[Elecraft] K3 mobile

[Ian White GM3SEK]

David Cutter wrote:
>? Cheap RTVs such as used in bathroom sealant has a vinegar smell and 
>will attack copper.  In a small confined space inside a radio that's 
>not good.  In an open outdoor environment where the gas can escape it's 
>not such a problem. 

The problems about "confined space" are only relevant while the silicone 
is curing. Acetic acid is released during this process and normally it 
escapes easily into the air - hence the vinegar smell. After 24 hours, 
fully cured silicone rubber doesn't smell (or even taste :-) of 
anything.

The situation to avoid is an almost completely enclosed volume such as a 
balun box. If the acid vapor has no means of escape, it remains trapped 
inside to cause corrosion.

> Dow Corning and others make RTVs with benign contents, but at a 
>considerable increase in cost. 
> 
They also don't cure as fast as the acid-based material, and tend to 
have a much poorer shelf life.

>Surface mount components have practically no exposed copper and pcb 
>tracks are not exposed, so, I wonder whether the acetic acid attacks 
>the tin coated contacts.
> 
The main problem on a circuit board is the huge number of junctions 
between dissimilar metals. In the presence of free acetic acid, these 
will corrode badly.


Coming back to the problem of securing toroids, one of the best 
materials is hot-melt glue. This is an excellent RF insulator 
(polypropylene) and works well in any location that doesn't get hot. A 
small blob would easily secure an edge-mounted toroid.

I guess the only reason why Elecraft recommends against it is because 
there's always sure to be someone who thinks "more must be better". 
Polypropylene has a significant dielectric constant so it *will* 
increase stray capacitance in critical tuned circuits.



-- 

73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek