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.