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 <[EMAIL PROTECTED]> To: Discussion of precise time and frequency measurement <time-nuts@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" <[EMAIL PROTECTED]> To: "Discussion of precise time and frequency measurement" <time-nuts@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). > ---------------------------------------- > > _________________________________________________________________ > Be the filmmaker you always wanted to beearn how to burn a DVD > with Windows. > http://clk.atdmt.com/MRT/go/108588797/direct/01/ > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ > time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.