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Project logs / How to find the problem?[sparkybg's HAKKO classic and T12/T1
About a year ago, I started to make a soldering iron based on Sparky's classic soldering iron. but unfortunately after about 3 month, I gave it up because I got busy. now I just started it again. I did everything and everything is working very well save one part. thermocouple and Op-Amp! I just have problem with this part. this is a picture of my board:
This picture shows the +input of Op-Amp:
This picture shows the -input of Op-Amp:
And this picture shows the output of Op-Amp:
The problem is that the output of thermocouple isn't correct. e.g. I measured +&-input and output of the Op-Amp and those green and black wires that are the output of the handpiece's cable and are +&- of the thermocouple. I measured all of these due to GND and the result was this:
Green wire: 0.333v
black wire : 0v
the +input of the Op-Amp: 0.275v
the -input of the Op-Amp: 0.011v
the output of the Op-Amp: 3.2v
The supply is 3.3v. as you can see the result isn't correct. What's the problem in your opinion? please feel free to ask any question.
Another question, How can I find the problem? I just have a VC9805 Digital Multimeter. I really stock. I attached the schematic+two layers of the PCB.