IBM 6150 Hardware Maintenance Manual page 128

Step 15
TNL SN20-9850 (05-87) to 75X0152 or 75X0166
The number 99 in the two-digit display indicates the keylock is in the
Locked position. Look at the keylock.
IS THE KEYLOCK IN THE LOCKED POSITION?
NO Record SRN 11-630-100, then go to "Step 36" on
page 3-0030-36.
YES Set the keylock to Unlock, then go to "Step 2" on
page 3-0030-2.
(5/87) Problem Isolation Charts 3-0030-15