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

Neville Michie namichie at gmail.com
Tue Aug 19 19:46:14 EDT 2008

this is a second attempt at an answer, the first seemed to evaporate.
Mercury-in -glass thermometers have formed the basis of a system of  
used constant temperature systems of very high performance.
A major producer of contact thermometers was Jumo (maybe  German).
These thermometers had a thin wire that went down the capillary and
contacted the mercury at the set temperature. The system could be  
accurate to
0.005 degree. The control algorithm is alien to modern EEs but used a
large thermal mass and a fixed rate of heating to produce a slow  
temperature ramp.
Fast response by the thermometer switching kept overshoot down to  
A cycle time of 5 or 10 seconds kept a very low amplitude temperature  
ramp running up and down
with mean temperature held quite close.
Locating the thermometer close to the heater caused a little over  
control which reduced overshoot and cycle amplitude.
The thermometers had up to 30 mA run through the wire, but more  
modern units reduced this to 1mA.
If the tip is corroded on your thermometer contact, maybe a higher  
resistance measuring circuit may
still operate reliably.
Good Luck with the unit,
Neville Michie

On 20/08/2008, at 9:17 AM, Mark Sims wrote:

> I suspect the only place you would find a replacement themoswitch  
> is in another unit.  I also doubt that any current mechanical  
> switch will be anywhere near stable and accurate enough.
> I stand by my original comments:  Built a solid state functional  
> replacement in the same form factor as the original unit.  It will  
> be hidden inside the oven assembly where only you will know of the  
> dastardly deed that you did.  If you ever find a replacement  
> switch,  you can install it and bask in its originality.  Until  
> then,  bask in its solid state patch enabled oscillatude.
> A non-original part that restores a device to operating condition  
> is far superior to a dead unit...  particularly if the part is not  
> visible.  There are lots of zillion dollar antique cars winning  
> best-of-show with modern internal engine components (not to mention  
> bondo and fiberglass under the paint).
> ----------------------------------------
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