[ICOM] IC-751A IF Shift Control: conversion to PBT

[John Geiger]

Now here is a related question I have:  In some other
Icom radios of that era (Icom 751 and 735), the QST
review makes it sound like the PBT works as it should
on CW-it narrows the passband.  But they also state
that on SSB it functions as an IF shift, just moving
the passband around.  Is this true?  And if so, why
does it change from CW to SSB, making it less useful?

73s John W5TD

--- Adam Farson <farson at shaw.ca> wrote:

> Hi Lee,
> 
> The IF Shift implementation is simply dreadful.
> Activating IF Shift bypasses
> the 9 MHz filter, and shifts the 455 kHz filter
> passband relative to virtual
> carrier. Removing the 9 MHz filter from the IF
> signal path compromises
> close-in dynamic range and adjacent-channel
> selectivity very badly.
> 
> Reinstatement of PBT restores the 9 MHz filter;
> rotating the "IF Shift"
> control now shifts the 455 kHz passband within the 9
> MHz passband, thus
> narrowing the effective IF bandwidth. All owners of
> Series II IC-751A's (new
> logo, IF Shift) are strongly urged to reactivate PBT
> - if this has not
> already been done. 
> 
> The W2ISB PBT Mod for the IC-765, which has the same
> effect, is more
> complex, as it involves circuit changes. It is
> downloadable here:
> 
> http://www.ab4oj.com/dl/main.html
> 
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
> 
> -----Original Message-----
> From: icom-bounces at mailman.qth.net
> [mailto:icom-bounces at mailman.qth.net] On
> Behalf Of Lee Mairs (SAG)
> Sent: 03 November 2006 11:44
> To: ICOM Reflector
> Subject: Re: [ICOM] IC-751A IF Shift Control:
> conversion to PBT
> 
> I'm interested in why one would like to do this
> change.  It seems to me that
> I remember some sort of patent issue behind the
> change from PBT to IF shift,
> but it is way back in deep core last activated about
> 1988. 
> 
> Any other highly recommended mods for the 751A?
> 
> I have a 751A from that era that I use daily for
> Navy MARS nets.  I did the
> Wilcox fix for bad batteries years ago, and recently
> had the capacitors
> changed in the PLL section.
> 
> 73 de Lee
> KM4YY/8
> 
> Adam Farson wrote:
> > Hi Eddy,
> >
> > The mod is very simple, and reactivates Passband
> Tuning in the Series 
> > II IC-751A (new logo, IF Shift, 1989 vintage).
> Here is the link:
> >
> > http://www.ab4oj.com/icom/751a_pbt.html
> >
> > Cheers for now, 73,
> > Adam VA7OJ/AB4OJ
> >  
> 
> 
> 
> Scanned for viruses by Blue Coat
> http://www.WinProxy.com/
> ----
> 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/
> 



 
____________________________________________________________________________________
Access over 1 million songs - Yahoo! Music Unlimited 
(http://music.yahoo.com/unlimited)