Hi everyone,
Last week i've found and bought an old jbc ad 2000 soldering station. I've tried to use a T245 type handle, i've had the same problem as mentioned above, at 350 degrees (C) setting tip temperature was only about 200 degrees.
I've opened it up, checked the pcb, i've found out that the controller chip was actually a 28 pin pic mcu.
When i followed the the track from positive thermocouple pin of the handle on pcb, i've found the thermocouple amplifier (op07) The positive feedback resistor (at the left) of it was 1M Ohm. Amplified output was connected to one of pic mcu's adc channels.
I've attached a k-type temperature tester to tip of the iron, powered the station, at max setting tip temp was 190 degrees (C), TC amp->pic adc input voltage was 4,5V Dc.
I've paralled the feedback resistor with 1Meg resistor. With new feedback ratio, at maximum setting tip temperature was 350 degrees.
That was my quick and painless solution, i hope if someone still (or will) have the same problem uses it.