Hi, I've done some tracing. As suspected there is indeed a ~5V supply that is used for charging the joycons and PWM on the fan. It should be really simple to fix.
See attached diagram. The square black IC on the left of the SoC is a boost regulator that pumps up the ~3.7V from the battery to 5.3V using a MAX8969EWL53+.
Please check whether the following (with battery/USB detached):
- resistance to GND between the 5.3V point near the MAX8969, should be ~3k Ohms
- continuity between 3 marked 5.3V points (capacitor near SoC, pin on fan connector, capacitor near right joycon switch), should be ~0 Ohms
If that's good, then check, with battery attached. Wait for the switch to be fully booted, else the regulator is not enabled:
- voltage on the 3.7V point marked near the MAX8969, should be around 3.7V
- voltage on the 5.3V points, should be 5.3V obviously
If there is no short on the 5.3V rails to GND but also no power, then reflow this chip. If that does not help, we must replace it.