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How to Troubleshoot VHF/UHF transmitters

By Jim Arcaro WD8PFK

In this article we'll continue our series on electronics troubleshooting, with a look at VHF/UHF transmitters. Most problems fall into a few basic categories: (1) low or no power output; (2) no modulation; (3) incorrect frequency of operation; and (4) problems with the CTCSS encoder.

As usual, start by connecting the transmitter/transceiver to an adequate power supply. A transmitter rated at 45 watts cannot properly operate from a 12 volt, 3 amp supply! The transmitter will probably blow the fuse in the supply the second it is keyed. Of course, when testing any transmitter, it is recommended that you connect it to a dummy load or terminating wattmeter to avoid interference with other stations sharing the frequency.

After the unit is properly connected, performance tests can be made to determine where the problem is. Power output can be checked with a wattmeter, frequency of operation with a frequency counter or error meter on a communications analyzer, and deviation with a deviation meter. These three tests will give you a good idea of where to begin.

No or low power output
The first thought you may have when experiencing low or no power output is that the final amplifier transistors have blown. Not so fast! I recommend that you, the technician, go through a complete transmitter tune up first. For a fixed or base station this may not be necessary, but for a mobile unit it is a must. Mobile rigs get bounced around daily, and the detuning effect can be just as effective on the transmitter as on the receiver.

I remember one unit quite well that was in use in a commercial dump truck. After keying, the wattmeter showed no power output for about 30 seconds to one minute before slowly coming up to rated power. By following the manufacturer's service manual and doing a complete tune-up, the rig was back in working order in less than half an hour. No parts needed to be replaced; just a little TLC brought it back to life.

Of course, you may find a unit that will not respond. In cases of fairly low output, the use of an RF voltmeter is almost a must. The probe, or “sniffer,” is placed on the base of the transistor and the transmitter keyed, then moved to the collector, and the corresponding amplification (or lack thereof) noted.

You should be aware that in some of the stages required for frequency multiplication, the output might be slightly less than the input. This is normal and should be noted. Also note that many transistors of the RF variety open when they fail. This is in direct contrast to small signał audio transistors that usually short from collector to emitter. An open transistor will not conduct and thus won’t generate much heat. So after the transmitter has been keyed for a short time, you can use your own heat probe – your index finger – to feel the case of each device after the key has been released!

The subject of transistor casings brings to mind an interesting variation. I once needed to replace a transistor whose metal case was not the collector. Instead, it was the emitter! Some time spent with a replacement guide showed that the Philips ECG 341 would do quite nicely – and it did.

No modulation
This problem, in most cases, can be traced back to the microphone – a broken wire in the cord, a damaged element, possibly a broken key switch, or a dead battery. Replacement mikes are fairly inexpensive unless they incorporate a tone o pad. Before replacing the mike, try another, with or without the tone pad, just to make sure you are on the right track. Also make sure the connectors are tight and not loose or spinning around. Sometimes it can be just as simple as that!

Along with no modulation, you can consider the symptoms of low deviation and over deviation. Many times these problems are caused by improperly setting the controls due to the use of uncalibrated test equipment. It is recommended that you check a few good signals before adjusting a deviation control.

At peak voice level, which can be simulated by soundly pronouncing the word 'four' the transmitter should produce less than 5kHz of deviation. If the transmitter is equipped with CTCSS encoding, the voice should cause 4.5kHz or less with the encoder disabled. The encoder alone should produce about 500Hz of deviation without voice, for a total deviation of 5kHz or less when talking loudly into the mike.

Important point – a transmitter that is over deviating may not “fit” the modulation acceptance bandwidth of the listener's receiver. This makes for choppy audio, excessive deviation and poor communications. If this happens to you, tell the talker to back away from the microphone. Six inches away from the mouth is about right.

Incorrect frequency
Most crystal-controlled transmitters will drift off frequency with time and age. They will also respond differently to changes in surrounding temperature. A warm transmitter on the workbench may not work very well at all in the trunk of a care when it’s ten degrees outside.

Most transmitters have some method of adjusting the operating frequency slightly, be it a capacitor or coil adjustment. If it is not possible to adjust the center frequency back to within a couple dozen hertz or so, it may be time for new crystals. Check the radio on other frequencies.

If it's off on one, the crystal is your culprit. If it is a synthesized radio, go over the manufacturer’s instructions for aligning the VCO. A VCO that is way out of adjustment also can cause no power output. In another instance, I fixed a synthesized radio that had been modified to work the five kHz split frequencies. This radio was sometimes intermittent and had driven a few of us techs crazy. Against our better judgement, we even replaced the microprocessor chip. No luck with freeze spray either. Then, on a bet, i decided to resolder the factory installed modification jumpers. That solved the problem. Hours of wasted time due to a poor soldering job. I'll never let myself forget that one!

Problems with CTCSS
I discussed some problems with CTCSS in the January|February issue. Basically, if the tone is more than a few cycles off frequency, it will not trip the decoder at the repeater or base station. Use the frequency counter with a low pass filter to measure the tone, and check it for deviation, too. If the module is not performing correctly, I recommend replacing it. The newer modules are smaller, better designed, and create fewer problems. Some of them even have built-in speech inversion for use in secure systems, such as police departments.

I hope you enjoy these columns and am always glad to hear from readers with suggestions for future columns. So let me hear from you. ‘Til next time, 73.

Cleveland Institute of Electronics
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