Tech Tip:

How many of you use a DIPLEXER/TRIPLEXER? This is the magical box that allows you to run a dual or
tri-band mobile radio with a single antenna (assuming your radio has two or three antenna
connections). Some radios have this built-in but many don't.

Why do I bring this up.......

I have a Kenwood TM-741 Tri-band mobile that I use for most of my hamming. If you've been in my
shop, you will see it just above my bench. For quite some time, I was having intermittent operation
on 2 meters, and the radio tested fine. The easiest way for me to find the problem, was to replace
the radio with my MFJ-269 antenna analyzer and start wiggling wires to see were the problem was,
while monitoring the SWR scale on the MFJ.

Sure enough, I found the problem with the tri-plexer. It turned out to be the 2 meter input's PL-259
had an open in it. Replacing it solved my intermittent problem BUT while I had the analyzer
attached, I noticed that the SWR was above 2:1 everywhere. This is not good! It could be the
antenna, feedline or something else? The antenna is a Diamond X-510 mounted at the 70 foot level,
and is being fed with 7/8" heliax. Putting the MFJ on the feedline showed me that the antenna system
is fine, so there must still be another problem with the tri-plexer?

I put a 50 ohm dummy load on the antenna port of the triplexer and swept it again with the MFJ. Sure
enough, the SWR was about 2:1 all over the pass band of the unit. Was it always like this? I don't
know, as I've never checked it! Taking the back off the triplexer rewarded me with a bunch of open
coils and capacitors arranged separately for each band (one section for 6 meters, one for 2M and one
for UHF). See the pictures for the internal view. The section on the left is for 6M, the middle is
for 2M and the one on the right is for UHF.

Each section is set up as an LC passband for each band and the only adjustments to be made would be
the open coils, as it uses fixed capacitors. While setting the MFJ for 146.000, I started to "play"
slowly with the coils, first collapsing them and then spreading them, all the while watching the SWR
on the MFJ. Some had little effect, while some made a dramatic effect, good and bad. All the work
was done with a fiberglass rod, which is RF inert. After twiddling a bit, I managed to get the SWR
down to 1.1:1 over the entire 2M band!

While I had it apart, I checked the other 2 bands and sure enough they were off as well, although
not as bad. Does it improve the performance of the radio? Not much, but I'm certainly happier
knowing it's working the way IT WAS SUPPOSE TO! Mass production has it's advantages (cheaper) but
many times, little time is spent for the alignment process.

Tedd Doda, VE3TJD

Lazer Audio and Electronics
Baden, Ontario, Canada