[Icom] Another 756Pro mod needed- MW sens

Wed, 22 Jan 2003 22:30:08 -0000

----- Original Message -----
From: "Adam Farson" <[email protected]>
To: <[email protected]>
Cc: "GEORGE T. BAKER" <[email protected]>
Sent: Wednesday, January 22, 2003 21:09
Subject: RE: [Icom] Another 756Pro mod needed- MW sens

> Hi Don,
>
> It is all a question of design intent. The IC-756Pro and Pro II were
> conceived as HF amateur transceivers, and therefore the designers
optimised
> their performance in the HF amateur bands so as to achieve the highest
> possible performance in accordance with the design intent, within the
> budgetary constraints assumed at the outset of the development programme.

What I was asking for costs less not more - they went out of their way to
write code so the preamps can't be engaged below 1.6 mhz and to bypass the
preamp chain alltogether.
>
> As is customary in modern receiver design, the RF selectivity is provided
by
> a bank of LC filters, rather than a tuneable preselector. All these
filters
> are bandpass, with one exception; the 0 ~ 2 MHz low-pass filter. I would
> think that the designers selected an LPF, rather than a BPF, to ensure
good
> image and out-of-band signal rejection in the 160m amateur band, whilst
> allowing for medium-wave (MF) broadcast reception. This choice was a
> conscious compromise, which carried with it the risk of front-end overload
> on 160m due to nearby strong MF broadcast signals - a common occurrence in
> most populated areas. Icom's receiver designers could have put in a 1.8 ~
2
> MHz BPF, but this would have precluded general coverage in the MF range.
> Would you have preferred such a BPF?

Well it has such a BPF - I am talking about the 756PRO -  a 1.8 to 2.0 BPF
just for 160m, but this filter stays in down to 1600.00, and then we engage
the .03-1.6 mhz LPF  I expect the gain of the  built in preamps would roll
off somewhere well short of 30 khz, but even if they make it partway thru
the MW band, that is more than we have now.

Allowing the preamps to function here (and removing the fixed attenuator)
have zero impact on 160m.  If the PRO II only has a 0-2 mhz LPF, then that
is a step backwards. Way back.

>
> To address this problem, and noting again that the designers viewed MF
> broadcast reception as incidental to the primary mission of the Pro/ProII,
> they configured the radio's firmware to switch additional attenuation into
> the receiver's RF signal path below 1.6 MHz.
>
> I ran a "quickie" sensitivity test on my IC-756Pro2, using an HP8640B as a
> signal source. These are the results:
>
> AM, 30% modulation. Signal level in �V/10dB S+N/N. Preamps off; RF ATT
off.
>
> 1.8 MHz:  2.5�V
> 1.7          2.8
> 1.6          3.0
> 1.3          4.0
> 1.0          5.0
> 0.6          8.0
>
> I would submit that these figures are none too shabby, even for the MF
> broadcast band. At 0.6 MHz, the degradation is 10dB relative to 1.8 MHz.

Well they are none too good either.  Quite inadequate for serious weak
signal work down there.  The PRO has only adequate sensitivity ANYWHERE if
you use engage preamp 1, and I nearly always use preamp 2.  I can listen on
80m with a full size 4 ele yagi, with preamp 2 engaged, and the S meter will
stay at zero if there is no abnormal atmospheric level. I can hear the noise
floor decrease if I remove the antenna, so I know I have enough sensitivity,
but only just.

>
> A tuned active or passive preselector can always be interposed between the
> antenna and the receiver input for enhanced sensitivity and
unwanted-signal
> rejection. Considering that the IC-756Pro/Pro2 was not designed as a
> general-purpose or LF/MF communications receiver, the addition of such an
> accessory should be no hardship.

The 9.5 db fixed attenuator in the LPF requires an external preamp to have
an extra 9.5  db of gain/headroom to overcome this loss. When you are
dealing with strong signals, another 10 db of IMD free headroom is hard to
come by. Which is why I removed the attenuator.

>
> The alternative - for serious LF/MF AM listening - is to use a receiver
> whose design is optimised for the relevant frequency range, preferably one
> fitted with a synchronous detector.

And one exists?  Forgetting about AM sync for a moment, what recent model
from anybody is optimised for MW/LF - and why would they want to appeal to
such a small market?   Even if the R&S ESH-2 and ESH-3 were "affordable"
both indicate significant attenuation is applied in the lower part of their
coverage.  They switch at 150 khz, so for my needs, that is quite OK but
perhaps not for the LOWFER enthusiast. For a lot less $, my old Racal
RA-17C13 with the 237B LF convertor is likely one of the most sensitive
LW/MW/HF general coverage sets ever made.

My point is that the PRO already has most of the requirements bult in to do
a very decent job throughout its entire tuning spectrum - but some design
engineer decided we didn't need the preamps below 1.6 mhz and spent money to
make that happen.  I was just hoping someone had come up with an easy way to
reverse this.

73 Don
VE6JY

>
> Best 73,
> Adam, VA7OJ/AB4OJ
> North Vancouver, BC, Canada
> http://www.qsl.net/ab4oj/
> Icom FAQ site:
> http://www.qsl.net/icom/
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]]On
> Behalf Of VE6JY Don Moman
> Sent: Wednesday, January 22, 2003 09:32
> To: [email protected]
> Subject: Re: [Icom] Another 756Pro mod needed- MW sens
>
>
> Well, an external preamp may well have to be used, but it irks me to have
to
> do so. I purchased a quite capable general coverage receiver that is at
> least 20 - 30 db less sensitive below 1600 khz than it should be.  Just
tune
> across a moderate signal on 1600 khz and watch the signal drop as you go
> past 1599.99.  They didn't even get the specs in the manual right - 1.799
> should have been 1.599 mhz. Yes I know they have a standard disclaimer
that
> receiver specs are not guaranteed on certain frequency bands.  And I have
> several 756Pro's that all behave the same.
>
> There is no technical reason or need to reduce gain in that region. In
years
> past, when radios had much less dynamic range, somebody noticed they could
> get overload on mw easier than any other band, so in their "wisdom"
decided
> to make that harder to have happen and embarrass the dealer,
> manufacturer,etc.
>
> I have a rural location with low noise and am about 100km from the nearest
> MW transmitter (however they are still very strong, producing several
volts
> of RF on some of the wires). Weak signals from Europe or Australia are
> weak - and the 756Pro hasn't got what it takes to hear them.   My mw
> sensitivity  modified Kenwood R5000 is superb down there, and the R71A not
> far behind. Even the Collins HF-2050, which isn't super sensitive, still
> does quite well. But they don't have that nice spectrum display.......
>
> 73 Don
>
> ----- Original Message -----
> From: "George, W5YR" <[email protected]>
> To: <[email protected]>
> Sent: Wednesday, January 22, 2003 15:41
> Subject: Re: [Icom] Another 756Pro mod needed- MW sens
>
>
> > Seems to me that the easiest "mod" would be to use a simple external
> > preamp in the Receive Antenna input . . .
> >
> > Gain is reduced in that frequency range for a number of reasons . . .
> > I am curious as to what  application requires additional gain below
> > 1600 KHz.
> >
> > 73/72, George
> > Amateur Radio W5YR -  the Yellow Rose of Texas
> > In the 57th year and it just keeps getting better!
> > Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
> > K2 #489  IC-765 #2349 IC-756 PRO  #2121 IC-756 PRO2 #3235
> >
> > ----- Original Message -----
> > From: "VE6JY Don Moman" <[email protected]>
> > To: <[email protected]>
> > Sent: Wednesday, January 22, 2003 2:21 AM
> > Subject: [Icom] Another 756Pro mod needed- MW sens
> >
> >
> > > The stock 756Pro receiver is rather numb on mediumwave (<1600 khz)
> > since
> > > neither preamp functions down there and there is a fixed attenuator
> > in that
> > > BPF stage. I have removed the fixed attenuator but it was only a
> > small
> > > improvement. What we really need is to enable the preamps to
> > function down
> > > there. This should be, at the very least, a menu option. Of course
> > it isn't.
> > > I am always annoyed when manufacturers decide what gain I need to
> > use....
> > > and ICOM has had a long history of doing that.... but at least
> > earlier sets
> > > were easier to modify.
> > >
> > > Anyway has anyone managed to enable the preamp function below 1600
> > khz? If
> > > so, I'd sure like to know more...
> > >
> > > 73 Don
> > > VE6JY
> >
> >
> > ----
> > Your Moderator: Dick Flanagan W6OLD, [email protected]
> > Icom Users Net: Sundays, 1700Z, 14.315 MHz
> > Icom FAQ: http://www.qsl.net/icom/
>
> ----
> Your Moderator: Dick Flanagan W6OLD, [email protected]
> Icom Users Net: Sundays, 1700Z, 14.315 MHz
> Icom FAQ: http://www.qsl.net/icom/
>
> ----
> Your Moderator: Dick Flanagan W6OLD, [email protected]
> Icom Users Net: Sundays, 1700Z, 14.315 MHz
> Icom FAQ: http://www.qsl.net/icom/