[ICOM] Preamp and receiver question

[Gary P. Fiber]

John,

I don't have all the answers but here are a couple.

None Icom HF and multiband Radios meet specifications without the preamp 
enabled. I think if you look at the published specifications it will say 
"with preamp enabled", at least that' the way it was from 1989 until 2001.

When I worked at Icom we had a former Kenwood tech working there, he 
worked there and on the IC-765 and 781's while they were still in 
production. He stated the preamp in the Kenwood was on all the time and 
when you disabled it you actually inserted an attenuator in line which 
was different than the way Icom operates their preamp.
I really don't know but he was very certain about that. Maybe the FT-450 
is built similar ?

For what ever reason the Original IC-706 preamp was noisy almost 
annoying at times. I never saw any sort of noise figure specification 
published from the factory but I had one of those and when the preamp 
was enabled the noise came up a lot too. You might do better using an 
outboard preamp, though you would need one that bypasses in the transmit 
mode.

Seems to me the noise blanker in most Icom's are designed for pulse type 
noise, like ignition noise. Most other noises like electrical and 
atmospheric noises an external device like a DSP noise blanker does 
better. The old NIR-10 DSP Filter might be a good device to have about, 
even though it was an audio type filter, though I am sure other outboard 
types are much improved.

I know the noise blanker circuit will cause clipping on strong signals 
and that in turn will cause you splatter around the desired receiving 
frequency likely due to the close to a more square wave output it seems 
to generate.

I suspect you could possibly get the preamp into some sort of non linear 
operation due to strong signals and cause you some grief while receiving 
due to the gain.
Now I am trying to remember did the original IC-706 have an RF Gain 
control, don't have the manual handy, but if so you could play that 
against the preamp and see if you can get the desired signal out of the 
noise while enjoying the receiver front end gain the preamp provides.

And  for admitting owning a rig for 3 months.......... we will certainly 
be marking that down in the record books John ;)

Gary K8IZ

On 6/29/2011 9:52 AM, John Geiger wrote:
> 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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-- 
Gary P. Fiber K8IZ

General Radio Telephone Operators License PG-19-6691,
with Shipboard Radar Endorsement.
Washington State Resident.

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