This may be the blanking shutters. The shutters are used to cover unwanted scan lines or scan line start/stop anomalies.
I believe the following 2170D-3 service codes control this.
HBLK (horizontal blanking shutter) 0-off, 1-on (Toggles the left and right shutters.)
LBLK (left blanking shutter) 0-63
RBLK (right blanking shutter) 0-63
VBLK (vertical blankiing shutter) 0-off, 1-on (Toggles the top and bottom shutters.)
TBLK (top blanking shutter) 0-15
BBLK (bottom blanking shutter) 0-15
You already know the next part about writing things down.