Russ, 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. Phil ----- Original Message ----- From: <[EMAIL PROTECTED]> To: <[email protected]> Cc: <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> > To: Discussion of precise time and frequency measurement > <[email protected]> > 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" > <[email protected]> > 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 -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ >> time-nuts >> and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
