Hi Gary, as you are monitoring the match with a "Jim and Lech" match
indicator, I doubt there is much problem there. I have had another thought,
which refers to the physical wiring against the "schematic circuit"
There is a very high current flowing out of the FET sources, and back to the
power supply. I wonder if you have the ground connection of the drive
circuitry connected close to the sources. I am not sure what might happen if
some of the large currents were conducted through the ground rail common to
the gate drive circuit. It might just be possible for this to jack the drive
supply volts up to level to puncture the gate oxide, under some
circumstances. I have lost CMOS memories in a micro-processor control system
because my ground via the thermocouples was better than that of the
industrial 5kW (3 phase) oven power feed, that it was controlling. I put
100ohms in series with the ground leg of the thermocouple and the problem
went away.
Winding the mains volts to the power supply with a variac will provide some
small step changes as the brush bridges turns but these should be quite
small. It is possible that the PSU might "bounce". I have seen an analogue
power unit give unstabilised volts for some microseconds at switch-on,
before the op-amp took control and wound the drive to the emitter follower
down. It might be worth sticking a big 50 volt zener across the power unit
to clip any transients. The FETS should stand four x supply volts. What sort
of PSU is it ?? (if its a switcher you can probably forget that comment). I
suppose it is a pointer when you say you had just wound the volts up when it
failed. That rather takes the blame away from the aerial, and tuning.
Wild thoughts but they may strike a chord.....
Cheers de Alan G3NYK
----- Original Message -----
From: "Gary - G4WGT" <[email protected]>
To: <[email protected]>
Sent: 04 July 2005 20:54
Subject: LF: RE: G0MRF 300w Tx PA mods
Dave & Alan,
Thanks for your comments, I will try to answer all the questions.
Dave wrote :-
"Have you any SWR protection on the TX?" - Yes, the G0MRF 300 watt Tx has
SWR protection.
"Are they 100V devices?" - The Mosfets are STW34NB20, Vdss 200V Id 34A.
"It may be that you just momentarily over-Volted the FETs" - The PSU is
variac controlled but max volts out is 45V, I increased the voltage from
34V
to 36V using the variac, the indications on Jim's LF Tuning Meter were
showing no cause for concern, the phase meter needle was very slightly
towards positive (not known if it is voltage or current phase error).
Alan wrote :-
"It is his opinion that problems occur almost entirely due to aerial
"effects". Unfavourable reactance / impedances can lead to very high
voltages appearing across the FETs" - I anm beginning to believe that this
is the problem area.
"You mentioned that you have had some "arcing" or corona effects" - The
arcing I mentioned was with the previous antenna & I have taken steps to
remedy that.
"unless your cores are getting very hot" - The blown air exiting from the
case is always cool indicating that nothing is heating up inside.
I will have to order some replacement mosfets, then I will take additional
steps to confirm that the antenna is ok. My original feed arrangement was
using a monitor line output transformer core for a matching transformer
which I think was saturating because it got quite warm & the antenna
current
would not increase beyond about 1.6 amps no matter how much power I threw
at
it. I now use a smaller grounded coil (110mm dia) on the input to the main
loading coil (280mm dia) & tap up that, using that method I was able to
achieve the best match ever since experimenting on LF with the aid of the
transmitters built in SWR meter & Jim's LF Tuning Meter.
I will post findings as I progress.
Regards,
Gary - G4WGT - IO83qp
Web : http://myweb.tiscali.co.uk/wgtaylor
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