Dear all,
I have been using the Den-on SC-7000 desoldering gun for years and have been very satisfied with the tool performance.
The only issue this tool has is that aproximatelly once in every two years, the control pcb fails.
It is easy to spot it because there is a led on the rear of the tool that light when the heater element is ON.
When the tool is operating normally the led blinks rougly once every two or three seconds (after the desired temp has been achieved).
However when the failure starts, the blinking is erratically and the desired temp is not achieved.
Until 2020, this problem was solved by replacing the control pcb which was sold by an US representative.
However from 2021 on the replacement PCBs have some fabrication issue and after they are installed, the heater stays on all the times.
I have bought more than 5 pcbs all whith the same problem.
I contacted Den-on support and after some discussions sent me more 5 pcb factory tested, but unfortunately after installed the problem is the same.
It is worth mentioning that I have tried 3 different tools.
So I decided that I should try to do some reverse engineering and try to find fix the problem myself, but I can't figure out how the circuit works.
The simplified diagram is attached below
And the photos ofthe control pcb are as follows.
As can be seen there is an epoxy covered assembly but I managed to remove the epoxy potting it one of the assemblies to see what is inside it (unfortunately broke the ceramic base).
The fotos show that the assembly contains a Lm2903 dual comparator and four transistors.
What confuses me is that there is NO thermistor or temperature sensor on the tip!!!
Can anyone suggest a theory of operation and or some adjustment I might do to solve the problem?
Thanks