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Re: LF: output transformer problems

To: [email protected]
Subject: Re: LF: output transformer problems
From: Rik Strobbe <[email protected]>
Date: Tue, 18 May 2004 22:39:49 +0200
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Hi Dick,

I forgot what design you are using, but I'm a bit surprised of all the trouble you got with the output transformer and LPF. I have built several variations on the G3YXM design (http://www.wireless.org.uk/136rig.htm) and it all worked without problems. Output transformers were on double-U core salvaged from old TV sets and the LPF has "air coils" (would on plastic "plumber tubes"). Only critical thing is the ratio of the output transformer, as the LPF is 50 Ohm in/out the transformer has to convert the low impedance at the drains to 50 Ohm. If the ratio is too low the FET's draw a lot of current and you get little output. If you are not sure what the ratio should be it is better to start with a far too high ratio (then you will have little output power but also little drain current) and gradually decrease the ratio (remove windings from the "antenna-side" of the transformer).
73, Rik  ON7YD

Citeren Dick <[email protected]>:

Hello all,

due to circumstances I'll have less time to work on the transmitter.

Meanwhile I tried several other transformers, as well as some
other things, like disconnecting the Scottky diodes and the 470nF
C's .
Now I have 6R8 from the TC4426 direct to the gates, which
are grounded with 47k resistor.
I tried all kind of things around the FET's with other C's to ground
or
with C-R to ground etc.
Some progress has been made, but still difficult.

The ouput filter seems to have a quite large influence on the over
all
drain current.
Also the value of the capacitor that grounds the centre tap is
important.
In my case I found better results at 330nF.

Several available cores tested, like 3C11 (white) and 3E1 (green)
both 36mm.
Also tried an T106-26 from an old computer SMPS.

Wonder what current I should have when applying lower supply
voltages
to the FET's.
For example. what currents to others have at 15V or 20V or so.

73, Dick, pa4vhf







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