[VHFcontesting] Larcan Amp Mod Question

[David Olean]

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 at 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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