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

Vince Fiscus, KB7ADL vlfiscus at mcn.net
Thu Jan 21 08:27:54 PST 2010

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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