[ICOM] Preamp and receiver question

Thu Jun 30 13:25:09 EDT 2011

John, I agree with Tim 100%,
Back a few years ago, I bought a new 706g in Tokyo while there on business and hand carried it on the plane. Yep, it was the JA model, but the only difference was it only covers the bottom 1/2 of 2M. Didn't care since I have a ton of 2M FM radios.
After I returned to the USA, I did some subjective weak signal / MS signal comparisons with my other equipment. At that time I had:

Icom 575H
Drake TR-6 with modern Fairchild JFETS (very well tuned radio!)
Yaesu FT-901 with transverter
A ton of 6M receive converters, feeding a score of different receivers.

These were all connected to a 6M7JHV at 72' fed with 7/8" hardline. Pretty quiet rural location.

The result? The 575H was sold immediately (and it was cherry).
The 706g could hear anything any other combo could. I later bought a new 756PRO in Tokyo and the 706g is equal to that as well.

It WAS strange using the 706g to feed my 1500W amp for MS...... ha ha But it worked perfectly.
BTW, while in Tokyo, I bought several filters for the 706g. They are waaaay cheaper there, than here in the USA... Still haven't figgered that out.

ron
N4UE

-----Original Message-----
From: Tim Duffy K3LR <k3lr at k3lr.com>
To: 'ICOM Reflector' <icom at mailman.qth.net>; Yaesu_FT-450 <Yaesu_FT-450 at yahoogroups.com>; 50mhz <50mhz at mailman.qth.net>; vhf <vhf at w6yx.stanford.edu>
Sent: Wed, Jun 29, 2011 11:09 pm
Subject: Re: [ICOM] Preamp and receiver question

John,
There is nothing wrong with the pre-amp in the IC-706. I have one - it works
erfect. 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
ot 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
irections were fine. All preamps can overload and amplify noise. This is
ot about noise figure or pre amp design.  
The IC706 hears everything better than the FT450. 
The IC706 is not your problem.
73,
im K3LR
-----Original Message-----
rom: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
ehalf Of John Geiger
ent: Wednesday, June 29, 2011 11:53 AM
o: ICOM Reflector; Yaesu_FT-450 at yahoogroups.com; 50mhz at mailman.qth.net;
hf at w6yx.stanford.edu
ubject: [ICOM] Preamp and receiver question
I have a receiver question for those on the list who know a lot more about
his stuff than I do, at least from the electronics standpoint.  I know more
bout 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
or me HI HI.  On 6 meters I have some noise source to my southeast (right
t the Carribean) that really raises the noise level on it.  It hits S8 or
o if the beam is pointed in the right direction.  I didn't remember
he FT450 I had being that noisy, so I had a friend bring his FT450 over,
nd 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
or each radio, the 706 (preamp on) has a 3db advantage in sensitivity.  The
DS for it is -139dbm vs -136dbm for the FT450 (preamp on).  That should be
/2 S unit difference or so in picked up noise.  The S meter calibration is
ery different.  For the 706 with the preamp on on 6 meters, a S9 signal
akes 3.3 microvolts, which is insanely liberal, but that is what it is.
or the FT450 under same conditions (preamp on, 6 meters) it takes a 33
icrovolt signal for S9.  That is a factor of 10, which should show up as a
0db difference on the meter-correct?  If that is correct, we have a 13db
ifference in readings between the 706 and 450 (3 db for increased MDS, 10db
or the meter difference).  That should be 2 S units or maybe 3-4 given the
on linear calibration of most meters.  But I am seeing a 7 S unit
ifference or so.
So here is my big question-can the preamp device in the 706 (or any other
adio for that matter) become overloaded and start to generate increased
oise?  I think it can, correct?  If you put enough signal, local noise,
hatever into the receiver, won't the preamp at some point become swamped
nd start to create noise on its own?  Could that be where this extra noise
s coming from?
You really can't run the 706 original on 6m without the preamp as it is
retty deaf without it.  The noise blanker on either rig doesn't do much on
his noise, so it isn't that the NB in the FT450 is taking it out while the
06 isn't.
73s John AA5JG
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