Re: Little ceramic trimmer caps that always break... A fix.

Chuck Harris

Honestly, does it really matter that I mistyped FG504?

I am talking about ceramic trimmers that exist in
hundreds of Tektronix instrument models.

-Chuck Harris

Colin Herbert via wrote:

Do you mean an FG504? I can't find any FG540 on TekWiki.

-----Original Message-----
From: [] On Behalf Of Chuck Harris
Sent: 19 August 2020 05:15
To: TekScopes
Subject: Re: [TekScopes] Little ceramic trimmer caps that always break... A fix.

Just to clear up some confusion:

This note is about subminiature ceramic disk type
trimmer capacitors. Tektronix equipment from the
1980's onward is littered with these little timebombs.

It is a "modernization" of the old style ceramic
trimmers that had a silver plated half circle on
the lid that was soldered to the adjusting screw.

In this case, the silver plating is inside of the
trimmer, and the ceramic disk is held on by very
temperamental magic. The moving ceramic disk sticks
to its ceramic base, and the magic disappears.

-Chuck Harris

Chuck Harris wrote:
Hi All,

Just a note. As happens to all of us, one of those
little ceramic trimmers in an FG540 was broken. The
screw turned and turned, but the ceramic disk on top
didn't. It was frozen solid.


I didn't have a spare, so I removed the trimmer, and
found the ceramic disk frozen solid to the ceramic
base... no trimming going on here!

First, I put the capacitor into some IPA to soak for
a bit. There was more than enough stinky flux on
the cap, so I think the flux was gluing the cap stuck.

After it soaked for a while, I was able to turn the
disk with my fingers... don't use tools, it will break!

I put it in for some more soaking, and twisting, until
it was good and clean.

Next, I took a small dot of cyanoacrylate glue, and
spread it around the screw. A small dot, not a flood.
[If you can't control your glue bottle, put some CA glue
on a piece of plastic, and use a toothpick to bring a dot
to your trimmer.]

Then finally, I gave a rag a squirt of "Kicker", a CA
glue accelerator, and put the rag near the trimmer... The
kicker's fumes are enough to harden the glue almost
instantly. If you don't have any kicker, just let it set
a while, it will harden from the moisture in the air.

I gave the screw a twist, and all was back to normal.

-Chuck Harris

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