[50mhz] [ICOM] Preamp and receiver question

[Tim Duffy K3LR]

John,

There is nothing wrong with the pre-amp in the IC-706. I have one - it works
perfect. Over 100 countries worked on 6 meters with this 706 barefoot. 

S-Meter calibrations are all over the map with various radios- you know that
- comparing a IC-706 to a FT450 - just is not fair at all - the FT450 does
not show S9 at -73 dBm like the 706 does (50 uV as measured here in the lab
- the 706 is a top performer on 6 meters). 

If you fix your noise problem you will be fine. Even you said other
directions were fine. All preamps can overload and amplify noise. This is
not about noise figure or pre amp design.  

The IC706 hears everything better than the FT450. 

The IC706 is not your problem.

73,
Tim K3LR

-----Original Message-----
From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
Behalf Of John Geiger
Sent: Wednesday, June 29, 2011 11:53 AM
To: ICOM Reflector; Yaesu_FT-450 at yahoogroups.com; 50mhz at mailman.qth.net;
vhf at w6yx.stanford.edu
Subject: [ICOM] Preamp and receiver question

I have a receiver question for those on the list who know a lot more about
this stuff than I do, at least from the electronics standpoint.  I know more
about receiver performance and less about how we get there.

I have an Icom 706-had it over 3 months now which is approaching a record
for me HI HI.  On 6 meters I have some noise source to my southeast (right
at the Carribean) that really raises the noise level on it.  It hits S8 or
so if the beam is pointed in the right direction.  I didn't remember
the FT450 I had being that noisy, so I had a friend bring his FT450 over,
and it was showing a noise level of S2 or so in the same direction.

Now I know that different variables are in play.  Checking the QST review
for each radio, the 706 (preamp on) has a 3db advantage in sensitivity.  The
MDS for it is -139dbm vs -136dbm for the FT450 (preamp on).  That should be
1/2 S unit difference or so in picked up noise.  The S meter calibration is
very different.  For the 706 with the preamp on on 6 meters, a S9 signal
takes 3.3 microvolts, which is insanely liberal, but that is what it is.
For the FT450 under same conditions (preamp on, 6 meters) it takes a 33
microvolt signal for S9.  That is a factor of 10, which should show up as a
10db difference on the meter-correct?  If that is correct, we have a 13db
difference in readings between the 706 and 450 (3 db for increased MDS, 10db
for the meter difference).  That should be 2 S units or maybe 3-4 given the
non linear calibration of most meters.  But I am seeing a 7 S unit
difference or so.

So here is my big question-can the preamp device in the 706 (or any other
radio for that matter) become overloaded and start to generate increased
noise?  I think it can, correct?  If you put enough signal, local noise,
whatever into the receiver, won't the preamp at some point become swamped
and start to create noise on its own?  Could that be where this extra noise
is coming from?

You really can't run the 706 original on 6m without the preamp as it is
pretty deaf without it.  The noise blanker on either rig doesn't do much on
this noise, so it isn't that the NB in the FT450 is taking it out while the
706 isn't.

73s John AA5JG
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