Cushcraft Trap - Coaxial Capacitor Repair

There are occasions that the coaxial capacitor on Cushcraft trap (R5, R7, etc.) fail due to a breakdown of the insulator between the outer aluminum tubing and the inner capacitor rod. Since parts are no longer available from the manufacturer, home repair is the only option. Following is how I accomplished an permanent fix for this failure. Why these traps fail is somewhat unclear but in my case, it appeared to be a crack in the insulator possibly caused by flexing.

Some coaxial capacitor have only a single rod as opposed to the dual rod version shown here. The repair process is the same, just simpler mechanics.

    

Picture 1: 17 meter Coaxial Capacitor failure (note the white silicon above the insulator that someone had tried as a fix ... see below).

 

Picture 2: Burned insulator after being removed from outer tube.

 

Picture 3: Repaired insulator (the dark area is really clearer that the original insulator).

The Process:

1) Remove the complete damaged trap from the antenna.

2) Measure and record the distance from some reference point (a bracket, screw or mark you put on the outer capacitor tube). This is critical as you need to restore this measurement when you reassemble the capacitor after fixing it. I'd suggest you also measure the how far the insulator protruded beyond the end of the outer tube even though there is still heat shrink over it (see picture 1). It will likely be 1" and is not critical to the operating of the trap but best to restore things as they come from the factory.

3) Carefully remove the shrink tubing that is the moisture seal on the effected capacitor. Depending on the trap in question (single inner rod or dual inner rod), this could be one or two seals.

4) Measure (again) and record the distance from the top of the outside tubing to the top of the insulator and from the top of the outside tubing to a reference point you choose on the inside rod. This will give you a more accurate measurement for reassembly but there is a chance you will move the parts in disassembly and if that happens, you can fall back on the measurements recorded in step 2.

5) Remove the damaged insulator and using whatever tools you have available (I used a Drumell Tool), remove any carbon residue on or burned into the plastic (see Picture 2). Also, clean any carbon residue of the outer tubing and the inner rod.

6) Put the rod back in the insulator to prevent the hot glue (more on glue in step 6) from plugging the insulator hole.

7) Using a preheated hot glue gun, melt the plastic of the insulator and apply new glue into the space cleaned out in step 5. Let cool completely.

8) File the new glue smooth with the form of the insulator and test to make sure it will slide into the outer tubing. You may have to add a bit more hot glue if there are some missed spots (lower than the insulator profile).

9) If your repairing a dual rod trap, this is the point to put new heat shrink tubing over both outer tubing units. Make sure they are long enough to cover the outer tubing, the protruding part of the insulator and at least 3/4" of the inner rod. If it's a single rod capacitor, the tubing can be applied in step 12.

10) Place the repaired insulator(s) and rod back on the inner rod but be sure to put the repair at the bottom end of the rod so that the repair is not in the same physical place as it was when the trap failed.

11) Reset the insulator and rod back to the factory measurements recorded in step 2 and/or 4.

12) If you are doing a single rod repair, place heat shrink tubing as discussed in step 9 over the outer tubing. For either a single or dual rod trap, move the heat shrink to cover the outer rod, insulator and rod so that there is 3/4" overlap at either end. Heat to shrink making sure that you didn't move the rod in placing the heat shrink tubing.

13) Replace trap on the antenna, reinstall the R7, retune and have fun.

R7 returned to (high power) service!

Using other methods such as filling the burned out portion of the insulator with silicon may work for a time but silicon doesn't permanently bond to this type of plastic and a the resultant gap will allow moisture in and result in another failure, especially under higher power.

 

LINK: John's (EI7BA) great site on more R7 and other Trap repairs plus more.

            http://www.iol.ie/~bravo/index.htm

 

Gerry VE6LB  ve6lb@rac.ca       VE6LB Home Page: http://www.qsl.net/ve6lb/