[amsat-bb] Re: Landwehr 2m Pre-amp issue

Thu Jan 21 08:36:46 PST 2010


One thing to check is the DC feed, especially the ground return.  I use a
separate shielded cable, old RG-58 actually, and over the years I have had
the DC ground open up, or become intermittent.  Both times, it introduced
significant RF noise.  You can use the signal cable for the ground, but in
my case that required a slight rearrangement of the DC grounds.


-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Vince Fiscus, KB7ADL
Sent: 21 January, 2010 10:28
To: amsat-bb at amsat.org
Subject: [amsat-bb] Landwehr 2m Pre-amp issue

I was suspecting my 2m Landwehr was having problems one day, don't know 
when it really started having problems, but last night I checked it 
out.  Last night when I was listening to AO-7, when I turned the pre-amp on 
I got about S 7 of noise and the signals from the satellite were buried in 
the noise, and I couldn't tell that the signals were boosted much, but I 
could still hear them with difficulty through the noise.  Turning the 
pre-amp off made hearing the signals easier, due to lack of 
noise.  Obviously since I could still hear stations, RF is still going 
through the device.  Checking it out on stronger fm signals, did seem to 
amplify them more, but I could tell there was still noise.

I don't know much about gaAsFet's but what I guess I'm asking, is when they 
Fail, could they fail in the noisy position, or when then they fail do they 
die with no amplification? I guess it could be possible some other 
component around the gaasfet failed making the device run wild.  Maybe 
dirty relay contacts?  The RF sensor still works still works fine.

I won't be able to get my hands on it for a while as there is too much snow 
on the roof yet, and I don't have weak signal noise generators to try to 
isolated the problems.  Landwehr gives you a schematic but they didn't like 
to reveal too much. The pre-amp is about 16 years old.  My 70cm Landwehr 
which is the same age and gets much more use is chugging away just 
fine.  The 2m pre-amp is under an eggbeater. I don't need a 2m pre-amp on 
LEO's with the beam.

Give me some things to check, Thanks, 73

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