[Collins] Help 75A4

[Greg Mijal]

I think for this meter fix I would put a high impedance volt meter or O 
scope across the terminals of the meter and see if there was voltage there 
when the meter stops reading.
73's
Greg
WA7LYO
Kinston NC
----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at storm.weather.net>
To: <gzook at yahoo.com>
Cc: <collins at mailman.qth.net>
Sent: Tuesday, February 03, 2009 1:17 PM
Subject: Re: [Collins] Help 75A4


> 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
>
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