SL-C9 ACE assembly
This is the ACE (Audio, Control, Erase) block used in the SL-C9 / SL-C20 / SL-C24 / SL-C30 / SL-C40 / SL-F25 / SL-F30 / SL-HF100 / SL-HF150 / SL-HF950. It reads the linear audio track from the video tape in a similar way to a audio cassette player. Because the speed of the tape moving across the head is slower than that of an audio cassette, the quality of sound reproduction is not as good. Manufactures later solved this limitation by adding hi-fi sound heads to the video head block. These rotate at a much higher speed and so allow for high quality sound.

The control track is also read by the ACE head. This track is used by the video servo circuits to sync up the video picture during playback.

Poor quality or low level audio: The head often becomes worn with old age. When this happens the sound quality suffers and the VCR can fail to operate. One common symptom is that the real-time tape counter stops rotating. When this happens it shows that the control track is not being read correctly. Under these fault conditions, the picture can also start to roll.
There a few things to try to improve the pick up on a worn ACE assembly (assuming that there are no faults with the circuit):

Luckily, it is quite rare that the unit needs to be completely replaced. It is usually sufficient to clean the head with alcohol and a lint free cloth. There are also various adjustments which can be made. These inlclude height, azimuth and tilt.

Adjustments on the head should only be performed with the use of an oscilloscope and a test tape. In the case of the ACE above, tilt adjustment is performed using the small grub screw on top of the unit next to the screw with the spring. The aim of this adjustment is to tilt the angle of the ACE so that it makes better contact with the tape. The face of the ACE when new is flat but with age it wears into a curved shape making this adjustment necessary.