: Connected to incoming single-phase power through the main machine switch and emergency stop loop.
| Step | Observation / Measurement | Likely Cause and Corrective Action | | :--- | :--- | :--- | | | Tripped RCD/MCB at power-on, or visible burning on board | The power stage has a major short. Test the five S6025L thyristors for shorts. Replace any faulty thyristor. Inspect the AC input for a dead short. | | 2 | No response; no motor voltage (0V) | Check input fuses. If blown, proceed to Step 1 to locate root cause. | | 3 | Erratic speed control or motor doesn't start/stops | Test all potentiometers for intermittent wiper contact. Replace any that are faulty. Calibrate new pots. | | 4 | Motor hums but doesn't start; current unstable | Inspect the R23 current sense resistor for correct low resistance. If open or high resistance, replace. Test capacitors C9 and C15. | | 5 | Motor can be started by touching a pin on the LM324 IC | A classic sign of a failed LM324 operational amplifier. Replace the LM324 IC and check surrounding passive components for damage. | | 6 | Motor runs but lacks full torque or speed | Test the bridge rectifier diodes and thyristors for one-way conduction only. Check for low output voltage from the control section. |
Instead of complex microprocessors, the board relies on an analog timing circuit (often utilizing an operational amplifier array or a specialty SCR gate-trigger IC). This block reads the speed command from the operator’s external potentiometer and translates it into synchronized firing pulses for the main power semiconductors. 3. Power Switching Stage (SCR Topology)
bus power to the motor hundreds or thousands of times per second.
