Is this burned or normal? (858D hot air gun temp sensor)

[Willen]

Hi,
I recently bought a cheap hot air gun rework station. Maybe Chinese clone of 858D. I recently knew that the 858D is one of the popular rework station around the world.

Just using it few hours, now shows "732" or "733" few seconds then shows "H-E"or "S-E". Device is not working. Sometime it heats horribly let's say within 5 sec then turns off. Sometime not heating.

Fan is working. Heater is fine with 70R resistance. Tilt sensor is fine. Cables from device to blower are fine. Soldering joints looks fine. Adjusted connectors and looks fine. Heater driver TRIAC is fine.

I just got an unusual heat sensor at the top end of heater coil. It looks like burned and melted and shorted. Continuity reading says it is totally shorted. I removed rensor, same result. Replaced sensor with 1.5k resistor, same result. Bypassed tilt sensor, same result. Any idea please? The device is close to the design: https://blog.enbewe.de/posts/2018-06-21-saike-858d-original-schematic/

 

[JimB]

That looks like a thermocouple, in which case it is normal.

A thermocouple is made from two wires of different metalsl and they are welded together.
As the junction heats up, a few mV is produced in the junction.
The voltage/temperature graph is well defined and stable for a given pair of metals.

JimB