Envelope-to: dave@picks.force9.co.uk
Delivery-date: Mon, 04 Jul 2005 21:04:35 +0100
From: "Dave Pick" <dave@picks.force9.co.uk>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 4 Jul 2005 21:04:11 +0100
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Subject: LF: RE: RE: G0MRF 300w Tx PA mods
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Gary

That's strange.
I would be happier with some 50A FETs, I rather like the IXYS IXFH58N20 but
the ones you are using should be OK...
I suppose the FETs are well clamped down with not too much heat-sink paste
applied?
Err...
Can't think of anything else but I am puzzled... I've done lots of
arcy-sparky tuning up type things with my 1kW transportable TX and it has
never blown the FETs. It does however, have the SWR and overcurrent
protection set on a knife edge!

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 20:54
To: rsgb_lf_group@blacksheep.org
Subject: LF: RE: G0MRF 300w Tx PA mods

Dave & Alan,

Thanks for your comments, I will try to answer all the questions.

Dave wrote :-
"Have you any SWR protection on the TX?" - Yes, the G0MRF 300 watt Tx has
SWR protection.

"Are they 100V devices?" - The Mosfets are STW34NB20, Vdss 200V  Id 34A.

"It may be that you just momentarily over-Volted the FETs" - The PSU is
variac controlled but max volts out is 45V, I increased the voltage from 34V
to 36V using the variac, the indications on Jim's LF Tuning Meter were
showing no cause for concern, the phase meter needle was very slightly
towards positive (not known if it is voltage or current phase error).

Alan wrote :-
"It is his opinion that problems occur almost entirely due to aerial
"effects". Unfavourable reactance / impedances can lead to very high
voltages appearing across the FETs" - I anm beginning to believe that this
is the problem area.

"You mentioned that you have had some "arcing" or corona effects" - The
arcing I mentioned was with the previous antenna & I have taken steps to
remedy that.

"unless your cores are getting very hot" - The blown air exiting from the
case is always cool indicating that nothing is heating up inside.

I will have to order some replacement mosfets, then I will take additional
steps to confirm that the antenna is ok. My original feed arrangement was
using a monitor line output transformer core for a matching transformer
which I think was saturating because it got quite warm & the antenna current
would not increase beyond about 1.6 amps no matter how much power I threw at
it. I now use a smaller grounded coil (110mm dia) on the input to the main
loading coil (280mm dia) & tap up that, using that method I was able to
achieve the best match ever since experimenting on LF with the aid of the
transmitters built in SWR meter & Jim's LF Tuning Meter.

I will post findings as I progress.

Regards,

Gary - G4WGT - IO83qp

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