Droppy Head Syndrome. The ERS210 models suffered from a clutch failure in the head mechanisms requiring repair or replacement of the affected part. It is usually caused by a “slip clutch” in the neck servo slipping. The clear signs of DHS are that AIBO’s head will not lift up his head and a whirring sound is heard.
There is also PAS, Pan Axis Syndrome affecting the Servo that rotates the head left and right. The head tilt and even the jaw motors are also of the same type and subject to this issue, but it is much less frequent in them. Repair is commonly carried out by “pinning” the affected clutch body. Have a look Aibo Clutch
Remove the battery cover and press the release hole for the Head Block Unit with the Limb Removal Key or a big paper clip. Then you can send me your defective Aibo Head Block by mail.