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Hi Dick,
Very useful information and encouraging
too that your TX is so similar to mine.
I will try that resistor mod with my last few IRF520
FET's and I have now ordered
a TUBE of IRF630's (200 volt devices) and a few
TC4426 driver IC's !!
I shall get this rig working or die of old age in the
attempt - hi!
I had ferrite beads on the FET gate leads and I thought
that would have cured any
vhf parasitics. Obviously not
!
I think that my PA cooling will be ok as I have an
efficient fan and large heat sink.
73 Chris G3XIZ
Dear Chris,
I use four IRF520 FET's in parallel push pull class
B in my transmitter. The FET's are run at 40 V. The gates are
driven via a transformer. At first I suffered from blown FET's till I found
that was due to parasitic VHF or UHF oscillations. It was cured by inserting
stopper resistors of 120 ohm directly at the gates. The circuit, with the
modification, can be found at page 58 of G3XDV's book LF Today.
After the modification at irregular intervals one of the FET's
became a short circuit. This was found to be caused by insufficient
cooling of the heatsink. Replacing the small fan by a larger one
cured it. The TX can now be run during the whole night without any
problem.
73, Dick, PA0SE
At 12:40
28-9-05, you wrote:
Thanks for the advice
Jeff,
All points noted.
I must confess that all of the published
designs seem to use a logic driver
IC.
I must be being awkward using
an inductor :-(
Maybe I'd better stick to a proven design..
I'll
take a photo soon and risk ridicule - HI!
73 Chris
G3XIZ
-----Original Message-----
From: F6CWN [mailto:[email protected]]
Sent: 28 September 2005
10:21:AM
To: [email protected]
Subject: LF: Re: Advice will
be gratefully received
Hello Chris,
At first,"RIP poor
MOS"!
I prefer use MOS with higher VDS.
The driver must absorb
energy, because the Ciss capacitance, i use TSC426 or
TC4429
driver.
It sis better to separate the cooling radiators to minimize the
reaction
between the two parts
of the bridge (capacity Drain / Ground,
the ground is "hot").
A problem with reactive loading, the coils of the
LPF are reversible.
A little photography of your PA can
help...
73: Jeff / F6CWN/p
----- Original Message
-----
From: "OSBORN, Chris" <[email protected]>
To:
"LF Group (E-mail)" <[email protected]>
Sent: Wednesday,
September 28, 2005 10:43 AM
Subject: LF: Advice will be gratefully
received
>
>
>
> The PA FET's in my new home
brew transmitter keep exploding !
>
> I'm using push-pull
IRF520's rated at 100 V Vds and my supply is 30
> volts.
> I am
careful not to exceed the V gs.
>
> The gates are driven with a
toroidal transformer and the whole
> circuit is just a scaled up
version of my original TX, which works fine.
> It generates healthy
power into a 50 ohm dummy load but on
> connection to the aerial
circuit - bang !
> The aerial system is identical to that used with
the old TX !
>
> Has anyone any ideas on what's causing the
problem and how to cure
> it ?
>
> I'm running out of
FET's and my nerves are somewhat frayed (shell
>
shock!)
>
> 73 Chris G3XIZ
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