The output transformation occurs inside high-speed semiconductor switches mounted directly to the aluminum chassis plate.
The JYMC-220BI features several small trimpots on the circuit board to calibrate motor performance. If you replace components, you may need to adjust these using an insulated screwdriver: jymc220bi schematic full
: Three pins for the speed control knob (typically a 5k or 10k ohm resistor). jymc220bi schematic full