Date: Wed, 29 Oct 2003 23:45:38 +0100
From: "Marek Przybyszewski" <sq5bpm@wp.pl>
To: rsgb_lf_group@blacksheep.org
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Organization: Poczta Wirtualnej Polski S.A. http://www.wp.pl/
Subject: LF: Re: Transmit tests
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> Hi to all at LF,
> 
> I have completed the power supply upgrade to achieve the 
> full 300 watts from
> G0MRF Tx but when testing on full power (300 watts into 
> dummy load) one of
> the PA mosfets only lasted 5 seconds. So back to the drawing 
> board !!!
Hi Gary,

Although I do not feel too experienced, I will try to help you. 
If you have an oscilloscope available, check the transformer 
input waveform for possible high-voltage needles. Do it on low 
power and a dummy load and play around like this:

Connect a "Zobel network" to the primary winding of the output 
trafo. Details can be found on G3YXM page, the idea comes from 
there. Here I am using 2x11nF in series (it means 5.5nF) and, 
also in series, 4x6.8Ohm / 5W (should give around 27 Ohm / 20W 
resistor). In my TX the resistor is placed 1cm from the fan as 
it can get hot on very slow QRSS. In principle it is a C around 
4.7nF (high voltage rated) and R around 22 Ohm in series. 
Playing around with the values you can find a consensus between 
FET ringing and heat dissipated. When you find that there's no 
high-voltage needles, go to higher power still observing what is 
happening on the trafo.

I did burn IRF540 (100V D-S) with a 12V power supply - if I used 
this "Zobel solution" it would probably not have happened. I did 
not burn any IRFP450 (500V D-S) although many times the bang was 
very close HI.

Here I am now starting to build the VFO again. Recent simple LC 
VFO design was sweeping through a half of the QRSS subband and 
it was hard to complete QSOs with it. However, recently I've 
found that there are easily available 11.0592MHz crystals. 
Dividing by 80 and pulling down by not more than 4ppm should get 
me into the band. I hope the crystal will be capable to do that. 
If not - I will look for some synthesis design, but, even though 
this is probably the best solution, I am still trying to avoid 
it...

best 73! to the whole LF group

Marek SQ5BPM
SP5ZCC Scouts Radio Club
http://www.sp5zcc.waw.pl/

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