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RE: AIDC 3733 downconverter cold weather problems


-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of Wayne Estes W9AE
Sent: Wednesday, January 22, 2003 10:14 PM
To: amsat-bb@AMSAT.Org
Subject: [amsat-bb] AIDC 3733 downconverter cold weather problems

Bill K0QK wrote:

I have two 3733 downconverters that exibit the same problem.  They do not
seem to work properly when the temps drop below freezing.

What I have done is to place a series string of resistors around the
downconverter to act as a heater.  The downconverter is wrapped in
pipe insulation.  In cold weather the downconverter is over 100Khz low in
receiving the test source.  Once I heat it up I can bring the signal back to
a nominal 144Mhz on the receiver.  I can vary the voltage to the series
to control the temp.

Is anyone using the 3733 downconverter now ?  Have you experienced any temp
related problems?  Have you found a solution?  Is anyone willing to work
me on this type of problem?

Wayne W9AE replies:

I have a 3733 downconverter on my roof.  I used it last night when the
temperature was about 5F.  It drifted for a few minutes after I applied the
power, but then was fairly stable.  I have the stock crystal which gives a
123 MHz IF.  Soldered to the crystal is a 60C PTC thermistor that is powered
by 13.8V.  That keeps the crystal and the surrounding components very near
60C all the time, no matter what the weather is.  This time of year it would
be more stable if I wrapped the downconverter in pipe insulation.  The drift
is barely noticeable 5 minutes after I switch on the power, and after that
it gradually drifts another 1 kHz over the next 20 minutes.

I would highly recommend using PTC thermistors instead of resistors.  A PTC
thermistor is self-regulating to a certain temperature, so you don't have to
fuss with it.  They cost about $3 each from electronics houses such as
DigiKey.  I only use one, but you could potentially use more than one if you
think it is needed.  Be sure to get one that's made for 12V.  Then it's much
more convenient to supply power to it.  I think the maximum power
dissipation is about 2 watts or a little less.

K5GNA's "144 MHz" crystals are cut for lowest temperature coefficient at
25C.  So with that crystal you would probably want to use a "cooler" PTC
thermistor.  I'm not sure what temperatures are available.  Probably 40C or
50C thermistor would be better for the K5GNA crystal.  You want it to be at
least 40C so it can keep the temperature stable even in the summer.

Also, I disabled the 12V regulator in my 3733 downconverter to make it more
convenient to power with 13.8V to a power inserter.  My downconverter, PTC
thermistor, and a DEM preamp are all powered via the coax with 13.8V from a
power inserter.

I hope this helps.  Email me directly if you have more questions.

Wayne Estes W9AE
Mundelein, IL, USA

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