[Icom] IC-751A vs. IC-765

Thu, 31 Oct 2002 10:23:03 -0800

Hi Floyd,

Sorry about the sloppy slang - most un-British of me. My English teacher in
Bournemouth would have cringed.

To re-state: This causes severe attenuation of "highs" in the recovered
audio when the reconstituted PBT/IF Shift control is set at 2 o'clock or
higher.

I observed this on my radio, and on a couple of others in the S/N 2XXX range
on which I performed the PBT mod. An IF sweep on all these radios revealed 4
to 6 dB passband ripple on SSB.

This problem was not apparent on IC-765's with S/N > 3000. I have not
compared different rev. levels of the Main Unit schematics for changes to
circuit values in the SSB IF-filter terminating networks; that would be an
interesting investigation. To my knowledge, Icom never issued a Service
Bulletin for this issue.

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 Floyd Sense
Sent: Thursday, October 31, 2002 07:15
To: [email protected]
Subject: Re: [Icom] IC-751A vs. IC-765

Adam - can you elaborate on the second item you mentioned as shown below?
What do you mean when you say "PBT is cranked up" ?   I have an 1800 series
765 with the PBT mod installed and haven't noticed any attenuation of the
high frequencies, except as a result of narrowing the bandwidth with the
PBT.  I haven't measured the passband response of the SSB filters, but maybe
that would be interesting to do.  I have the audio mod installed as
described by Dean Straw a few years ago in QST.

I've never run across any info describing a correction to a termination
problem with these filters.  Do you know how it was corrected in the later
models?

K8AC
Floyd Sense in Angier, NC

----- Original Message -----
From: "Adam Farson" <[email protected]>
To: <[email protected]>
Sent: Thursday, October 31, 2002 12:17 AM
Subject: RE: [Icom] IC-751A vs. IC-765

<<snip>>
> 2. Mis-terminated SSB IF filters, causing up to 6 dB of passband ripple
> (corrected in S/N > 3000). This causes severe attenuation of "highs" in
the
> recovered audio when the reconstituted PBT is cranked up.
>
> These issues can be addressed by looking for an IC-765 with S/N > 3000 and
> performing the W2ISB PBT Mod. This is in a downloadable archive:
> <http://www.qsl.net/ab4oj/download/download.html>
>