I just wrote up this Telrex antenna Trap repair procedure for Bob, K6XX. If you have room somewhere on your web page maybe you could include it somewhere so other Telrex owners around the world can keep em ticking!

73, Dennis N6K

Hi Bob, Yes Telrex traps do tend to toast but not since I modified the 15 and 20 mtr traps on the driven element of my 7 element Monarch which covers 40 thru 10 mtrs. Never had a problem with the 10 mtr traps. I marked where the driven elements assemblies were inserted into the short stub coming off the boom and was able with my tower cranked down and another person on my patio roof supporting and pushing up on the ends of the driven element, to remove the 2 sides. (I borrowed my neighbors pool cleaning poles and mounted a donut shaped piece of aluminum to the end of the pole and had my friend capture the end of the element so he could lift and take the weight off my end near the boom).

The failure mechanism is that even though the doorknob cap used in the trap is rated at 5 to 7 kV ACROSS the cap, Telrex, in their infinite wisdom, had laid the doorknob cap directly on top of and touching the coil section of the trap! So, the caps appear to flash right up through the SIDE of the cap from the coil! The object of this repair exercise is to remove the old burned out doorknob cap but replace it with 2 doorknobs in series, allowing you to lift the straps that the ends of the doorknob cap is attached to. Two advantages of this type of repair are:

1. You now have double the voltage protection across the two 5 kV doorknobs caps.

2. The doorknob caps no longer lie directly on top of the trap coil. Now, on my Monarch, the value of the doorknob happened to be 33pF ( Since they seem to burn one side of the driven element at a time, I was able to chisel off the goop on the corresponding good trap on the other side of the driven element and read the value, maybe yours are different value.) Ok so here's the drill on extracting the old burned doorknob and then resealing the trap. Once you get the element on the ground I went to a machinist (who happened to be my father) and he made me a small sharp chisel shaped piece of metal. I used it and a small ball peen hammer and I CAREFULLY chiseled off that brownish colored goop used to seal the the trap. I exposed the doorknob capacitor (or what was left of it) and I also exposed the flat metal straps on each side all the way down to and including the nut and screw end of the straps.

Be very carful when chiseling around the coil not to dig the end of the chisel INTO the coil or flat metal straps. If you do hit the coil by accident, although you won't cut through the coil you will nick it pretty good. Take a file to it later and file off any sharp edges or burrs you create. Using a small wire brush, be sure to remove ALL traces of the black burnt carbon and other debris that the burned out doorknob capacitor deposited in and around the coil.

You may need a small chisel tool to clean in between the coil windings. Since it's hard to find the exact 32.XX to 33.XX pF cap I replaced the original cap with 2 doorknob caps in series using a short piece of threaded rod to screw then together. (Changing the value of the combo of caps by just a few tenths of a pF moves the resonant point quite a bit. 0.5pF too much or too little could move you 150 kHZ if I remember right!) I found plenty 50 to 75 pf 5 kV doorknob caps on the surplus market and put 2 caps in series of various values and was able to fine tune my antenna resonance points to exactly which part of the band I wanted. (This is NOT the fun part, as I started with some series combination value around 32.5 pF using my hand held capacitance meter, then, before sealing the traps, I reinstalled the driven element and cranked the tower up and measured resonant frequency.)

Changing the value of the combo of caps by just a few tenths of a pF moves the resonant point where I wanted it. I originally tried to use a grid dip meter t0 measure the resonant freq of the good corresponding trap on the opposite side of the driven element but that didn't work out for some reason (It's been a few years since I did the repair.) For some reason and maybe because I lifted the caps off of and away from the coil, my notes indicate that I wound up using combinations of caps that yielded capacitance in the 30 to 31.5 pF range, nowhere near the original 33 pF (marked value) they replaced!!

The opposite ends of the dual doorknob caps assembly are now connected to each strap and now it is easy to position the caps at least 1/4 inch away from the coil. Once you get the antenna resonant where you want it, seal the repair by first wrapping the coil from end to end with Kapton Tape which should be available from industrial electrical supply companies. I utilized a 1/2' width tape and applied 2 layers to the coil using an overlapping wrap. Then apply Dow Corning 738 Electrical Sealant to cover all exposed areas of the coil and doorknob capacitors. 738 is UV stable and has a dielectric property of 400 to 500 Volt per 0.001 inch. That will give you plenty of arc protection now that you have the caps 1/4" off the coil! ( The sealant did NOT affect the resonant point, luckily!)

Unfortunately 738 only comes in white which makes for a very ugly repair, but up at 50 ft or more, who's going to see it except the birds! It also has only a 6 month shelf life so a lot of places won't stock it and you may have to special order it. There's an industrial supply company in San Diego called Yale Enterprises (619 299-7710) that used to sell it in 3 OZ small tubes but you may have to buy the caulking gun size. I believe that one 3 OZ tube will easily have enough sealant to redo 2 traps. The small tubes used to cost @ $7 and @$20 for the caulking gun size. There's also a place in San Jose area called Anderson Supply that carries it but may make you buy the larger amount to meet minimum order. Caveat Emptor if you use any other type of sealant, make sure it's UV impervious, temperature stable and has enough electrical dielectric strength, voltage wise.

Also, a different type of sealant might shift the resonant point !!! After you repair the burned trap, go back and chisel out the remaining good trap doorknob and replace it or I can guarantee that in the dead of winter when your tower is covered with ice, the other side WILL fail. My Telrex Monarch continues to provide good reliable service, contest after contest on 40 thru 10 meters, pumping 1.5 kW into it for 48 hrs at a time! (I just found a note showing I repaired it 8 years ago!) If this 200 lB beam wasn't such a killer antenna on 40 mtrs with 3 almost full size elements, I probably would have not gone through this much trouble to repair it. BTW, it is was originally rated for 100 MPH wind and 5 KW and cost over $2000 in 1982. So who says a trapped antenna can't compete!

73, Dennis Vernacchia N6KI / ZF2AR

I put also 20m telrex manual on my web for download!
73 es best dx
de oe1ems







Last Update:March 10, 2007