Near the end of the CQWWRTTY test (30+ hrs), while running only 1000W, we
heard a buzz from somewhere near the AL-1200. I yelled wildly at Alan to
hit ESC to kill transmit. I then investigated and found a loose PL-259 on a
switch used to switch between a dummy load and the rest of the antenna
system. I could tighten it another turn or so. Back on the dummy load, and
finally the antenna, all seemed well. We worked a few more stations, then
heard the buzz again. However, this time I could see bright white light
emanating from the inside of the amp through the vent holes. We couldn't
find anything else wrong, and continued to use the amp for another 2 hrs or
so at 500W without any difficulty.
The pristine bandswitch, just replaced last week (!) is shown at
The tank coil taps for 15M and 10M are within the yellow circle, and note
the arrow pointing to a metal that appears to be part of the attachment of
another layer of the bandswitch.
The arced bandswitch is shown at http://www.qsl.net/wa9als/burned.jpg It
looks like the arcing was between the 15 and/or 10M tap attachment points
and 2 of those "anchoring tabs" of the bandswitch structure.
In contrast to the last time there was an "accident" like this, the
bandswitch connections appear structurally intact. Last time the 15M
attachment point was destroyed.
This happened at a time when the amp was running very hot. I think
Ameritron would say that a condition of high SWR developed. I want to
contribute it to the heat somehow! Can anyone imagine how -just- heat could
lead to the arcing, albeit obviously indirectly? Otherwise, I'm left with
trying to trouble shoot an antenna system, including a switch, connectors,
tuner (on direct setting), lightning protection etc etc that apparently only
malfunctions during certain conditions of QRO. Both times something like
this has happened, it's been after a long period of QRO contesting at KW+ on
RTTY. I guess I don't really think it's the antenna system - I think it's
inherent to the amp not being built for QRO RTTY contesting.
It's not something like C31XR elements blowing in the breeze etc - The SWR
on that is rock solid always and the same for the past 2 yrs.
Any constructive opinions greatly appreciated! Thanks
John - WA9ALS - VP5RY
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