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

[Adam Farson]

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
>  



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