Rework Station overheating iron

[ajp8868]

Hi All,

I bought this 853D in a broken state and one of the issues was the soldering iron, I thought it would likely be the thermistor as it just continues to heat up but reported a low temperature. I've replaced the ceramic with the correct one (as stated on WEP's site) but it's still reporting low temps which don't change until I take the iron out and plug it back in (but still low).

I have done some investigation and it's using an LM358 to detect the voltage difference and that then reports to the pin circled on the main chip.

As this board uses a few LM58s to detect the temps I tried swapping one round but it's the same, my next step is capacitors but the two smd capacitors tested ok.

Am I looking in the right place? Does anybody have one of these that could get some reference voltages?

Thanks!

[ajp8868]

Just an update on this, there seems to be no voltage reading on the the thermistor input or output when the iron is plugged in but it has 3.5v when it's not plugged in. I would have thought until the thermistor heats up and actually starts resisting more, the voltage would be relatively similar?

Can anyone help? 

[Rasz]

voltage drop all the way to zero sounds like you are connecting thermocouple = dead short

[ajp8868]

Ah thank you, that would make a lot of sense. The element I bought was an a1322, and the sensor pins test at 1.2ohms.

Would the a1321 be suitable? From what I've read these seem to contain a thermistor instead.