Since the temperature promised to exceed 100 degrees today, it seemed
like an ideal day to take off work and go out into the blazing sun to
work on a bad KT34XA. The symptoms on the tower were the SWR was bad on
20 most of the time, and varied with no particular pattern on the other
bands over time. Armed with K6LL's "checks all joints" trick, I first
checked all element halves. All under 0.5 ohms, most measuring 0.1 ohm
with a Fluke 77.
I was then thinking of what others had said about all other problems
being caused by a bad balun. I was going to pull it and check it with a
200 ohm resistor and an RF-1, but I got to looking at the phasing straps
and the matching bars. I realized that if you were to measure the DC
resistance between the element halves of the rear driven element, you
should see a DC loop along the straps, out the front driven, up the
matching bars and through the balun coil. I measured this and saw 70
ohms! A sharp rap on the matching bar with a screwdriver brought this
down to less than one ohm, but it was clear a bad connection was at hand.
I disassembled the connecting straps between the front driven and the
bar(s) and cleaned with steel wool and reapplied penetrox, then put back
together. All is well now. BUT...
One thing I didn't mention was there were the #6 sheet metal screws
applied into the elements as others had suggested as a way to
"bulletproof" those mechanical connections. The screws were all tight,
but the connection had failed anyway. I guess nothing is foolproof.
I hadn't seen this test in print before, it is probably old news but I
will take full credit for discovering it since it was new to me! ;=>
73, Jim N6IG
PS: I went ahead and tested the balun anyway with the resistor and the
RF-1, good match from 2 to 30 MHz...
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|