In case the meter is issuing a "#4 - GAIN ERROR - SYSTEM UNUSABLE - CONSULT MANUAL", a gain adjustment in the hardware might be required.

This can be done by adjusting the P4 potentiometer (near the MU-metal can) in the MCA PCB (ASSY 87030, on the left side of the instrument).
When doing this, you can put the meter in "ANALYSIS" mode and use the 662keV peak of a Cs-137 source as a reference. Use the side buttons to put the cursor on the peak, adjust until spectrum readings have the peak in the correct place (662keV).

After this adjustment, the stabilization procedure should now pass.