If you ever notice that the voltage of the switch power supply keeps dropping (or even flatlines) when connected to the mainboard, but restores when the mainboard is removed, troubleshoot using the following steps.

1. Remove all plugs on the mainboard except the power supply plug and measure the voltage of the power supply plug again.
2. If the voltage returns to normal, then it means one or more of the accessories connected to the mainboard were short-circuited; plug them back one by one to sort out the faulty one(s). (The accessories that could go wrong include: the X/Y-axis limit, autofocus sensor, and open-cover protection sensor.)
3. If the voltage does not return to normal after removing all the mainboard's plugs, replace the mainboard with a new, identical one.