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 will rise
rapidly. Any slowing edge will have to be on the drain side in the form 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 question,
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 concentrate 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 -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
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 types
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 Schäfer <[email protected]>
> *To:* [email protected]
> *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 reduced
> 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 <[email protected]>
>> *To:* [email protected]; Chris Osborn <[email protected]>
>> *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 frequency.
>>
>> 73s
>> Roger G3XBM
>>
>> On 22 November 2010 20:21, Roger Lapthorn <[email protected]
>> <mailto:[email protected]>> 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
>>
>> --
>> 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
>>
>>
>>
>>
>> --
>> 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
>>
>
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