How to solve if the Spindle analog output no change?

Problem: Input S500 under MDI mode is not working, spindle speed is not changed, firstly we doubt the parameters are not correct, but after check the analog output and shift speeds parameters are normal.(from http://www.cncmakers.com)

Use universal meter to measure the analog output voltage while input the S command, and there was nothing change, so it should be the CPU board of the CNC controller or some wiring were open circuit.

Then open the CNC controller cover, there was IC burned smell, the IC767 was burned, this is the problem, IC767 controls the analog output, replace a new IC767 the CNC controller resume to work, spindle analog output back to normal.