, but these simple tips may be just the thing to help you get your system back on the air in a pinch.

Next month, it’s back to the most basic of basics. You’ll want to direct your friends to this series to learn what exactly makes up a repeater. And in the future, some tips from the field and stories from some of the dustiest most spider-infested cabinets on the sites. Ewoo! My skin is crawling already….

Gummers^(TM) is a trade mark of Paul Shinn