[ICOM] IC735 VCO caps... the real story.

[D C *Mac* Macdonald]

The best explanation that I've heard was that the plasticizer
(the chemical compound) that keeps the plastic flexible) slowly
leaches (evaporates) out of the plastic dielectric.  Some call
this process "out-gassing" which is reputedly the cause of the
film that accumulates on the insided of automobile windshields.
This happens even in autos NOT driven or otherwise occupied by
active smokers.
 
This has always been a believable expanation to me, whose
64 1/2 Mustang continued to be plagued by windshield film
even long after I quit puffing on the cigarettes!
 
73 - Mac, K2GKK/5
(Since 30 Nov 53)
OKlahoma City, OK


_____________________________________
Date: Sat, 27 Feb 2010 16:32:00 -0800
From: gfiber at comcast.net
To: icom at mailman.qth.net
Subject: Re: [ICOM] IC735 VCO caps... the real story.

My experience,

I am unaware of any cut off serial number for the IC-735 VCO trim 
caps.That rig never came with ceramic dielectric trim caps so far as I 
remember.

I am not sure we at Icom America ever actually discovered the problem 
with the plastic trimmer caps. One school of thought was the wax used to 
deaden microphonics within the VCO had got into the original caps when 
the factory poured it in. The DC voltage is so low within the VCO 
circuit I am sure its not due to the caps being over voltage being 
applied. But still the plastic trim caps do go bad and replacement fixes 
the problem either with new plastic dielectric or ceramic dielectric 
caps. I do not remember the factory ever changing to a ceramic based 
cap. Remember there were some 40,000 IC-735's produced world wide and 
its very likely a lot of them have never had this problem as some here 
on this forum attest to.

Several Icom's used the same design. The IC-M700 marine SSB transceiver 
used the same VCO and no trim cap issues with those so far as I 
remember, they also came standard with the CR-64 as the Part 80 to end 
technical rules state a marine SSB transceiver needs to be with in 20 Hz 
of the transmit frequency its supposed to be transmitting on, much 
tighter specs than an amateur transceiver has. Still that's not the 
answer to the caps going bad. I suspect it may be the wax though it 
seems to me the VCO never gets warm enough for the wax to melt during 
operation to migrate up into the caps.

IC-735, 745, 751 / A, 761. R71, M700. M700TY, 271 A /H , 471 A/H all 
used the plastic dielectric trimmer caps some more extensively than 
others. I may have missed a rig or two also.
 
Gary K8IZ