Dave's account is spot on. I didn't remove the high current 50V to the FETs
during receive, I just removed the bias which is what the protection board
seems to do if high SWR is encountered. Since FETs aren't supposed to have
the same thermal runaway issues that bipolar transistors can have, I didn't
think it was necessary but I defer to the real engineers opinion on the
subject. My Larcan has run happily for almost 10 years without removing the
50V on receive, just the bias and I've never noticed any noise generated
by the amp.
Terry Price - W8ZN
Directive Systems and Engineering
703-754-3876
On Tue, Sep 6, 2022 at 11:56 AM David Olean <k1whs@metrocast.net> wrote:
> Hi Dave
>
> The small wire supplies bias to the FETs. I wrote up a conversion paper
> on the Larcan, and had a few issues with removing DC from the big FETs
> when they were not keyed up. I used big FET switches to remove DC during
> receive periods. See below....
>
> "A few words about the Larcan protection board. It was designed to allow
> hot switching of the individual amplifier modules in a TV transmitter.
> We do not need hot switching, so much of the design sits unused, but it
> does have a high VSWR protection function that will remove bias to all
> of the FETs if a high VSWR is encountered. It does add a bit of
> protection, and I left things as is for that reason. Part of the hot
> swap circuitry involved charging electrolytic caps before the actual
> drain voltage is applied to the FETS. A Step Start voltage comes in
> early on PIN 8 of the Larcan protection board. There is a second 50 volt
> connection back to the 50 volt drain circuits via PIN 2.R5, a 10 ohm 1
> watt resistor limits capacitor charging current and provides a soft
> start. That other connection on PIN 2 must be dis connected if you use
> the W6PQL 50 volt DC FET switch to disconnect the drains from 50
> volts.Otherwise, current from PIN 8 will flow through R5 and try to
> power the six FETs when you are in receive or standby!Make sure that you
> disconnect PIN 2 where it connects to the RF board on the big Larcan RF
> board, or you will burn up R5 and have voltage on the FETs while R5 is
> cooking!"
>
> Terry, W8ZN left the 50 volts DC on his FETs all the time, so did not
> have the problem with R5 on the Larcan protection board. He rigged up a
> small PC board to remove bias during receive, and disconnected that
> small wire that went to PIN 8, and ran it to his bias circuit as I
> recall. His conversion is on the Directive Systems website under TECH
> Notes.
>
> 73
>
> Dave K1WHS
>
> On 9/6/2022 11:16 AM, k3sk@buckwalter.co wrote:
> > I'm in the process of completing the changes needed for my 220MHz amp.
> I'm
> > using the Larcan 1KW version and on the original DC power plug there is a
> > wire on one of the small pins that goes to Pin 8, "Step Start" on the RF
> > Module VSWR Control Board. W8ZN's conversion package says to cut that
> > wire when replacing the connector. Does it get reconnected anywhere
> or is
> > it not needed?
> >
> > de K3SK
> > Dave Buckwalter
> > FM07 - Farmville VA
> >
> >
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