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 - 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.

Does anyone have a recommendation on a simple non-programmable Heat/Cool thermostat? Or a brand to avoid?

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In sci.electronics.repair, on Tue, 24 Jun 2025 09:45:43 -0500, Fred <fred@farkle.com> 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

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micky says...

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.

Thanks very much for the reply. Yes, my conclusion was that
while a mercury bulb could last forever, a reed switch is
going to wear out eventually. From contact pitting I would
think, or perhaps an arm would break.

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.

I haven't done that yet, but will give it a try if I can get
the front cover off - it's not clear how that's supposed to
happen. Probably just a friction fit.

But I don't remember or never knew if AC starts promptly
when the thermostat tells it to.

Yes, the blower motor starts immediately when the thermostat
switches. I really don't think there's a problem with the
AC itself.

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

Yeah, but if I'm going to replace it, I think I'll just get
a "modern" one at Home Depot for less money. I don't need
it to be programmable, but I would like to be able to adjust
the hysteresis.

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On 2025-06-25 8:45 a.m., Fred wrote:

micky says...

> 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.

Thanks very much for the reply. Yes, my conclusion was that
while a mercury bulb could last forever, a reed switch is
going to wear out eventually. From contact pitting I would
think, or perhaps an arm would break.

> 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.

I haven't done that yet, but will give it a try if I can get
the front cover off - it's not clear how that's supposed to
happen. Probably just a friction fit.

> But I don't remember or never knew if AC starts promptly
> when the thermostat tells it to.

Yes, the blower motor starts immediately when the thermostat
switches. I really don't think there's a problem with the
AC itself.

>> 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

Yeah, but if I'm going to replace it, I think I'll just get
a "modern" one at Home Depot for less money. I don't need
it to be programmable, but I would like to be able to adjust
the hysteresis.

I have a variety of Reed Switches for sale for classic arcade games, but
I expect it would cost less to just replace the thermostat than buying
and shipping a NOS reed switch...

https://flippers.com/catalog_oc/index.php?route=product/search&search=reed%20switch

John :-#)#

--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
#7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

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