See CTN:2394 for the symptoms and investigation. Open this post at CTN:2395 to see the chain of previous work on this circuit.

Fixes:

• I replaced the OPA140. This didn't solve the issue.
• I also replaced the IRF630 MOSFET. For the heat sink, I just used the mica pad and no extra thermal glue.
• I retouched the solder joints at some places.

Present State:

• As usual, the sense resistor (1 Ohm) is getting very hot. The MOSFET is not unusually hot after about 10 min of supplying 1.5 A of current.
• This circuit should be ideally printed on PCB with thick traces and a proper design for heat dissipation. We need to get out of this workaround protoboard circuit! Can be a starter project for a new student or an undergrad.
• The signal input and heater output all are unnecessarily complicated different kinds of connectors. Particularly from DAC to the box, it could be a much simpler twisted pair of wires.
• I noticed now that the Sorenson power supply was supposed to be exclusively used for these can heate drivers. It is presently being used for driving EOMs also. I'll switch the EOM drivers to the other Kepco supplies.
• I checked for oscillations in the heater circuit as mentioned before. Seems like the steps taken before are still working file and I couldn't see any oscillations across the sense resistor.