In this article, we will continue to study the electronic troubleshooting series and introduce VHF/UHF transmitters. Most problems can be divided into the following basic categories: (1) Low or no power output; (2) No modulation; (3) Wrong operating frequency; (4) There is a problem with the CTCSS encoder.

As always, first connect the transceiver to a sufficient power source. A transmitter with a rated power of 45 watts cannot operate normally with a 12 volt, 3 ampere power supply! During the second lockout period, the transmitter may blow the power fuse. Of course, when testing the transmitter, it is recommended to connect it to a virtual load or terminal power meter to avoid interference with other stations sharing the frequency.

After the device is properly connected, performance tests can be performed to determine where the problem is. You can use the power meter to check the output power, the frequency counter or error counter on the communication analyzer to check the operating frequency, and the deviation counter to check the deviation. These three tests will give you a good idea of where to start.

No output power or low output power
When you encounter low power output or no power output, the first thing you may think of is that the final amplifier transistor is exhausted. Not so fast! I suggest that you, as a technician, debug the entire transmitter first. For fixed or base stations, this is not necessary, but for mobile units, it is necessary. The mobile drilling rig is beating every day, and its detuning effect is as effective on the transmitter as it is on the receiver.

I remember using a good device in a commercial dump truck. After inputting, the power meter has no output power for about 30 seconds to 1 minute, and then slowly rises to the rated power. By following the manufacturer's service manual and conducting a full commissioning, the platform can resume normal operation in less than half an hour. No need to replace parts; just a little bit of TLC can rejuvenate it.

Of course, you can find a unit that will not respond. If the output is low enough, you should almost use an RF voltmeter. The probe or "sniffer" is placed on the base of the transistor, the emitter is locked, and then moved to the collector, and the corresponding magnification (or lack of magnification) is recorded.

You should note that in some steps required for frequency multiplication, the output may be slightly smaller than the input. This is a normal phenomenon and should be noted. Also note that many RF transistors will turn on when they fail. This is in sharp contrast to the small audio signal transistors that are usually short-circuited between the collector and emitter. The transistor in the off state will not turn on, so it will not generate much heat. Therefore, after locking the transmitter for a short time, you can use your own thermal probe (index finger) to feel the status of each device after releasing the key!

The theme of the transistor case reminded me of an interesting change. I once needed to replace a transistor whose metal case was not the collector. On the contrary, it is a launcher! It took a while for the replacement guide to show that the Philips ECG 341 will do well-and it does.

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

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