ARTICLE FROM APRIL 1997

TUNING FM RADIOS, PART 2

This month I'll discuss tuning of FM receivers and transmitters further. Even though emphasis is on the Motorola MITREK and MAXAR, keep in mind that a good deal of this applies to other FM radios as well.

EQUIPMENT
It would be wonderful if all of us had a commercial FM service monitor in our shacks for tuning our radios. Lacking one, with some ingenuity you can still do some meaningful work. One affordable and very valuable piece of test equipment you need is a good old fashioned AC voltmeter (or audio dB meter). The type with a moving needle meter movement is best, not a digital one. For tuning the Motorola radios you need to have a ordinary zero to 50 uA panel meter as well or you can use your VTVM.

This is one application where the old fashioned meters with moving needles work best. They make peaking or nulling of circuits very easy. Not all of us can follow the rapidly changing digits on a digital meter when tuning a radio! Fortunately old AC voltmeters (even HP models) show up at hamfests on a regular basis. Keep an eye out for them.

QUIETING
The simplest way to tune a FM receiver is by "quieting." With no signal you get a loud rushing noise from the detector (speaker) with the radio unsquelched. As the unmodulated signal becomes stronger the rushing noise quiets. Thus a strong signal that has driven the receivers limiter stages into limiting is said to be "full quieting."

To measure quieting connect your AC voltmeter across the speaker leads. open the squelch and adjust the volume control and the voltmeters range switch till the meter reads "0" dB on the decibel scale (if the meter has one). By setting the range switch correctly you should be able to get a reading without the speaker being so loud as to drive you out of the shack.

Note the position of the range switch. In my example lets say that I can get a level of zero dB on the -10 range position. This tells me that on this position my reading of zero is really -10 dB audio level or about .25 volts AC. As you increase the signal going into the radio, this reading will drop as the radio quiets. You can increase the sensitivity of your meter by going to a lower range (-20 dB) or you can decrease the level of the signal going into the radio to keep the meter on a usable part of the scale.

For comparison purposes most technicians quote the sensitivity of a FM radio in microvolts that give 20 dB of quieting. For example, it you had a calibrated signal generator you would set your meter and volume control to give the "zero" dB reference level. Then you turn on the signal generator and increase its output till the meter drops 20 dB in level. The output level of the signal generator is the sensitivity of the radio.

OTHER SIGNAL SOURCES
This takes me back to the times when I had no signal generator to tune my radios with. The first and most obvious signal source is off the air signals, such as the local repeater. Another often overlooked source is your HT. Set on low power and used with an attenuator it can serve well when you need a strong signal.

A case in point is when you have to retune the helical resonators on the front end of a receiver. When untuned they have so much "off channel" attenuation that they can make a perfectly good radio seem to be completely dead. A HT can easily provide the strong signal you need to bring them into preliminary alignment.

From this point on what you need is weak signal that you can slowly attenuate as the radio comes into tune. One method is the old fox hunters trick of the "wave guide beyond cutoff" attenuator for your HT. This is a lot simpler than the name makes it sound. All you need is a scrap of any kind of metal pipe or tube that is big enough to insert your HT into with a cap on one end.

As you lower your HT into the tube, it begins to act as a wave guide that is too small for the frequency that you are transmitting on. That is, instead of conducting the signal as a true wave guide, it begins to cut if off. The amount of attenuation you get is dependent on the length of the tube and the position of the HT in it. Obviously you need some remote non- conductive way to key your HT and a string to move it up and down inside the tube. HINT: place a 4.7K resistor and a small toggle switch across the external speaker mike connections. This is how most HT's are keyed by their external microphones.

XTAL OSCILLATOR
Another weak signal source could be harmonics of a simple crystal oscillator. This method was very popular before programmable signal generators made the scene. This method works very well if you have several radios to work on that are on the same frequency. For example a crystal that gives you a harmonic on 146.52 would be perfect for all of your two meter programmable radios. It could just as easily be on 146.01 (or .07) if you needed to tune your packet radios. The point being that just two or three crystals could handle all of your needs.

A quick check of the JAN catalog shows that transmit crystals for the old style crystal controlled FM rigs cost $11.00 each for 10PPM tolerance. Most of these radios used the 8th harmonic, that is the crystal frequency is multiplied times eight to get the operating frequency. For our 146.520 signal, we would need: 146.520 / 8 = 18.315 Mhz.

The oscillator circuit published last month will generate plenty of harmonic energy from any crystal. It can be equipped with a home brew attenuator and after calibration be used for checking receiver sensitivity. This is the same method I used when I was in high school. To tune my six meter converters, I took the 8.40 Mhz crystal from my transmitter. The sixth harmonic of 8.4 Mhz is 50.4 Mhz (the old six meter AM calling frequency).

I plugged the crystal into the coil socket of my grid dip oscillator and then carried the grid dipper up to our garage to in order to attenuate the signal. The drive control on the grid dipper provided fine control of the signal level. Normally I started off with the oscillator sitting next to the dead radio. As I tuned it into life I had to carry the oscillator to the garage and them make several trips back and forth to reduce the output as the radios sensitivity increased. Not very convenient, but it worked!

A home brew oscillator in a shielded box with a ARRL handbook step attenuator sure would have made things a lot easier for me.

TRANSMITTER
Transmitter tuning is a different story. Assuming as we discussed last month you have a frequency counter and thus a way to tune the transmitter on frequency, the next problem is tuning for power output. You have two problems. One is tuning of the low level stages, the other is setting the transmitter power output.

The older Motorola radios like the MAXAR, MICOR and MITREK all have metering positions for all of the transmitter stages. Detailed instructions for tuning them are in the service manuals. It is absolutely vital that you have these manuals in order to tune these radios. Especially when tuning the multistage helical resonators.

These radios were intended to be tuned with the aid of dedicated test sets that plugged into service jacks in the radios. A function switch allowed the technician to easily flip between stages tuning for the correct current at each stage. Well, we don't have these test sets available so we have to improvise.

In almost all cases an ordinary DC voltmeter or a micro amp meter will work just fine if you understand how the metering circuit works. On all three radios I use a old zero center 25-0-25 microamp meter that I bought at a hamfest. The advantage of the zero center being that you don't even have to worry about hooking up the leads correctly. I increased its usefulness by adding a range switch and some shunt resistors. My meter now has ranges of 25uA, 100uA and 1 mA. full scale. This handles just about everything.

HINT: The lead of a ordinary two watt carbon resistor perfectly fits the metering socket of a MICOR and MITREK radio.

Calibration of the meter is not that important. Most stages of these radios were intended to be "tuned for smoke," that is tuned for maximum output. What is important is that you have a sensitive meter so you can see the often tiny reading a untuned stage gives you when your starting out.

NEXT: Follow the instructions in the manual! There is a rational method behind the way the Motorola engineers want you to tune these radios. This is true for any FM transmitter. Always have a dummy load connected to the transmitter. Always monitor the total current the radio is drawing from the power supply. Always start with the lowest power stage first and work your way towards the power amplifier stage.

Get yourself orientated inside the chassis before you start. There is nothing more frustrating than wasting time trying to tune the wrong coil. The first time I tune a unfamiliar radio I locate each of the adjustment points in the chassis and clearly mark them (L3,L4 etc..) with a pencil. This saves me a lot of time and confusion.

TEST BENCH
Spend some time setting up your test bench. Eliminate as much haywire as possible! A clean professional setup prevents needless frustration and prevents shorts and damaged equipment. When tuning the Motorola radios get an extra main connector plug if you at all possibly can and wire yourself up a simple control head with a speaker in it. A transmit PTT toggle switch is wonderfully handy as are terminals for your AC voltmeter.

WARNING: The MITREK radios use a floating push pull audio output stage! There is 7 volts to ground on both sides of the speaker. Grounding either side will burn out one of the two (or both) of the expensive audio output IC's. Connect your AC voltmeter from one side of the speaker to ground. DO NOT connect the ground lead of the voltmeter to the speaker terminals.

When you start tuning be careful that you do not end up barking up the wrong tree. You can get false peaks (weak ones) that make the radio appear to be "almost" working and lead you astray. Keep in mind that when adjusting slug tuned coils that turning the slug clockwise screws it lower in the coil form. This lowers the slug into the coil REDUCING the resonate frequency of the circuit. Turning the slug out (counter-clockwise) lowers the inductance increasing the resonate frequency.

For example, if your tuning a radio from the 150Mhz public service band to the two meter band, the coil slugs have to go deeper into the coil forms, not out. The same for the helical resonators. The screws turn inward to lower the resonate frequency, outward to raise it. It is vital that you tune the radio in a orderly planned way, never start tuning coils helter-skelter. Never forget this!

Ferrite slugs in the coils are easy to break. Try to have the right alignment tool if at all possible. Never force a slug to turn, you will break it. If a slug jams, take the coil form off the board, disassemble it and try to find out why. You may have to pick wax or glue out of the form to free the slug. Gentle heating with the tip of your soldering iron or even a tiny drop of oil will sometimes free jammed slugs.

FINAL STAGES
FM radios have class C amplifier stages. Class C amps have a tendency to "come alive" suddenly when the drive from preceding stages reaches a critical point. One second the wattmeter reads zero, then with the slightest turn of a slug the wattmeter pins. Other then its emotional affect, pinning the wattmeter needle is of little use to you.

Before starting to tune up a FM transmitter, I always locate the radios power output set control and turn it down. Tune all of the multiplier and driver stages first using the metering pins per the instructions. When your ready for the P.A. stage, then turn the power set control up part way. After you get an indication of power out of the P.A., and your sure the radio is tuned properly, then go ahead and set the power out to the desired safe level.

When tuning P.A. stages, it is very useful to have a amp meter in the power supply lead. Never tune a FM amplifier for maximum output. Always tune for the best efficiency. That is, the desired power output at the least PA collector current. This requires watching both meters at the same time while you tune. Not an easy trick I admit. It takes time to tune for maximum efficiency and this is one reason it is important to do it at a low power level where you can do it safely.

As you tune the P.A. you will clearly see that at some power levels much more current is drawn by the P.A. transistors than others. In most cases I have seen the a increase in power of only 5% can easily increase P.A. current by 20% or more. Obviously your far better off NOT to have that 5% extra RF power output due to the stress it puts on your power supply and P.A. This is clearly a case where less is more.

Although I have said this many times before, I will state it again. For Amateur Radio purposes ALWAYS derate the power output of the transmitter! Commercial transmitters like the MICOR and MITREK are designed for very low duty cycles. Commercial users simply aren't rag chewers! If you have a 50 watt radio, use the power set control to reduce it to 25 watts. Tune for the best efficiency and your radio will last a lot longer.

NEXT MONTH
This article is running long, so I'll save my discussion of power measurement, wattmeters and dummy loads till next month. Till then, if you have any questions or need clarification of this material, please contact me. I would be very happy to help you.

David Metz, WA0AUQ
Comments to davemetz@muscanet.com
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