Full Erase Head
This full erase head has been used in all models since the SL-C9 onwards. It erases the video, hi-fi and linear tracks of the tape. It is not used if insert editing is being used.

It is situated just after the tape exit point from the supply reel (i.e. before it reaches the complete drum assembly).

If the full erase head (FEH) is not functioning properly then this can been seen by patterning moving down the screen for about the first 10 seconds of a new recording (although this can also be due to incorrect back tension. It is more pronounced when the new recording is made over an existing recording.
This part rarely fails. The more likely cause of the problem is that the bias oscillator which drives the head is not working. This is usually due to a broken transformer, drive transistor or capacitor.

Bias oscillator circuit
Bias oscillator circuit

The head can be cleaned with a little alcohol and a lint free cloth. Clean the head using a left to right motion not an up and down one.

This fault is particually common in the SL-HF950.