[R-390] Cleaning sealed pots?

[John KA1XC]

Don't use Deoxit for cleaning pots, it's a contact cleaner and is designed
to dissolve metal oxides (guess what some pot elements are made of ) and
leave behind a light lubricant. The R-390 pots are very susceptible to this
kind of solvent damage and of course I found out the hard way. I make a
habit of measuring the pot elements before and *after* I work on them, and
watched one of the front panel's controls double in value after I Deoxited
it. :^(

So the next time I decided I'll try some CaiLube, after all it's designed to
be used on controls, and after an extremely gentle application to the
element of  a replacement used Limiter pot (500K) that I was preparing, I
saw its value go from about 800K to 3 Meg after just a few rotations of the
control shaft. :^(

The original Limiter pot I was replacing was completely shot, its element
measured 75 Meg, almost not there. In the same radio the IF Gain pot on the
IF deck measured 10X greater than it was supposed to be; in each of these I
suspect solvent application as the cause of the damage.

I don't know what to recommend as a good cleaner or lubricant now, it almost
seems like voodoo. Some audio guys swear by WD-40, and others swear at it.
Other remedies I've heard included Vaseline as well as some kind of silicone
gel that also provides mechanical damping, and another fellow in one of the
radio newsgroups makes his own secret homebrew formula that he sells. One
thing is for certain - always measure the part after treating it to see if
its value was affected.

Often the 390 pots are not just a little dirty but actually worn out. I've
had a couple of RF Gain pots that were mechanically worn out at the 10
o'clock position that they normally sit at; there was simply no more
resistive element at that one spot.

I'd definitely like to find a good replacement source for the Audio/Line
pots, but 2.5K panel pots with .25" shaft and audio taper are near
impossible to find;  if they were 5K it wouldn't be so bad.  I've actually
thought of trying a dual 5K pot with all the connections paralleled.

John
KA1XC

 > Good evening all
>
>
> I am working on a R-390, with sealed pots for the AF and line gain pots.
Both
> are very erratic, obviously need to be cleaned, but, they are the high
> quality sealed units. The quality is nice, but how do you clean them? Can
one very
> carefully drill a hole through the side, with a bit of grease on the bit,
and a
> drill stop, and then spray in a bit of Deoxit? They don't look like they
> would be easy to disassemble either.
>
> Any thoughts would be appreciated.
>
> Randy
>
>
> --- StripMime Report -- processed MIME parts ---
> multipart/alternative
>   text/plain (text body -- kept)
>   text/html
> The reason this message is shown is because the post was in HTML
> or had an attachment.  Attachments are not allowed.  To learn how
> to post in Plain-Text go to: http://www.expita.com/nomime.html  ---
> _______________________________________________
> R-390 mailing list
> R-390@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/r-390
>