Why does not this hair straightener work?

[saaluc]

Get a continuity tester and draw the schematic of the SCR drive, there seem to be more components involved.
For example what are the diodes around the SCR for?

1. I have a continuity tester but am not able to draw the schematic. Can you guide me by asking a series of specific questions?

2. I do not know what are the diodes around the SCR for, but I can explain what is before and after them, etc.

3. Can you set aside or ignore those two test that I have done (The pin No. 7 of the MCU is connected (through a resistor and a diode) to the Gate of the SCR; this pin and the Gate have 4.8 volts, even if I have not pushed the on/off button of the device. I put the negative probe of the multimeter on the Gate of SCR, and the positive probe on the Cathode of the SCR, there is no voltage (0.000)) and independently guide me diagnose the fault(s) of this device?

[saaluc]

_And the cathode-GND path?_

I will mark it on the picture for you.

[timeandfrequency]

OK, let's keep these early measurements aside.
But after that, we do not own any magic wand or sorcery trick to repair a defective board. Unfortunately.
If a simple solution existed, I would have already shared it here.


There are just parts and those are connected by traces that provide electrical connexions.
If no obvious burnt part can be seen, no defective parts can be measured, no wire disconnected, no weak solder joint, the path to success is much steeper : it's all about understanding how the circuit works.
And this means reverse enigineering it.
But we don't have the schematic, so you need to draw it.

Other members asking for help on any electronic repair forum have to do exactly the same work : you can check some examples here and here.

You may draw the schematic by hand or use a CAD tool :
https://easyeda.com/en
https://www.digikey.com/en/schemeit/project
https://www.circuit-diagram.org/

[saaluc]

_And the cathode-GND path?_

Please download the attached picture and open it 100% size.
Pay attention to the light blue circle, red line, dark blue line, purple rectangle.

1. The light blue circle marks the "via" which is connected to negative pin of the black capacitor. I think there can be no path from cathode to this "via" because of the diode next to the cathode.
2. The red line is from pin 6 of microcontroller to cathode.
3. The purple rectangle marks the resistor and the capacitor connecting cathode and gate.
4. The blue line marks the path from cathode to the heating element on the other board and finally to ACN (AC Neutral).

I hope these informations help you.

Will it help to mark also the path which goes through the heating element of board No. 2?

[saaluc]

OK, let's keep these early measurements aside.
But after that, we do not own any magic wand or sorcery trick to repair a defective board. Unfortunately.
If a simple solution existed, I would have already shared it here.


There are just parts and those are connected by traces that provide electrical connexions.
If no obvious burnt part can be seen, no defective parts can be measured, no wire disconnected, no weak solder joint, the path to success is much steeper : it's all about understanding how the circuit works.
And this means reverse enigineering it.
But we don't have the schematic, so you need to draw it.

Other members asking for help on any electronic repair forum have to do exactly the same work : you can check some examples here and here.

You may draw the schematic by hand or use a CAD tool :
https://easyeda.com/en
https://www.digikey.com/en/schemeit/project
https://www.circuit-diagram.org/

What I really mean by asking "Can you set aside or ignore those two test that I have done and independently guide me diagnose the fault(s) of this device?", is:
Are my tests and checks necessary and sufficient?
If I had done no tests and checks, what tests and checks would you recommend? would you recommend the same tests and checks that I did?

[inse]

Are the LEDs back on again?
Are the wire connections between the two boards OK?
Is there any noticeable change in Gate-Cathode voltage of the SCR if the heater is switched on?