[time-nuts] Restoring GR 1120-AB Frequency Standard
magnus at rubidium.dyndns.org
Thu Aug 21 13:00:16 EDT 2008
Mike Monett wrote:
> wa3frp at aol.com wrote:
> > I didn't believe that the thermoswitch was the problem, at first,
> > chiefly because of the simplicity of operation. Eventually, after
> > checking wiring, a carbon resistor that is in series with the
> > thermoswitch, and components around the inner oven control
> > circuitry, I removed the thermoswitch to the bench.
> > After hooking up to a ohmmeter and using a 60 watt light bulb as
> > the heat source, I found that I could duplicate the a pulsating
> > open / close as before. I first focused on the bulb leads and
> > eventually completely removed the old leads and rebuilt each one
> > and did all new soldering under magnification. The problem remains
> > the same.
> > I'm ready to move on at this point noting that this component
> > failure has me stumped and that the fault is most likely internal
> > to the thermoswitch (as strange as this seems). Years ago, when I
> > first saw how internal temperature worked using the mercury
> > thermometer switch, I remarked that it was one component that
> > would never fail. HA! That statement came back to haunt me.
> I have been following this thread with some interest, as I expect to
> have similar equipment in the future. What is amazing is how you
> discovered the problem!
> Like you, I would not have believed a mercury switch could fail. But
> a quick search showed the contact can oxidize, and gave several
> patents aimed at solving the problem:
> 1. Reduction of oxides in a fluid-based switch - US Patent 7071432,
> Often, oxides may form within the switch and inhibit proper
> functioning of the switch. For example, the oxides may increase or
> decrease the surface tension of the liquid metal, which may increase
> or decrease the energy required for the switch to change state.
> Oxides can lead to poor switch performance, and even switch failure,
> because they lessen or prevent a switching fluid from wetting
> surfaces it is supposed to wet.
Hmm... but capacitive sensing should still work well.
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