I have read your first comment and you have said that there's some instances that the unit power up by repeatedly turning it on and off.
This kind of symptoms is an indication of out-of-regulation condition in the power supply section. It was caused by leaky/dried filter electrolytic capacitor in that section. It will dropped the supply voltage and will disable the unit. In the first stage of the symptoms, warm-up will make itself stable but it will get worst, time will come that the unit will not even power up at all like what you've experienced right now.
Leaky capacitor has a physical indication of fault like bulged/dome at the top and can be easily identify like in this sample picture.
Dried electrolytic capacitor has no physical indication of fault, you must have a capacitor checker/tester to determine its condition.
Check the filter electrolytic capacitor most especially in the supply line of the relay, and in the voltage regulator IC's circuitry.
Hope I helped you.
Have a nice day!
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