{Collins} Cleaning ceramic trimmers



I recently purchased a KWM-2A that had low output and no PA grid current
on most bands.
After scouring the archives and the schematic and attempting a
re-alignment the ceramic trimmer caps were identified as a big part of
the problem. There is a wealth of information in the archives on dealing
with these troublesome components but I would like to offer a few more
points that may be helpful.
 
1.	Ceramic trimmer caps that are "stuck" exhibit clear symptoms.
When attempting to adjust them instead of peaking the signal they will
switch between two distinct levels as the trimmer is rotated. Also,
close visual examination of stuck trimmers will reveal that the thin
disc just below the top ceramic rotor rotates together with the rotor. 
2.	If you or a prior owner attempted to align a unit with stuck
trimmers more then likely it is now way out of alignment. 
3.	It is possible and I think preferable to disassemble the
trimmers without un-soldering any connections. Long needle nose pliers
work well to pull back the c-clip like retainer. In all cases I found
that there was enough slack in the attached leads to do this. 
4.	Separating the stuck disk from the rotor without cracking the
thin disc can be a delicate operation. Soaking in hot water seems to
make it easier (as reported in the archives). 
5.	Cleaning the components can be accomplished in many ways. I used
warm soap and water and a tooth brush followed by a through rinse with
filtered water and then drying and polishing the mating surfaces by
sliding them on a clean paper towel lying on a hard flat surface. 
6.	In my opinion there is no need to use adhesive between the thin
disc and rubber bushing and I doubt that this was done when they were
first built. The friction between the rubber and the disk is sufficient
to hold them together.  The key is to minimize the friction between the
top rotor and the thin disc by thoroughly cleaning their mating
surfaces. 
7.	Note that there are different trimmer capacitance values. Keep
track of which go where. 
8.	It is possible but risky to un-stick the rotor and the thin disk
without disassembling the trimmer. I attempted this on the trimmer on
the PTO to avoid taking the PTO apart. Using an exacto knife I carefully
turned the rotor while gently pressing the blade edge in the gap between
the rotor and disc. (In my case the disk cracked on one side but was
still functional.) This process does not of course clean the mating
surfaces but it does allow you to adjust the trimmer. No doubt it we
become stuck again sooner then if it was disassembled and cleaned. In
light of the risk and the un-cleaned surfaces this is not a fix to take
pride in. 
 
Question: I did not see any physical differences between trimmers with
different values.  The thin discs all seem to be the same thickness and
the plated areas on the ceramic seem to all be the same. How are the
different value obtained?
 
George, W3WO (formerly KG6GA)
 
 
 






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