[Drake] RE: Reply to two posts - power supply electrolytics and plate cap cement

[David Harmon]

Understood....thanks Barry!

Dave

-----Original Message-----
From: Barry L. Ornitz [mailto:ornitz@tricon.net] 
Sent: Tuesday, May 27, 2003 10:39 PM
To: David Harmon; drake@mailman.qth.net
Cc: dmartin@visuallink.com; wagner@dma.org
Subject: Re: Reply to two posts - power supply electrolytics and plate
cap cement


David Harmon, K6XYZ, wrote:

> Barry....very interesting. As you probably already know, JB
> Weld is a steel filled epoxy and withstands heat pretty 
> well. I have previously used it on large model aircraft 
> exaust systems on the tuned pipe and it lasted for years of 
> flying. Do you recommend adding anything to the JB Weld? I 
> am not sure of your wording below. Of course, the glass must 
> be contaminate free for this stuff to stick.

I apologize if my wording was confusing.

The reason for adding chalk or zinc oxide to straight epoxy 
was to reduce its thermal conductivity.  The metal powder in JB 
Weld does just the opposite.  I have not had any personal 
experience with JB Weld at high temperatures so I cannot say 
which might be better.  You do have to worry about the 
different thermal expansion between the metal and the glass 
with JB Weld and other epoxies too.

I have had quite a bit of experience with the high temperature 
silicones.  They work quite well which is why I prefer them.  
They remain somewhat flexible so the issue of differential 
thermal expansion does not come into play here.  A tube of 
the red Permatex costs about the same as a package of JB Weld. Just
remember that a thick section will not cure properly and 
that even a thin layer under a metal cap may take a few days 
to fully cure.  

Trying to use either an epoxy or silicone and using the vacuum 
tube before the adhesives have fully cured is a sure-fire 
method for disaster.

        73,  Barry     WA4VZQ     ornitz@tricon.net