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Re: Preamp bypass circuit



> Date:          Fri, 16 Feb 2001 10:53:03 -0500
> From:          Al Lawler <al.lawler@oracle.com>
> To:            Doug Braun NA1DB <na1db@yahoo.com>
> Cc:            Doug Faunt N6TQS +1-510-655-8604 <faunt@panix.com>, amsat-bb@AMSAT.Org
> Subject:       Re: [amsat-bb] Preamp bypass circuit

> 
>   One thing to bear in mind when talking about
> sequencers is that some radios (such as the  ft736
> and uniden hr2510) throw stray RF out the antenna 
> ports even when the radio is not transmitting,
> so even a sequencer doesn't guarantee long life
> for anything on a shared t/r path.
> 
>   -al
>   WB1BQE
> 

Al makes a very good point here.  Some radios send out
a brief transmit pulse when you turn them on.  The Kenwood
TR-751A is a perfect example.  Everytime you power it up,
it burbs out a pulse of RF.  I burned out the mixer diode in
my DEM 2.3Ghz converter this way while testing it.  Lucky
for me the fix was trivial.

I have never found a good internal mod for the 751A to
cure the transmit on power up problem.  When I used it on
my converter, I had a RF relay in line with the normally 
closed port connected to a small 50 ohm load.  A simple
RC delay network across the coil delayed the relay 
closing (connecting the radio to the converter) for just
long enough to avoid the RF pulse.  The relay was powered
by 13.8V from the radio via an accessory plug I installed
for a pre-amp squencer.  

--73-- David WA0AUQ

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