Some people have experienced a problem with the AM-6155 conversion for 220 MHz. The problem is related to the amount of drive required to obtain the 400 to 500 watts of output these units are capable of generating. If you fail to achieve this power level with a maximum of 4 to 5 watts of drive, then you may have a problem with the input tuning circuit. Also, if the unit used to work just fine, but now puts out hardly anything, the problem is the same. Or if your power output gradually drifts down as you operate the amplifier, same problem. Check the VSWR. I bet it's not very good.

All these problems can most likely be traced to a non-resonant input circuit. This is caused by the wrong or a changed value of the 33 pf input tuning capacitor ( silver mica added to the input circuit during conversion ) that was originally specified in most of the conversion information that I remember seeing. Due to the normal tolerance of these capacitors some of the conversions work just fine. But if your capacitor was a little on the high side, then the grid might not have ever tuned properly. What can happen when there is an off resonant condition? You increase the drive power to say 8 or 9 watts to achieve the desired output power. The extra heat and higher voltage developed across the mismatch "cooks" the capacitor and it changes value over time. Suddenly an amplifier that "worked" for years, stops working.

The solution is very simple: Replace the 33 pf with a 20 pf to 22 pf capacitor (silver mica). You should observe that the grid now tunes with the variable input tuning capacitor at least partially engaged (you will probably find that the best tuning used to be achieved with the variable at minimum!). The tuning will be a lot sharper, but the input drive power should be lower for the same power output and long turn power stability should be much improved.

Give it a try it might just work for you too.

Sometime back I wrote a little article on the input tuning problems that sometimes occur with the venerable Fair Radio amplifiers. Here's another problem area that has been observed on an amplifier used on 220 MHz (Could also show up on 2 meter amplifiers). Erratic input tuning is once again the symptom. It may also be your problem.

The input circuit has a 'pedestal' inductor that turns out to be made in two pieces, joined in the center by threaded stud. Over time this connection can work loose or, more likely, become resistive by the build up of corrosion. This will cause the inductor to change effective length and become non-resonant. "Flaky" or no input tuning is usually the result. Sometimes a check with an ohm meter will reveal the problem (pedestal should be ground), but the only sure way to check it out is to disassemble the unit and clean the threaded joint. This is not a simple task and requires the complete removal and disassembly of the grid circuit. It is removed by the same procedure that you would use if converting an amplifier to 432 MHz. A little anticorrosion compound, such as that used by electricians on aluminum house wiring, should prevent future problems.

Years ago when I ventured into the world of 6 meter FM I needed a simple, light weight antenna that I could mount at the top of an antenna mast. The Ringo vertical was the first thought, but this a relatively large antenna to mount on the end of a 15 foot mast with other rather large antennas already installed. I needed something lighter and smaller.

The 6 meter FM crowd often use unmodified 5/8 wave two meter antennas for mobile installations. I don't fully understand why this works, but it does work quite well. The logical extension of the thought process is to use a 5/8 wave 2 meter base style antenna for 6 meter tower installations. Actually, it really defies all logic if you look at the base matching network of the typical 5/8 2m vertical. However, there is a way to make at least one manufactures model work very well as 1/4 wave antenna for 6 meters.

I purchased a Comet Model CA-ABC21 5/8 wave GP vertical antenna which is a fairly well made unit with ground radials and a universal mount that will fit just about any mast size, including 2 inch. The unit is a bit pricey at about $50.00, but it does provide a quick and dirty way of getting on 6 meter FM. By the way, the Japanese seem to have problems with SO-239 connectors. Something was lost in the translation for I had to remove some material from the bottom of the connector to make a standard PL-259 screw on. But it can be made to work with Mr. File.

I tried the antenna on 6 meters without modification and could not find a match anywhere in the 6m band. This was not entirely unexpected because these antennas have a base loading coil that is fed 1 to 2 turns up from the ground end and obviously made to match 2 meters. However, the length of the whip part of the antenna approaches a 1/4 wave in the 6m band. The solution was to remove the ground connection from the matching network and thus form a base loaded 1/4 wave antenna. The slight loading by the remaining coil turns resonated the unmodified whip at about 52 MHz (VSWR 1.1:1). The VSWR was less then 2.0:1 across the whole band. Following is the simple modification for this particular antenna, but the basic idea could be applied to any similar antenna that allows you to get at the matching coil without destroying the antenna.

The SO-239 connector can be screwed out of the base if you remove the single set screw that keeps it from rotating (metric set screw, of course). Carefully grasp the threads of the connector with pliers to aid in its removal. (Note how far the connector was screwed into the body before you remove it. This will make sure you don't screw it in to far when you replace it.) Once unscrewed you will have the entire matching coil and connector assembly in one piece. Find the place on the coil where the feed cable is tapped on to the coil about 2 turns up from the ground end. Simply clip the coil on the ground side of the tap and at the ground end where the coil is attached to the connector body. That's all there is to it. Screw the assembly back into the body of the antenna, careful not to go to far and damage the split brass post that makes electrical contact with the whip part of the antenna. Replace the set screw and complete assembly of the antenna according to the original instructions. The radials are of course too short for 6 meters, but the antenna seems to work just fine. It doesn't work bad on the low end either. CQ CQ No lids, No Kids, No Space Cadets.

Hurry please, my medication! Have you looked at the price of a HAM-M (HAM IV) lately? My goodness it's enough to cause massive heart failure. Those of us who have been around long enough remember that for the princely sum of about $100, one could move into the 'big time' from a U-100 or CDR clicker to a, WOW!, HAM-M rotator. This was about 25 years ago when a Ben Franklin was worth something and the democrats were running the country (no connection). Sometime between then and now the HAM-M has become a big buck item, commanding a large chunk of change. And to add insult to injury, it has not changed one iota. It's still exactly the same rotator as my vintage units. What has changed is the control box (for the better) and the fact that for somewhere between $350 and $400 smackers you don't even get the lower mast support assembly! I must be getting old. Another positive, however, is that the unit does come with a replacement south centered meter face if you want to change from the north centered scale factory installed.

The point of this article is that I thought one of the really nice improvements to the control box that I heard about over the years was the addition of a switch that allowed you to check full rotation calibration without turning the rotator to the stop. You know what, that feature has been removed from the latest models! Meter calibration was always a problem with the old units; you had to run the antennas all the way to the stop to set the full scale meter reading with the pot that was on the back of the box. Voltage regulation of the meter circuit was either non existent on the very early models or not very good on some of the later models. The calibrate pot is now on the front panel (another nice improvement) but the calibrate switch has been deleted. Major bummer. The meter circuit is better regulated these days, but still I find that it does change with time and line voltage.

For those of you who have likewise missed an era, the function can be added back to the new control boxes with a minimum of effort. I also added a brake light that adds a little piazza to the front panel. Following are the simple modifications:

Locate the wiper arm of the calibration pot (wire goes to ground). Lift the ground end of the wire and connect it to the center arm of a single pole, double throw, momentary push button (or toggle switch) that you have installed on the front panel. The normally closed contact is connected to ground and the normally open contact goes to the backside of terminal #7 on the 8 terminal rotor cable strip. That's it. When in the normally closed position the meter indicates rotor position as always, and when in the normally open position, the meter goes full scale right where you can use the calibration pot to set to full scale. There is a slight error (maybe 5 degrees) because of the difference in ground paths, but you can soon remember a mental compensation factor and put it right on. A nice spot for the switch is dead center between the calibration pot and on/off switch.

Another simple addition is a brake light indicator. I found a small red lamp in the junk box that mounts in a 1/4 inch hole. The voltage is not to important because a resistor can be added to drop the voltage to a value that will give long bulb life for the particular lamp you find. The light is connected between terminal #2 of the rotor cable strip and ground. The area just to the left of the calibration pot is a good mounting location. Adjust the voltage across the lamp with a series resistor that drops the 24 VAC of the brake circuit to about 70 % of the rated lamp voltage ( R=E/I). Use the lamp current from a catalog or variable power supply to determine the current for the lamp being used.