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From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <AANLkTim5BtnMWs8s0bnMh8z8piL6y1sRh=gECjPrcwRT@mail.gmail.com> <AANLkTingecRKSX1+H4xqEz6GnX4O1DsLrSA+PBVw+UgZ@mail.gmail.com> <373372.70115.qm@web28506.mail.ukl.yahoo.com> <4CEAE16C.5030101@iup.uni-heidelberg.de> <294616.39798.qm@web28506.mail.ukl.yahoo.com> <4CEBEEDF.6000301@iup.uni-heidelberg.de>
Date: Wed, 24 Nov 2010 10:26:41 -0000
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Subject: Re: LF: VLF G3XIZ - QRT
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Chris I dont think slowing the drive edges will be the answer. This is=
 a
switching PA and needs to switch quickly otherwise the dissipation wil=
l rise
rapidly. Any slowing  edge will have to be on the drain side in the fo=
rm of
a low pass filter. Unstable loads will lead to excessive voltages at=
 the
drain which cause breakdown even if the voltage rating is 4 times the=
 supply
(the usual spec.) Why a transformer the tranformer arced is the  quest=
ion,
there should not be that high a voltage there ....are you transforming=
 the
output impedance to the impeadance seen at the base of the loading col=
? If
so you will need thick insulations and no sharp bends (which concentra=
te the
field) I wonder whther the transformation could be done in two stages?=
? the
transformer and a tap on the loading coil??

Alan G3NYK

----- Original Message -----=20
From: "Stefan Sch=E4fer" <Stefan.Schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, November 23, 2010 4:42 PM
Subject: Re: LF: VLF G3XIZ - QRT


Hi Chris,

You are using a class D PA? Maybe you don't even need a output
transformer if you apply a coupling winding on the coil.
Why are the FETs blew up? Overvoltage or overtemperature? And what typ=
es
of FETs are you using?

73, gl

Stefan/DK7FC

Am 23.11.2010 13:57, schrieb Chris Osborn:
> Hello Stefan / LF,
> All was going last night well until I heard sparking from the aerial
> distribution box each
> time the carrier was turned off.
> After about 45 minutes the PA FET's blew up.
> I did some investigation and found that the output transformer has
> developed some
> short-circuited turns and so is now useless.
> The loading coil seems unharmed so once I get a replacement
> transformer and connect in some
> new FET's I'll be back in business.
> I'll slow down the rise and fall times of the pulse train from the
> driver unit to the FET's and hopefully cure the sparking.
> (Mal)
> Yes, N.O.V. holders must get prior permission before they transmit=
 and
> must stay by the telephone for the duration.
> Thanks to all those who looked for my signals.
> 73
> Chris G3XIZ
>
>
> --------------------------------------------------------------------=
----
> *From:* Stefan Sch=E4fer <schaefer@iup.uni-heidelberg.de>
> *To:* rsgb_lf_group@blacksheep.org
> *Sent:* Mon, 22 November, 2010 21:32:28
> *Subject:* Re: LF: VLF G3XIZ - QRT
>
> Hi Chris,
>
> Sorry to hear that. Just checked to mails after beeing abscent the
> weekend and wanted to wish good luck too now but just read your mail
> below.
> I assume the breakdown immediately de-tuned your system which reduce=
d
> the HV? So the damage is limited to a few turns? Has it happened at
> the 0.2mm dia wire?
> BTW i am using 0.4mm dia wire, so not really much more...
>
> Good luck in the next tests!
>
> 73, Stefan
>
>
> Am 22.11.2010 22:15, schrieb Chris Osborn:
>> Thanks for looking Roger and chaps but game's over for tonight.
>>
>> I've insulation breakdown somewhere in the aerial circuit and I can=
't
>> fix it until daylight.
>>
>> Maybe try again next weekend.
>>
>> 73 Chris G3XIZ
>>
>>
>>
>> -------------------------------------------------------------------=
-----
>> *From:* Roger Lapthorn <rogerlapthorn@gmail.com>
>> *To:* rsgb_lf_group@blacksheep.org; Chris Osborn <g3xiz@yahoo.co.uk=
>
>> *Sent:* Mon, 22 November, 2010 20:49:24
>> *Subject:* Re: LF: G3XIZ - no copy yet on 8.9719kHz
>>
>> Just swapped from the loop to the earth electrode - band noise
>> stronger - and there /may/ be a hint of a signal on Chris's frequen=
cy.
>>
>> 73s
>> Roger G3XBM
>>
>> On 22 November 2010 20:21, Roger Lapthorn <rogerlapthorn@gmail.com
>> <mailto:rogerlapthorn@gmail.com>> wrote:
>>
>>     Have been looking for 20 minutes so far but nothing seen from
>>     Chris at about 34km distance.
>>
>>     Will continue to look for the next 2 hours.
>>
>>     73s
>>     Roger G3XBM
>>
>>     --=20
>>     http://g3xbm-qrp.blogspot.com/
>>     http://www.g3xbm.co.uk <http://www.g3xbm.co.uk/>
>>     http://www.youtube.com/user/g3xbm
>>     G3XBM   GQRP 1678    ISWL G11088
>>
>>
>>
>>
>> --=20
>> http://g3xbm-qrp.blogspot.com/
>> http://www.g3xbm.co.uk <http://www.g3xbm.co.uk/>
>> http://www.youtube.com/user/g3xbm
>> G3XBM   GQRP 1678    ISWL G11088
>>
>