Hi All at LF,
As previously reported, after a PA MOSFET failure in the G0MRF 300w Tx I
decided to carry out the mods to the PA as described by Jim M0BMU. I have
wound the exsisting toroid with the same amount of turns as shown in the mod
using the home made litz wire for the primary (as decribed by Jim) & because
I had no solid enamelled copper wire for the secondary I made litz wire for
that also & the transformer completed (not so easy on a toroid as on a
straight core). After carrying out the other changes I connected it back
onto the board. Having also upgraded the power supply I decided to bring up
the volts slowly using a variac on the mains input. Checked that all
appeared ok & keyed up - NOTHING !
I carried out some tests & found the base resister o/c on a MOSFET, keyed up
again & low & behold - OUTPUT - into the dummy load, was I releaved.
So I continued to gradually increase the variac setting to produce higher PA
volts & when maximum was achieved I was measuring a DC input of 645 watts
(43 volts 15 Amps) into the 50r dummy load, although only key down for
seconds the 25 watt dummy load was cooking & no fan cooling on the PA but
the mosfets were not even warm, the PA transformer was slightly warm.
Next is to borrow a better dummy load & carry out some prolonged tests.
I will keep you informed of progress & also upload a picture of the toroid
to my web site (I can't make it mall enough to attach).
Regards
Gary - G4WGT
Web: http://myweb.tiscali.co.uk/wgtaylor
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