To: <[email protected]>
Sent: Sunday, May 09, 2004 10:02 PM
Subject: LF: TX-ouput transformer problems

Hello all,

I completed my G0MRF-transmitter, and today I've done the
first tests on it.
All is working fine, except the output circuit.

As far as I can see now, the outputt-transformer on the FET's is
not good.

I'm using an old core from the HV-circuit of a computer monitor.
I got this tip from Jim, M0BMU a while ago.
Core size is about 63x39x14mm (2 "C"shaped halves, held together
with a metal spring clip)
I payed a lot off atention to have no air gap.

Used 10 turns on the primary side, centre tapped.
(That means 2 times 5 turns.......)
Secundairy coil max. 21 turns with taps, all as described in the
original design.
Well, at 15V drain voltage I already had 9Amp........
With 40V over 50 Ohm.......
FET's got hot........
I changed primairy to 2 times 7 turns instead of 2 times 5 turns,
and that gave me 15V, 5Amp with also around 40V over 50 Ohm.
So, higher efficiency......

I wonder if anyone has experience with these core's and have hints about
number of turns to be used, before I start to get into endless rewinding
coils etc.

73

Dick, pa4vhf




Hello Dick and group,

I also was curious abt the output xformer, but what caused the trouble in my
case was the timing of the PA-FETs. While one was still conducting, the
seconde one was already conducting. The result was heavy ringing, hot FETs
and miserable efficiency. One can prove that problem by watching the Voltage
at CT. In my case it dropped rapidly seen as a negative pulse.
Removing the two schottkies did not bring the highest performence, but now I
can use it as a transmitter at least. 230 Watts out into a dummy at 60V DC
and 5 Amps.
Good luck and let us know abt your experience.
vy 73 de Peter, DJ9DW.