[ICOM] IC-745 Freq Drift

Wed Jan 12 15:00:13 EST 2005

Reid,

Although not documented, the IC-CR64 is usable in the
IC-745, as the 745 PLL Unit is virtually identical to
the version used in the later IC-751/751A/761. I have
done many CR-64's in the IC-745. Some PCB artwork
versions require a jumper to provide power to the
IC-CR64 in the 745, but that is easy.

The only way to accurately set the reference
oscillator is by using a frequency counter.
Suggestions about using any "on-air" sources as a
"standard" are questionable at best, due to additional
frequency uncertainties introduced by the RIT
calibration, (yes, even when it is turned off!), BFO
Frequency, etc. They each have to be set independently
to ensure correct results.

The reference oscillator in the IC-745 (as well as the
751, 751A, and 761) runs at 30.7200 MHz and is set by
adjusting a small "can" inductor while measuring with
a counter. In the 751A, you actually measure the 2nd
harmonic from the ref osc doubler at 61.4400 MHz while
adjusting the reference osillator. The 745 has a
separate 2nd LO Unit that generates the 61.4400 MHz
signal because the doubler circuitry used in the 751A
version of the PLL Unit is not installed on the IC-745
PLL Unit.

There is no "internal PS Fan" in the IC-745. While
similar to the IC-740 and it's fan-cooled IC-PS740
internal power supply (virtually identical to the
IC-PS35), the IC-745 does not use one with the PS-35.
The thermal problem associated with the 2nd LO Unit is
due to heat build up while using the internal power
supply, which is adjacent to the 2nd LO Unit.
Actually, the problem is already there, and the
IC-PS35's heat just exacerbates it. The cure I
described, published on Adam's web site,  virtually
eliminates this problem inherent in all 745's, whether
you use an IC-PS35 or not.

If the problem you are now experiencing has just
begun, the cure is probably not related to the known
2nd LO Unit issue. 

Hope this helps.

Thanks for joining my Yahoo Group, see you over there!

73, Frank W3UHF

--- Reid Bishop <rbish at attglobal.net> wrote:

> 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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