Mac Mini shut down suddenly when in use, failed to power up afterward

I tried changing power cable, disconnecting all peripherals, even reset system management controller (leave the power cable unplugged for > 15sec) etc, none worked. So I brought it to repair store, but they can’t be sure if it is the embedded power supply or the motherboard and insist non-refundable diagnostic payment to find out.

My question: Is there a way for me to identify what goes wrong, before deciding my next move? Where it’s me fixing it myself or sending to repair center.