IBM 6150 Hardware Maintenance Manual page 270

Step 7
1. Set the power switch on the system unit to Off.
2. Connect one of the cables to the system unit.
3. Set the power switch to the system unit to On.
4. Wait 15 seconds, then answer the following question.
bJD
IS THE POWER-ON INDICATOR ON?
YES Repeat this procedure.
NO The last device or cable attached is failing. Exchange the failing
device or cable, then go to "Step 17" on page 3-8800-17.
(9/86) Problem Isolation Charts 3-8800-7