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

wa3frp at aol.com wa3frp at aol.com
Wed Aug 20 02:34:28 EDT 2008


Hi Neville,

Voltage across the thermoswitch contacts in less than 10VAC.   Current 
is 6-8 mA.  Once in failure mode, the mercury column passes the 78C 
point without causing circuit closure.  Instead, a rapidly pulsating 
open / close with an internal resistance of 50K - 300K is observed. 
Circuit closure eventually happens at a temperature above 82C. After 
this overtemp condition, the thermoswitch will perform normally for 
numerous cycles before going back into failure mode.

Once I detected the intermittent thermoswitch failure, I increased 
voltage and current to see if this would restore reliability.  Failures 
went from minutes and hours to about 12 hours and occasionally days but 
not good enough to declare success. I haven't increased voltage and 
current further as I feel that it better to replace the failing 
component.


Thanks,

Russ


-----Original Message-----
From: Neville Michie <namichie at gmail.com>
To: Discussion of precise time and frequency measurement 
<time-nuts at febo.com>
Sent: Tue, 19 Aug 2008 7:46 pm
Subject: Re: [time-nuts] Restoring GR 1120-AB Frequency Standard



Hi,
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
thermostats
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
millidegrees.
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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