This phenomenon shows that aperture's failure to fully open. Chassis, check the light ring plate no distortion, no dust, on the basis of judging fault should be out in the aperture of the automatic control circuit. Check Q701, R725, IC704, no voltage found IC704 6 feet. The foot malfunction with the external circuit. Carefully check, found that the R736 Ω (87 k) has been open. After the replacement R736 troubleshooting.