Envelope-to: dave@picks.force9.co.uk
Delivery-date: Mon, 04 Jul 2005 18:24:45 +0100
From: "Dave Pick" <dave@picks.force9.co.uk>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 4 Jul 2005 18:23:55 +0100
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In-Reply-To: <NKEMLGOEKANPNJOBFBAFGEJBCFAA.g4wgt@tiscali.co.uk>
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Subject: LF: RE: RE: G0MRF 300w Tx PA mods
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Gary.

Have you any SWR protection on the TX?
I would doubt if the cores would saturate so quickly that you didn't notice
the heat and drop-off in power before it went pop!
SWR protection is essential with these PAs because an arc on the antenna
system will suddenly strike, causing poor SWR. The trip will both save the
PA and stop the arc from doing too much damage to the antenna.

If you do have SWR protection then it may be that you just momentarily
over-Volted the FETs. Are they 100V devices? I always reckon on an HT of no
more than a quarter of the FET max rating...

73

Dave G3YXM. 

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Gary - G4WGT
Sent: 04 July 2005 15:29
To: Steve McDonald
Cc: LF (Rsgb LF Group)
Subject: LF: RE: G0MRF 300w Tx PA mods

Hi Steve & LF,

On 28th January 2005 you included the following statement in your e-mail.

""I think Scott has found that anything over 43V at 15A is getting very
scary as the cores will saturate all of a sudden...""

It seems that t has just happened to my PA, although my readings were
showing 36 volts at 14 Amps (504 watts), I was testing again & tweaked the
volts up a bit from 34 volts at 12 Amps (408 watts), it only stayed on about
30 seconds & failed.

During testing after the mods were carried out I bench tested into 50 ohm
dummy load & achieved 645 watts so I felt that I could procede a little
higher into the antenna, the antenna readings looked fine using Jim's
(M0BMU) LF Tuning Meter. So I wonder, was it core saturation??

The core I am using is the original which came with the kit from David
(G0MRF).

Any thoughts or ideas ??

Regards,

Gary - G4WGT - IO83qp

Web : http://myweb.tiscali.co.uk/wgtaylor


-----Original Message-----
From: Steve McDonald [mailto:jsm@gulfislands.com]
Sent: 28 January 2005 22:50
To: g4wgt@tiscali.co.uk
Subject: Re: G0MRF 300w Tx PA mods


Gary - it sounds like you are getting similar results as we are with our
'west coast half-killowatt' amps. I am wondering what toroid you are using
in the output xfrmr? Both Scott (VE7TIL) and I are using salvaged TV cores
which seem to be OK up to a limit...they all appear to be slightly
different. I think Scott has found that anything over 43V at 15A is getting
very scary as the cores will saturate all of a sudden...be careful with
yours and get as much cooling on it as you can. I am thinking of rewinding
another output xfrmr with the same turns ratio but with two cores stacked
together. Good luck with yours.

Steve / VE7SL

Web: "THE VE7SL RADIO NOTEBOOK" at   http://www.imagenisp.ca/jsm
[L.F. Loops] [Tuna Tin DX] [H.F. Maritime DX] [Crystal Radio DXing] [2200m
Band] * NEW [CQ Crossband - 2200m To HF] [NDB DX] [Boatanchors]

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