Den-on SC-7000 Desoldering gun control pcb defective

[rodd]

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

[fzabkar]

Pin #3 of the hybrid IC is missing from your diagram.

Photo from a different angle:

https://m.media-amazon.com/images/I/51pRgLOh1LL._AC_UF1000,1000_QL80_.jpg

[rodd]

Hi,
The diagram was taken from the operational manual provided by Den-ON.
Do you have any idea of how it is possible to control the tip temperature without a sensor?
Thanks

[fzabkar]

I would think that it would not be possible to control the temperature without sensing it. You could still vary the temperature and run the heater in open loop mode, but that's not the same thing.

[helius]

A coil of heating wire such as Nichrome has a temperature coefficient of resistance. It is therefore possible to sense the temperature and heat the tip using the same winding; but the control circuit must alternate between heating and measuring. I think it works by using a voltage comparator on pins 2 and 5 of the hybrid to turn on the SCR once every AC cycle (120 times a second). Pin 5 is the reference voltage, set by the temperature potentiometer, and pin 2's voltage depends on the temperature.

The schematic is also missing D4, in addition to pin 3 of the hybrid.