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

phil fortime at bellsouth.net
Wed Aug 20 03:26:40 EDT 2008

In other words, you sound like you are not 100% sure it's the switch. A 
shame you couldn't graph voltage and current going to the switch as it 
cycles. Is it possible a cap or something else is intermittently failing in 
the circuit. Again I'm assuming with that resistance there is something 
between it and the heater. Though I had one, I don't think I ever looked 
inside the thing.

I have seen some similar units (I think what you have) that use an almost 
paper thin column of mercury. I would guess that is how they can achieve the 
fine resolution/precision.


----- Original Message ----- 
From: <wa3frp at aol.com>
To: <time-nuts at febo.com>
Cc: <fortime at bellsouth.net>
Sent: Wednesday, August 20, 2008 2:34 AM
Subject: Re: [time-nuts] Restoring GR 1120-AB Frequency Standard

> Phil,
> The details of the thermoswitch failure are not completely clear. 
> Intermittently, the mercury column will pass the 78C point without causing 
> circuit closure. Instead, a pulsating open / close with an internal 
> resistance of 50K - 300K is observed. Circuit closure eventually happens 
> at a temperature above 82C. Once an external failure, i.e. lead wiring, 
> was ruled out, I used a borrowed microscope to view the internal structure 
> of the thermoswitch. Unfortunately, this did not reveal the failure 
> source.
> Russ
> -----Original Message-----
> From: phil <fortime at bellsouth.net>
> To: Discussion of precise time and frequency measurement 
> <time-nuts at febo.com>
> Sent: Tue, 19 Aug 2008 9:33 pm
> Subject: Re: [time-nuts] Restoring GR 1120-AB Frequency Standard
> Neville,
> good answer. Perhaps the original poster could supply a photo of his 
> switch,
> perhaps more suggestions could be made. I think we had an old gr-1100 at 
> one
> time, it was a 100kc unit. Needless to say it was scrapped years ago and 
> I'm
> sure it's been melted down and sent back to the usa in tin cans or cars by
> now! I'm curious as to how the switch failed. I'm assuming it was a sealed
> unit.
> Phil
> ----- Original Message -----
> From: "Neville Michie" <namichie at gmail.com>
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> Sent: Tuesday, August 19, 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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