From: NONONOmisc07@fmguy.com   
      
   In sci.electronics.repair, on Tue, 24 Jun 2025 09:45:43 -0500, Fred   
    wrote:   
      
   >My upstairs thermostat is a Robertshaw thermostat vintage 1980.   
   >Yesterday I noticed it was getting a bit warm, and found that the   
   >thermostat had failed to turn on the AC.  I switched the thermostat to   
   >Off, then exercised the control dial back and forth a few times, and   
   >after that it has worked fine with no problem.   
   >   
   >The thermostat is a TX400 or CM260, and in place of a mercury switch, it   
   >uses a sealed reed switch and a magnet that's mounted at the end of a   
   >coil spring.   
   >   
   >Would it be a fair assumption that the problem yesterday was the reed   
   >switch just not making good contact  -   
      
   When I thought this was a mercury switch, before I  read it a second   
   time, I thought it likely a wire to the swtich had broken where it   
   flexed.  Even good wire could break after 45 years.  But a reed switch   
   changes the odds towards a failed switch.    If you didn't do it nicely   
   like thixs, you absolutely should turn the AC on manually in some way.   
      
   What happens if you use measure the voltage across the reed swithc when   
   the temp and setting calls for AC. It should be zero or teeny-weeny if   
   the switch is good. But 12v if the switch is no good at all.  Rotate the   
   setting to as low as it goes and make sure the vom registers the same.   
      
   Or short the right two contacts of the switch with a screwdriver blade   
   and a) see if ther'e s a spark, b) if the AC goes on.   
      
   > it's wearing out after 45 years.   
   >It seems unlikely that it would be something at the AC end, but I don't   
   >know much about how that works.   
   >   
   >I just wonder if I should preemptively replace the thermostat, or wait   
   >for it to mess up again, which may never happen.   
      
   YOu coudl run aq couple wires behind the switch across the AC terminals   
   with a toggle or slide switch at the end, wnad wne it doesn't go on when   
   it should, you could turn it on manaully and be sure that the AC is   
   really working.   
      
   Bear in mind that my oil furnace takes a minute or two or more to turn   
   on aftr it's powered up, and even then, it's only the burner that starts   
   right away, the ciruclation fan doesn't start until the burner gets hot   
   enough (and that fan continues after the burner turns off. The first   
   half of this effect is probably less if it's already powered on and the   
   thermostat sets it on, .   
      
   But I don't remember or never knew if AC starts promptly when the   
   thermostat tells it to.   
   >   
   >Does anyone have a recommendation on a simple non-programmable Heat/Cool   
   >thermostat?  Or a brand to avoid?   
      
   FWIW https://www.ebay.com/itm/277091077135  $55 -S&H after all t hese   
   years   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)