In contest RTTY duty, you must change the blower of the AL1200
to a high speed blower. The blower is fine for extended CW and
SSB duty at full power, but not RTTY contests. The manual should
warn about extended duty use in high duty cycle modes in the
When you reduce power on RTTY, you do not decrease heat inside
the PA proportionally to power. Heat decreases somewhat less
than the square root of the power decrease. That is because
efficiency falls as power is reduced, and there are also fixed heat
You'll find many components, like your external switch and even
some coax connectors, will overheat on RTTY.
> The antenna switch between the amp and either dummy load or antenna
> shows signs of heat/arcing, introducing the possibilities:
> 1. The loose PL-259 caused the whole problem, or
> 2. The switch wasn't designed for QRO RTTY (true), so it started to
> break down when it was hot enough, long enough.
Any loose connection or arc after the amp can cause a problem.
As a matter of fact, this is the most common problem causing tank
switch or capacitor failures. The send is improper loading, where
the loading is set too light for the drive power.
One of the most frequent intermittent antenna system failures are
lightning arrestors using gas discharge tubes. Connector failures
and poor connections are also common, but it is my experience
that premature firing of gas discharge tubes in the antenna system
cause most antenna system problems when using big amplifiers.
> Fortunately, I caught it soon enough that the tank coil tap
> connections to the bandswitch are still intact, and the amp still
> works normally. However, several of you who sent me a link that led
> to http://www.vcnet.com/measures/bandsw.html, and now I want to know
> how safe it is to use this bandswitch with the arcing residue on the
First, remember you can read anything on Internet. While some
technical information on that site is good, much is unreliable and
As to how long the switch lasts, it depends on how much or if any
metallic deposits have reduced the rating.
73, Tom W8JI
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