Hi,
I need help in repair of Weller WECP-20 soldering iron, particularly from the person
who has done the repairs on this.
Please look into the attached hand written scematic, copied from the bloger in this forum.
Both the ic,s LM358 and SL443Awere defective, hence I replaced them and added a 220 ohms resistor in series with the existing 240 ohms after half wave rectifier as suggested by some blogger. But the iron is not functioning.
I am finding that the SL443A is not functioning with the input at pin 8, of more than about 6 volts. I don't know whether it is right. I have replaced and tried with three numbers and all are behaving in the same manner.
The two stages of LM358 outputs at pin 1 and 7 is high at around 13 volts and output at pin 7 is not varying by varying the temperature setting pot. I have ensured that LM358 in good worhing condition.
Inputs at the first stege pin 2 is around 1.5 volts and pin 3 is around 6 volts, as such, naturally the out put at pin 1 is high at 13 volts.. The second stage input pin 5 is around 13 volts, because it is fed from fist stage output, pin 1. Second stage input pin 6 is varying from about 3 volts to 6 volts by varying the temperature control potentiometer. Naturally the output at pin 7 is high at around 13 volts and not being able to control..
I have checked the 5volts regulator 78L05 and it is working alright. I have also checked all the resistors and diodes, all are ok. Soldering iron element is also ok. But the temperature sensor in the soldering iron is showing very high resistance, more than 2 M ohms in cold condition and around 13 k ohms in hot condition. I didn't know whether it is correct value.
To check the output states of the LM358, I put a 1.5k ohms resistor from input pin 3 to ground and found that the, both the outputs at pin 1 and 7, go low, below about less than about 1 volts. Hence the triac is getting triggered and soldering iron is getting heated up, without the temperature control. So it can be deduced that the LM 358 is working, but dont know, why the circuitry is not functioning properly.
I request any person who has experience in this, to analyze the problem and provide me the solution, for which I will be very thankful.