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

[wa3frp at aol.com]

Hi Mike,

Yes I feel that you have nailed the root cause of the failure.  That 
explains the failures observed here.

Thanks and Best Regards,

Russ


-----Original Message-----
From: Mike Monett <XDE-L2G3 at myamail.com>
To: time-nuts at febo.com
Sent: Wed, 20 Aug 2008 5:14 am
Subject: Re: [time-nuts] Restoring GR 1120-AB Frequency Standard



  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.

  2. Preventing corrosion degradation in a fluid-based switch - United
  States Patent 6781074, 08/24/2004

  Liquid metal switches rely on the cleanness of the liquid  metal for
  good performance.  If  the liquid metal forms oxide  films  or other
  types of  corrosion  product buildup within the  switch,  the proper
  functioning or  performance  of   the   switch   may  degrade  or be
  inhibited.

  For example, the oxide film or other corrosion products may increase
  the surface  tension  of the liquid metal,  which  may  increase the
  energy required  for the switch to change state. Films of  oxide and
  other corrosion  products may increase the tendency  for  the liquid
  metal to  wet  to  the substrate  between  switch  contacts, thereby
  increasing undesirable  short circuits in  the  switching operation.
  Build up of oxide and other corrosion products may also  degrade the
  ability of  the  liquid  metal to wet to  the  switch  contacts, and
  thereby may increase the probability of undesirable open circuits in
  the switching operation.

  This is very interesting. Thanks for posting your experience.

  Best Regards,

  Mike Monett

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