[Icom] Sorry for the clutter

Fri, 07 Feb 2003 13:01:00 -0500

Hello All,

Here's part of my reply to an email I received from Fern regarding the 745, 
it may help others:

To answer your question regarding the reference oscillator on the IC-745, 
the answer is yes. The PLL Unit is identical to the IC-751A in the area of 
the reference oscillator and also the VCO. There are some other differences, 
mainly less component count, but they are essentially the same. The IC-CR64 
Hi-Stability Crystal Unit can be installed just like in the IC-751A, 
although you'll never find that in any 745 manual. It is not needed, 
however, for most applications. Actually, the main issue with drift in the 
745 is not the reference oscillator, as it's the same as in the IC-751A, and 
is pretty good. The 2nd LO Unit (on the right sidewall of the radio) is the 
culprit. The crystal trimmer is a 30 pF plastic trimmer, with a 6 pF fixed 
disk in parallel on the back of the PCB. This capacitance ratio is the 
culprit. I normally remove both of these components, and reinstall a 12 pF 
(White) ceramic trimmer along with a 24 pF 50v NPO fixed capacitance where 
the 6 pF used to be. There is still a total of 36 pF, but the largest 
capacitance component after the mod (the fixed disk) is much more 
temperature stable and this combination gives a good tuning window too. I 
have never seen one that would not tune the 2nd LO on frequency after the 
mod. After mod and alignment, the ceramic trimmer isn't typically near one 
end of it's range, either!

Additionally, I always add a grounding strap to the crystal, there is a 
conveniently placed hole in the PCB near the crystal that goes through to 
the ground plane for this, but be careful, there are actually two holes 
close together, and the other one is NOT ground. I normally use tinned 20 
AWG buss wire with a little bit of sleeving on it for this jumper.

The accompanying pictures should tell the story. I just finished this 745 
for a customer, and the 2nd LO mods are part of the standard package I do. 
Pictures 615, 637, and 633 show the 2nd LO unit before, and pictures 646, 
642, and 638 show the "after" configuration. This mod really helps the 
stability especially if you use an IC-PS35, which winds up being right next 
to the 2nd LO crystal when it is installed! Some people also cut out a 
little block of styrofoam, core it out in the middle, and slip it over the 
crystal for added thermal stability. I don't do this, as I don't use 
internal power supplies.

Best Regards,

Frank W3UHF

>From: Adam Farson <[email protected]>
>Reply-To: [email protected]
>To: [email protected]
>Subject: RE: [Icom] Sorry for the clutter
>Date: Fri, 07 Feb 2003 09:46:43 -0800
>
>Hi Fern,
>
>Pas de probleme - your posting contained some remarkably "good stuff" for
>IC-745 owners.
>
>Cheers for now, 73,
>Adam VA7OJ/AB4OJ
>
>
>
>----
>Your Moderator: Dick Flanagan W6OLD, [email protected]
>Icom Users Net: Sundays, 1700Z, 14.315 MHz
>Icom FAQ: http://www.qsl.net/icom/

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