Thermometer Soar TX-570 shows 10 degrees too low temperature

[AndersJ]

Suddenly it started showing about 10 degrees lower than it should.
Drifts a bit upon startup then stabilizes.
Any ideas any one?

Sorry, I cannot figure out how to rotate the image.

[mimmus78]

Check inside. There should be an ntc near the k-type connector. They need to know temperature at the k-type connector to guess the temperature on the other side of the junction.

[AndersJ]

The NTC is there, and reacts to heat, so it is alive.
Can't say if it has drifted or is otherwise malfunctioning.

[mimmus78]

Well, if you have a static offset you should check this part of circuit. Basically temperature shown on display is the sum of the temperature of the ntc + the temperature of the k-type probe. If you short the k-type connector with a single piece of wire, you should read the temperature of the ntc on display, this should be similar to temperature ambient (if meter is not manipulated too much).

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[AndersJ]

Shorting the connector does not affect the reading.
In ice it shows zero.
In boiling water it shows 50 degrees (C).
It seems to be span issue, since it shows about half of the expected reading.
At room temperature it displays approximately the expected temperature/2,
thus my incorrect and sloppy assumption about a 10 degree static offset.

A circuit diagram would help, but I have not been able to find one.

[jeroen79]

Are there no trimmers to adjust the reading?

If the problem started 'suddenly', did anything happen to the meter?

Can you do some measurements around the K-type connector?
Is it dirty? Does it have a higher contact resistance?
Does it cause a drop in the thermocouple's voltage?

[mimmus78]

Can you take a photo of the inside PCB?

If there are no trimmers and it doesn't use a standard chip it can be hard to proceed with repair.

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[mimmus78]

I didn't asked, you tried to replace this k-type probe.

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