[Icom] Another 756Pro mod needed- MW sens

Wed, 22 Jan 2003 13:09:26 -0800

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.

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?

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.

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 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.

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/