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Servicing FM Receivers

By Jim Arcaro WD8PFK

FM ReceiverThis issue we'll take a look at Servicing FM receivers, such as those found in a VHF/UHF transceiver. Let's assume you've been asked to service a rig for a fellow ham. You've never seen this piece of equipment before. Where do you begin?


Power up, then listen
First, connect the rig up to a 12V power supply. If you don't know which lead is ground, you can clip the negative side of the supply to the case. The positive lead usually goes to the volume control on/off switch. This lead usually can be easily located using an ohmmeter.

The next step is to make sure a good speaker is connected. Some rigs require an external speaker. Others require a special jumper connection to be made on the rear panel connector to bypass an internal speaker. Don't forget to test the internal speaker. On a particular brand of taxi cab radios, the internal speaker was known to go bad 95 percent of the time – so don't assume it's good!

Now power up the unit, turning the squelch down and the volume up. You should hear noise coming from the speaker. If not, you can test the audio Section by feeding a test tone into the center terminal of the volume control.

If you don't hear the tone, the problem is in the audio section. If you do hear the tone, the problem is in the front end, IF stages or demodulator.

Suppose the complaint was weak reception. The best place to start is at the RF amplifier. But don't go too fast. Did you hook up a good antenna? I always use my trusty magnet mount stuck to the top of the refrigerator. Did you examine the SO-239 antenna connector during your visual inspection? A friend once showed me a unit that had the center socket pin missing. No wonder the operator was having problems!


Diodes
Another consideration in weak reception is the switching diodes. These are usually special PIN types, used to quickly switch from receive to transmit. If they go bad, an unwanted attenuation of the signal can occur. Other diodes in this section should be carefully examined, also. I once found that a switching diode from the regulated power source was thermally intermittent.

The tool used here was a can of freeze spray. If you suspect the PIN diodes are bad, unplug the jumper from this section and connect the antenna right to the RF amp input. If the signal is stronger, you've found the culprit.

Of course, the MOSFET is becoming quite common as the RF amplifying device and is still prone to blow out due to static discharge. Usually, a high impedance voltmeter will tell you something is wrong here when compared to the voltage readings on the schematic.


Mixer and oscillator
A telltale sign that this section is defective is the absence of noise with the Squelch down and tuned to an unused channel. (You can qet a good idea of this by turning your TV to an unused channel. Notice the hissing sound. TV sound is FM, too, but the quieting effect is only apparent when a signal or carrier is present.)

If this is a crystal-controlled rig and only one frequency is dead, this is a Clue that the crystal may be cracked. This can easily happen if the unit was dropped, Smashed, kicked — or thrown against a wall! Crystals and crystal filters won't tolerate much abuse. If this is a new synthesized type of rig, you may try to substitute the crystal in the master oscillator. They are usually in the six to 10 MHz range. Do this just as a test, not as a permanent cure. The crystal must be cut exactly to proper frequency and capacitance.

If this trick doesn't work and you still Suspect the crystal, you may be able to take it to a local two-way or TV repair shop. Many of them now have frequency Counters with a built-in Crystal checker. This type of unit will check the crystal at its fundamental frequency — which will be a good go/no go test.


IF amplifiers, limiter and detector
Not much goes wrong in these sections, but I have experienced cases where the whole radio was completely Out of whack! It usually ends up that the slugs inside the coils have turned themselves due to vibration and spun around inside the coil forms, so that the radio is no longer in alignment.

The best way to do an alignment is with a SINAD meter and a communications monitor-type signal generator. Possibly you have made friends at the local two-way shop, and they'll let you use their equipment. (If you do a good job on your own equipment, they might offer you a part-time job!)

The signal generator’s output attenuator is connected to the antenna input, while the SINAD meter is connected across the speaker terminals. Start with the signal at about five microvolts, and reduce it until the SINAD meter reads about – 6 dB. Now follow the alignment procedure in the manual, always working toward the – 12 mark on the meter.

If you reach – 12, reduce the output from the generator and continue. I always go through the receiver twice, at two frequencies in the center of the band, if possible. (Obviously, this is not necessary with a single channel radio) When you're done, the signal generator should be around 0.2 to 0.5 microvolts for 12 dB SINAD, and good sensitivity.


Audio amplifier
You'll know the audio amplifier is bad when you can't achieve good SINAD at any signal level. This tells you that you have excessive audio distortion – probably one of the push/pull-type output transistors is bad.

I remember one particular unit that had been “played with” previously by a green technician. The problem was a burned resistor, or so he thought. When he replaced it, he put in one color-coded brown-black-green, instead of the 15 ohms it should have been. Then he wondered why it still didn’t work!

Replacement with the correct value brought Smoke again and led to the discovery of a shorted tantalum electrolytic capacitor, acting as decoupling for the power supply voltage. Until next time, keep those ears and eyes open. 73.

 

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