Repair help. Mosfet keep shorting
R3 and R4 are from a current threshold warning. When current trough the shunt (R3+R4) rises above the threshold of U3 the controller gets a STOP signal.
For example, when the spindle is jammed.

With this circuit bypassed, you can dump the full transformer current into the spindle and FET risking damage to both.

Here are some ways you can destroy this FET:
- Slow gate. if the 555 gate drive is too slow, or not high enough, it results in the FET being in the linear region too long until it overheats.
- Back-EMF from the spindle should go through D4 and be dissipated. If D4 is open, high transients will occur and the FET will be damaged.

D3 prevents the gate voltage from going above 18Vac rectified + Vforward, if that supply is available.

The IRF740 is a 400 Vds n-channel. Yours is an IRFZ44, with only 60V. For a circuit running at rectified 36Vac (50Vdc) that is very close.
Even with D4, you'll still be looking at transients. Since diodes take a small amount of time to conduct.

This would mean you most likely have burnt it by back-EMF or gate voltage.
Vgs is not allowed to be higher or lower than 20V or 12V from ground. (in this circuit) D3 clamps the gate voltage at rectified 18V with diode drop. This will be around 25V. Beyond maximum ratings.

My suggestion would be to replace it for a >200 Vds fet with Vgs equal or better than +-20V, and protect the gate with a 15V zener diode.