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I recently bought a nice used model 238 tuner from a private party. The =
day the tuner arrived I hooked it up and began to call CQ while running =
750 watts of RTTY on 40 meters into a 75-meter delta loop fed with 450 =
Ohm twinlead.=20
The manual explicitly warns:=20
"When using BALANCED LINE, if the SWR rises during a long transmission, =
it is an indication that a significant portion of the transmitter power =
is being lost in the balun."
Unfortunately, I visually detected the change in SWR at precisely the =
same instant in which my nose detected the acrid smell of smoke. As we =
all know, all circuits contain compressed smoke particles. Once these =
compressed smoke particles are liberated, the circuits no longer work. =
True to this theory, after I let the smoke out of the tuner it no longer =
worked.
Post mortem examination of the tuner uncovered two sources for the =
failure as well as two additional potentially weak spots.=20
The first failure point was one of the balanced output insulators =
passing through the back panel of the tuner. Arcing at the bolt-washer =
interface had melted a portion of the insulator. Upon cooling, this =
melted plastic had flowed around the washer, insulating it from the =
bolt. These lightweight plastic insulators were junked and replaced with =
substantial ceramic insulators. The second failure point was a burnt =
spot where the output lead from the inductor was attached to the =
inductor. A minute portion of the phenolic board had vaporized at the =
attachment point.
Testing also discovered that the spring that holds the wiper against the =
coil became very hot in use. The output lead from the inductor was =
removed from the burnt spot discarded. It was replaced with a slightly =
longer lead that attached to the opposite side of the inductor. =
Post-modification testing confirmed that the spring no longer became hot =
in use. The second potential weak point was the balun. It too became hot =
in use and seemed to be marginally sized to handle full legal power into =
balanced loads.
The dinky stock balun was removed and replaced with a hefty homebrew =
balun. The new balun is too large to reside where the old one was =
placed. The new balun rests on a Plexiglas bracket attached to the floor =
of the tuner. The new balun is parallel with and just inside the right =
wall of the case. It overhangs the phenolic shaft that connects the =
antenna selection knob to the antenna selection switch. =
Post-modification testing confirmed that the new balun did not become =
hot in use.=20
The modified tuner is now capable of legal limit output into balanced =
loads. Since junk-box parts were used, a dollar cost for these =
modifications is hard to compute. I would guess that even at retail, the =
cost should not exceed fifty dollars.
De N7TIN, John nr Seattle
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<P>I recently bought a nice used model 238 tuner from a private party. =
The day=20
the tuner arrived I hooked it up and began to call CQ while running 750 =
watts of=20
RTTY on 40 meters into a 75-meter delta loop fed with 450 Ohm twinlead. =
</P>
<P></P>
<P>The manual explicitly warns: </P>
<DIR>
<DIR>
<P>"When using BALANCED LINE, if the SWR rises during a long =
transmission,=20
it is an indication that a significant portion of the transmitter power =
is being=20
lost in the balun."</P></DIR></DIR>
<P>Unfortunately, I visually detected the change in SWR at precisely the =
same=20
instant in which my nose detected the acrid smell of smoke. As we all =
know, all=20
circuits contain compressed smoke particles. Once these compressed smoke =
particles are liberated, the circuits no longer work. True to this =
theory, after=20
I let the smoke out of the tuner it no longer worked.</P>
<P>Post mortem examination of the tuner uncovered two sources for the =
failure as=20
well as two additional potentially weak spots. </P>
<P>The first failure point was one of the balanced output insulators =
passing=20
through the back panel of the tuner. Arcing at the bolt-washer interface =
had=20
melted a portion of the insulator. Upon cooling, this melted plastic had =
flowed=20
around the washer, insulating it from the bolt. These lightweight =
plastic=20
insulators were junked and replaced with substantial ceramic insulators. =
The=20
second failure point was a burnt spot where the output lead from the =
inductor=20
was attached to the inductor. A minute portion of the phenolic board had =
vaporized at the attachment point.</P>
<P>Testing also discovered that the spring that holds the wiper against =
the coil=20
became very hot in use. The output lead from the inductor was removed =
from the=20
burnt spot discarded. It was replaced with a slightly longer lead that =
attached=20
to the opposite side of the inductor. Post-modification testing =
confirmed that=20
the spring no longer became hot in use. The second potential weak point =
was the=20
balun. It too became hot in use and seemed to be marginally sized to =
handle full=20
legal power into balanced loads.</P>
<P>The dinky stock balun was removed and replaced with a hefty homebrew =
balun.=20
The new balun is too large to reside where the old one was placed. The =
new balun=20
rests on a Plexiglas bracket attached to the floor of the tuner. The new =
balun=20
is parallel with and just inside the right wall of the case. It =
overhangs the=20
phenolic shaft that connects the antenna selection knob to the antenna =
selection=20
switch. Post-modification testing confirmed that the new balun did not =
become=20
hot in use. </P>
<P>The modified tuner is now capable of legal limit output into balanced =
loads.=20
Since junk-box parts were used, a dollar cost for these modifications is =
hard to=20
compute. I would guess that even at retail, the cost should not exceed =
fifty=20
dollars.</P>
<P>De N7TIN, John nr Seattle</P></FONT></DIV></BODY></HTML>
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