Topband: Verifying integrity of 75 ohm coax.

[Bill Aycock]

Mike--
Nothing you have described indicates abrasion, but, rather, indicates a 
chemical reaction, resulting in "Plating" of the silver. I suspect a 
sustained DC potential.
If something like "No-Oxit was there, that certainly is a possibility. You 
may have created a battery with dissimilar metals in a chemical bath. If so, 
metallic migration is a possibility, even with no signal.
Bill--W4BSG

-----Original Message----- 
From: Mike Waters
Sent: Sunday, November 24, 2013 1:37 PM
To: Tom W8JI ; topband
Subject: Re: Topband: Verifying integrity of 75 ohm coax.

On Sun, Nov 24, 2013 at 9:08 AM, Tom W8JI <w8ji at w8ji.com> wrote:

> ... But all this tells me is that some silicone dielectric greases may not
>> be abrasive.
>>
>
>   ... contact wear, which is not unusual for contacts that arc when
> switched ... caused by the arc carburizing the grease into a
> silica-carbide, which is a well-known abrasive.



Those switch contacts carried 5 VDC at a milliamp (at most!) to and from a
CMOS interface IC (similar to BCD, but it wasn't) on the logic board of the
computer that ran the machine.

And I don't think that's enough to make anything arc, do you? :-)

I wish I would have saved that switch that I greased, or took a good photo
of it. The circle of grease around the contacts turned bright silver,
almost white, with no visible traces of the underlying phosphor bronze
contact base metal. And from the amount and appearance of the resulting
"silver grease" perhaps the contacts had a heavier-than-normal thickness of
silver plating. Whatever the case, I never expected to see what I did, and
I'll never forget how the machine acted after so short a time, how the
switch looked when I took it out, and how amazed we were. If that grease we
had wasn't abrasive, I just don't know how else to explain what happened
there.

When I saw the silver-impregnated grease bridging  between all the
stationary contact fingers, and the odd way the machine acted, I assumed
that the grease must have been carrying some current between several
isolated stationary contacts. I don't remember checking to see if it was
conductive; but as full of silver 'powder' as it became, it sure should
have been.

I would appreciate learning exactly what silicone dielectric grease that
you successfully used. I currently use a clear liquid lubricant made by
DeoxIT (S-Shield?) on wiping switch contacts. I seem to recall that WB2WIK
said that white lithium grease was good on rotary switch contacts. I've
never tried it.

73, Mike
www.w0btu.com
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