Refurbishing a Cushcraft A3S, by George Szymanski, DU1GM
|Around ten years ago I
bought a second hand A3S triband HF yagi, packaged together with an
IC746, 12V PSU and rotator, and when I erected the A3S at my QTH in
Quezon City it made a remarkable difference in hearing and working
DX. Recently I had to remove the
A3S from the tower and dismantle it. Prior to putting it into
storage, I needed to clean the traps and elements as they were coated
to varying degrees with soot from the traffic pollution along
the busy ten lane Commonwealth Avenue which is not far away. Fortunately the grime was
easy to remove and the next stage was to test the traps.
The Cushcraft A3S Manual can be downloaded by clicking HERE
The A3S is a very popular yagi and if you see one for sale second hand at a good price you should snap it up! It is easy to refurbish as I will explain here.....
I found two of the traps had broken tabs at one end, probably caused by typhoons whipping the elements about over the years. The tabs provide an electrical connection for the outer tubing of the trap which forms the capacitance for the trap. Inside the tubing is a coil wound on a non conducting rod. The ends of this rod are inserted into aluminum tubes which protrude beyond the aluminum outer tubing of the trap.
The tabs proved easy to repair as shown in the photograph above. Without this repair the trap would not resonate on its correct frequency as the capacitance would be essentially disconnected.
I tested all the traps for continuity then tested them again with a GDO. All the traps, including the two repaired ones passed the tests. The GDO checks need to be done carefully. First of all, determine the trap type, then set the GDO to the approximate frequency required. Insert the GDO coil partially into the end of the tube with the tab screw.
Once you have found the dip, slide the GDO coil out of the end of the trap tube, checking the dip all the time, until you have just the slightest indication. This will be as close as you can get to the correct reading. Note that the trap and GDO should be well away from any conducting items which will affect the test. My GDO is an old type, Tradiper TE-15, 1970's vintage but still working, but the frequency stability and readout leave a lot to be desired! Nevertheless, it is the dip that is important and the actual frequency can be found by using an external receiver to find the GDO output. This is usually quite loud so no extra coupling is required, merely an antenna in the vicinity. You may find that the resonant frequencies you measure are not exactly the same as quoted by Cushcraft, but don't worry overly about this as long as the frequencies are nearby. It is important that the traps all read approximately the same according to their type number. When originally supplied, the traps, as well as the elements all had paper markings which weather off quite quickly. The traps can all be identified by the measured frequencies and the elements are obvious when their lengths are measured and compared to the manual.
If you find any traps that cannot be dipped then there may be a fault inside. You should first make sure the coil is ok by checking the resistance from one end of the trap to the other. It should show short circuit. The cover can be removed from the trap by undoing the screw at one end then tapping out the inner parts. Do not attempt to pry out the plastic supports, they should just be tapped out by inserting the handle of a screwdriver or mallet into the trap tube and tapping the other end to push out the inner parts. The other end support plastic will slide past the center punched points on the cover. The coil should be clean and it is covered with a black sealant. Check the screws which secure the coil ends and make sure there is no corrosion anywhere. You may find signs of insect infestation which should be cleaned out. In my antenna there were no signs of any insects.
There are three types of trap on the A3S, and these are listed here with the operating and GDO frequencies
The next stage was cleaning of the elements. This has been described in another article by DU1EV who cleaned a similar antenna. Lots of elbow grease, Scotchbrite or metal scrubbing pad and some laundry soap were all that was required in my case.
Here are some of the driven element parts during the cleaning process.....
After Mayet DV1JAZ and I moved out of Manila to Sariaya in Quezon province, about 80km south of Manila, I was able to reassemble the A3S. When you are in a position to do the same it is as well that you have a good supply of new worm drive type clamps which are used to secure the elements and traps as it is certain that the original ones may be missing or just plain worn out. In my case I had a few too little and had to make do with reusing some of the old ones which were still ok. Once the tower is built I will replace some of the older clamps with new ones before the antenna goes on to the tower.
After re-assembly, the antenna was fitted on to a temporary pole on one corner of the roof above the shack. I deliberately made this part of the roof a flat section so I could mount a tower on it! You can just make out the choke balun at the feed point of the yagi. This is nothing more than an exact copy of the choke as suggested in the A3S manual. As of now, all that exists of the tower is the base which is already fitted in to the roof slab, as we have other priorities right now. A junior op is about to come into the world! Not to mention that the house is not quite finished yet as can be seen in the picture...
Anyway, the antenna has proved its worth once more and I am busy working as many new DXCC as I can on 20m using JT65a. As time goes by conditions are slowly improving on the HF bands and I seem to have timed the refurbishment perfectly.
To see more pictures of the A3S in its temporary position please visit my web site:-
During 2011 I plan to build the roof tower myself and it will consist of at least two 10ft sections and, depending on my welding skills, I may end up with three sections. Look out for an article on making your own tower, and the pitfalls I may find in the process!
© Philippine Amateur Radio Association, Inc. 2010