[ICOM] IC-745 Freq Drift

[Reid Bishop]

Adam,

I don't hear any warbling sound or PLL dropouts, so you are probabaly
correct- it's not the VCO trimmers.  I should note that I do not have an
internal power supply, so the PS fan blowing on the 2nd LO osc crystal would
not be an issue.

Thanks for the link to the IC-745 VCO trimmer replacement.  That link also
includes an upgrade of the 2nd LO osc circuit, which just might be the
issue.  I've also applied to join the ICOM Classic group on Yahoo.

As far as determining the oscillator crystal frequency- can this only be
done with a counter?  You also mentioned the CR-64 OCXO- is this unit
installable in the IC-745?  I don't recall seeing this as on option for this
radio.

Adam, thanks for all of your input.

-Reid-  KC0TAR


-----Original Message-----
From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
Behalf Of Adam Farson
Sent: Tuesday, January 11, 2005 8:05 PM
To: 'ICOM Reflector'
Subject: RE: [ICOM] IC-745 Freq Drift


Hi Reid,

Unless you are hearing a warbling sound on receive, and complete PLL
dropouts on certain frequencies, the problem is probably not due to the VCO
trimmers. The Icomclassic group (http://groups.yahoo.com/group/icomclassic)
is another valuable source of IC-745 information.

The IC-745 VCO trimmer replacement and alignment procedure is here:

http://www.qsl.net/icom/ic745/main.html 

You can check the master-oscillator frequency with an accurate counter. If
the crystal has aged or become contaminated, drift such as you describe
could result; 350 Hz is rather extreme, and points to a defective crystal.
If you can let me have the crystal frequency, I may have a spare (a pull
from another radio) lying around. The CR-64 OCXO will be a definitive cure.

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 Reid Bishop
Sent: 11 January 2005 17:19
To: 'ICOM Reflector'
Subject: [ICOM] IC-745 Freq Drift


Hi,

I have a IC-745 that has developed a horrendous frequency drift as it warms
up during transmit.  After a minute or so of TX, the frequency starts to
drift as much as 350Hz - this is especially maddning when operating digital.
It seems that once it gets good and warmed up (digital modes, long tx, fan
coming on frequently), it remains stable.  During SSB, it rarely remains
totally stable, always floating 100Hz up and down.  This radio was just
purchased a couple months ago from a HAM who bought it new in 1984 and only
used it for one year.  It had been in a closet since (and it looks brand new
too...)

I have read in old lists that some folks have replaced the trimmer caps in
the VFO.  Has anybody on this list actually _done_ this procedure?  I took a
look in the service manual and checked ICOM's 745 FAQ, but can't seem to
correlate the two documents together very well.  I called ICOM today, and
they said the crystal oscillator was probably the problem, and I needed a
high stability oscillator.  They said 300Hz is not out of line for this
radio.  Somehow, I find that hard to believe.

Thanks!

Reid - KC0TAR


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