Re: VFO Stability

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From: JEVERHART@cayman.vf.mmc.com
Date: Mon Aug 07 1995 - 14:40:16 EDT


randyw@crl.com writes:

> BATHTUB CAULK will eat up copper and other metal type things. If it
> has a vinger smell it will attack metal...... I know from experience
> having sealed a PC Board in it. Looks nice, ran very good for about
> 90 days. The copper just disappeared from the board. de stan ak0b

Boy does he ever speak the truth! I have some related experience.

Back in the days when Iran was a valued ally (and very good customer for
military and other hardware), I designed some equipment for a company that
had Iran as a customer. Since the stuff was going to be exposed to the
elements (weather, but no like Dayton, so don't grouse!), we "potted" our
assemblies with a foam material. (Gee that's not a bad run-on sentence but
it's not world class like NILs's :-)). Anyhoo, the foam potting material
intentionally expanded as it set to ensure that it filled all of the nooks
and crannies to keep out nasty stuff like moisture and desert sand. Being a
conscientious engineer (how many of them do you know that could deliver that
word without a dictionary or spell-checker?), I carefully sealed relays and
potentiometers so that the potting foam could not get inside.

After several months of outdoor exposure the electronics stopped working! It
seems that the pots I had worked so carefully to seal and protect, developed a
severe case of corrosion. Chemical analysis showed that the corroded material
inside the pots was copper actetate. AHA! It seems that ordinary RTV and
bathtub caulk (and most ordinary silicone sealing caulks) generate acetic acid
during the curing process - that is what makes the familiar vinegar smell.
Better yet, the curing process requires the presence of water vapor in the air.
In the presence of a corrosive sensitive material like copper, this is a sure
recipe for disaster! My fix was very simple - I replaced the damaged
components and sealed the new ones with a specially formulated RTV,
specifically RTV 3140. The latter is not corrosive because it does not use
acetic acid in its chemical processes. But RTV 3140 is not commonly available.

The bottom line is that I do NOT recommend ordinary RTV anywhere that will
expose a corrosive metal to moist conditions. Use something more benign.

BTW, if you read thru all of this you definitely do not have Attention Deficit
Disorder but perhaps altogether too much free time on your hands (watches on
your hands instead of vice versa?) I must be reading too much of Nils' stuff
lately. It's starting to make sense.

72/73,

Joe E., N2CX


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