T-962A+ Reflow Oven Thermocouple Malfunction

[jionbha]

We've got a T-962A+ drawer reflow oven where I work that is malfunctioning. I'm tasked with taking a shot at getting it operational for two more weeks while its upgrade/replacement is stuck in shipment on a vessel at port.

There are two thermocouples hanging from the interior roof of the oven. In the picture below, the temperature reading can be seen ramping rapidly from 40°C to 180°C, plateauing, then returning to 40°C. The heater is never activated during this time, the measurement is completely wrong, and the interior is cool to the touch. The duration of the curve shown is approximately 15 seconds.

This error is sporadic, and I can’t yet consistently replicate it. The shape of the error’s curve has so far been consistent. The measurement pictured is during manual operation; the same behavior will occur mid-profile run, and this of course throws off a solder reflow profile.

Any obvious clues to a simple fix?

[jpanhalt]

Is it the original T-962? Have you done any improvements, like the Unified Engineering firmware, temp reference, and grounding?

Those TC's are relatively fine wire.  If they were open, and that was the only problem, I would not expect your results.  However, if you have not fixed the ground issue, you may just be picking up noise.  That is the first thing I would check.  If you suspect an open TC, then remove it and clean and re-weld/braze.  I suspect an open TC and poor ground might also give funny results, but I have no experience to base that on.

I did all the usual modes on mine, including the new firmware, and have seen no similar problems.

[SMdude]

It's possible it has a dry joint on the main board. Pull it apart and wriggle, heat, cool, pull it off and visually inspect.
Do so at your own risk, if you electrocute yourself, I never said nuffin 

[48X24X48X]

Most of these Puhui oven uses the MAX6675 thermocouple IC (I own the T-937). This IC was one of the counterfeit market favorite due to it's high price, so I won't rule out if it behaves oddly after some time. But, these chip can detect open condition whenever it happen. I'm not sure whether the default/stocked firmware takes into account these scenario though. Due to the extreme heat and countless cycles, as some others have mentioned the joints might become dry over time.

[jmelson]

Many thermocouple sensing ICs are set up to show high temperatures with an open thermocouple lead for safety reasons.  I'd check for an open or intermittent connecton on the thermocouple.

Jon

[jionbha]

Thanks all! I opened it to check the thermocouple connections as recommended. It was an intermittent connection scenario where the leads attached to the PCB. Fixed -- appreciated!