Fault Phenomenon: Hitachi ZX200 excavator boom descent speed is uncontrolled, accompanied by impact.

Troubleshooting Process:
– Checked throttle valve: descent circuit throttle valve improperly set.
– Checked counterbalance valve: counterbalance valve spool stuck in open position.
– Checked cylinder: cylinder internal leakage.
– Checked operation: excessive handle opening.

Root Cause: The counterbalance valve on the boom cylinder head end is stuck, unable to restrict return oil flow, causing loss of speed control.

Required Parts: Clean or replace counterbalance valve.