[Collins] Help 75A4

[Dr. Gerald N. Johnson]

On Tue, 2009-02-03 at 08:50 -0800, Glen Zook wrote:
> 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.
> 
Those makers do NOT KNOW what they are saying. That is very true for
voltages over a volt or so but at low voltages, like this meter circuit,
microphone circuits, or servo amp input metering, silver contacts will
not make just from mechanical closure. I've had this beat into me by
failed circuits several times, including the servo metering of the
Collins 821A-1, a church PA system, and my 51J.

When trying the servo input metering on the 821A-1, the symptom was that
if the manual pot was turned to cause a sufficient error, the meter
began to read and would read back down to zero until the meter button
was released. A definite threshold effect with breakdown and microweld.
For the 821A-1, I was able to have the silver contacts gold plated for
the prototype, then for production specified precious metal contacts and
had no more problems.

In the church PA system, I replaced the silver contact switch with a
switch having gold contacts and no more problem. It as a frustrating
search to find out why the microphones on that switch would not work
after switching. Circuit tracing with an ohmmeter made the selected
microphone work until the switch was moved again.

Same thing on the meter of my 51J, except I didn't trace the problem,
long, just looked for a better switch and cured the problem and I
believe that will solve the 'A4 too.

> 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.

Other that it will go open for small signals.

>   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
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer