[ICOM] Preamp and receiver question

[John Geiger]

Hi Gary,

Thanks for the information.  The 706 original does have an RF gain control.
Will try that with the preamp. I think you are right about the preamp being
a noisy device, the Yaesu FT857D suffers from the same problem.

73s John A5JG

On Wed, Jun 29, 2011 at 12:52 PM, Gary P. Fiber <gfiber at comcast.net> wrote:

> 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
> > ----
> > Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> > Icom Users Net: Sundays, 1700Z, 14.316 MHz
> > Icom FAQ: http://www.qsl.net/icom/
> > To support QSL/QTH.net: http://www.qsl.net/donate.html
> >
>
>
> --
> Gary P. Fiber K8IZ
>
> General Radio Telephone Operators License PG-19-6691,
> with Shipboard Radar Endorsement.
> Washington State Resident.
>
> Confidentiality Notice: This e-mail message, including any attachments, is
> for the sole use of the intended recipient (s) and may contain confidential
> and privileged information.
> Any unauthorized review, use, disclosure or distribution is prohibited. If
> you are not the intended recipient, please contact the sender by reply
> e-mail and destroy all copies of the original message.
>
>
> ----
> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> Icom FAQ: http://www.qsl.net/icom/
> To support QSL/QTH.net: http://www.qsl.net/donate.html
>