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From: "Gary - G4WGT" <g4wgt@tiscali.co.uk>
To: "Alan Melia" <alan.melia@btinternet.com>
Cc: "LF \(Rsgb LF Group\)" <rsgb_lf_group@blacksheep.org>
Date: Mon, 7 Aug 2006 21:13:03 +0100
Message-ID: <NKEMLGOEKANPNJOBFBAFKEEPCOAA.g4wgt@tiscali.co.uk>
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Subject: LF: RE: re PA FET
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Hi Alan & LF,

I have repaired the PA tonight & during the afternoon I had a look in the
antenna coil box etc, what I found was obviously the cause of the failure.

Possibly due to the hot weather the top of my loading coil container had
sagged & during the recent rain (yesterday) the indent had filled with water
causing a small pool. The other contributing factor was that I must have
disturbed the feeder to the vertical at the top of the coil & it was almost
touching the inside top of the plastic coil box so it had been arcing at
that point to the pool of water & eventually caused the demise of the PA.

The PA last night was running 38 volts at 11 Amps for about 400 watts, the
SWR was virtually 1:1 & no indications of any problem were seen when
periodically checked.

I have now corrected the container problem with a plastic support inside
pushing the top upward in a dome so no water will lodge & re-connected &
re-dressed the feeder wire.

I am going to run some tests & check the coil again whilst live.

I have also sent a copy of this mail to LF to keep others informed.

73

Gary - G4WGT.


-----Original Message-----
From: Alan Melia [mailto:alan.melia@btinternet.com]
Sent: 06 August 2006 22:53
To: Gary - G4WGT
Subject: re PA FET


Hi Gary Yes sorry to hear about the FETs and with no heating of the heat
sink. That suggest either a change in antenna tuning, a flash-over of an
insulator, or maybe a instant of overlapped drive. I guess the FET got a
high voltage....I wonder what you rating is an how big a margin you have,
with the supply voltage you are running. As far as I can make out the min
FET voltage is 4 times the supply voltage, but a mistune can easily exceed
even that. Have a look round the outside for distressed components in
daylight.

I did a quick look at Geoclock and Singapore has quite a good darkness band
I estimated 1930 to 2300z should be possible. Shanghi Is more difficult the
window is only about 1 Hour at present 2130 to 2230z. It will get better of
course from now on into the autumn.

Cheers de Alan G3NYK