Power supply GND needs to be connected to the SCR and the micro controller.
Just do a continuity check from the electrolytic capacitor.
Does any of the LEDs light up?
You said "Just do a continuity check from the electrolytic capacitor."
1. When I put one probe of the multimeter on the negative pin of the electrolytic capacitor, and the other probe on the cathode of SCR, there is no continuity beep but 91.4 ohm.
2. Is it not also necessary to locate the microcontroller GND (the datasheet cannot be found on the internet) and check its continuity to the cathode of SCR? If it is, can you guide me to locate it?
You asked "Does any of the LEDs light up?"
3. When I received this device, pushing the buttons could light up all of the LEDs
BUT the device did not work (elements were cold).
After I disconnected the yellow X2 capacitor, one pin of the VDR, one pin of the polyester capacitor, and one pin of the black capacitor, to check them out of the circuit, and then reconnected them and plugged the device, the LEDs did not light up any more.
But when I did voltage test on the push-buttons, the LEDs lighted up again. Probably they will light up again if I do voltage test on the push-buttons.
If you need more pictures, please tell me.