[Collins] Help 75A4

[radioart at frontiernet.net]

Hi Glen,

Don't forget carbon buildup due to organic materials in the air turning to carbon when contacts arc....  Usually this is the biggest contact resistance offender especially when switching small currents.

Dennis, k0eoo


----- Original Message -----
From: "Glen Zook" <gzook at yahoo.com>
To: collins at mailman.qth.net, geraldj at storm.weather.net
Sent: Tuesday, February 3, 2009 10:50:44 AM GMT -06:00 US/Canada Central
Subject: Re: [Collins] Help 75A4

Tarnish on silver contacts is not silver oxide but is silver sulfide.  It occurs gradually due to exposure to the atmosphere where minute amounts of various sulfur compounds naturally occur.  According to the Silver Institute tarnishing on electrical contacts has negligible effect on the conductivity.  This was based on over 20 years of experimenting and field observation.

Now there are definitely other things that build up on switch contacts which require removal ("cleaning").  These include dust, nicotine, and all sorts of pollutants.  Also, over time it is certainly possible for contacts to lose tension and therefore do not make positive contact.

However, the fact that the silver plating on the contacts tarnish is not cause for concern.  But, cleaning of the switch is definitely a good idea to remove the other pollutants that can cause intermittent operation.

Glen, K9STH

Website:  http://k9sth.com


--- On Mon, 2/2/09, Dr. Gerald N. Johnson <geraldj at storm.weather.net> wrote:

Likely the switch has silver contacts. They oxidize and won't conduct low voltages unless the oxide is broken down with higher voltage, like a volt. I had that in my 51J-3. I replaced the switch with on having precious metal (gold is good) contacts. No more problem.
 
Or it could be a bad solder connection in the S-meter circuit between the switch and the tube.


      
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