[time-nuts] Restoring GR 1120-AB Frequency Standard

[Magnus Danielson]

Mike Monett wrote:
>   wa3frp at aol.com wrote:
> 
>   >Phil,
> 
>   > 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.
> 
>   >Best,
> 
>   >Russ
> 
>   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,
>   07/04/2006
> 
>   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.

Cheer,
Magnus