If your radio stays in transmit mode constantly, the D1 diode is likely shorted, or the LOCK switch is stuck closed.
The complete schematic for the Yaesu MD-100/100A8X Go to product viewer dialog for this item.
Yaesu MD-100/100AX is a dynamic desk microphone known for its distinctive audio filtering capabilities and compatibility with various Yaesu transceivers. The internal circuitry includes active components such as to manage its high-emphasis and low-cut filtering functions. Circuit Schematic Overview