TM 55-1930-209-14&P-19 Table 9-3.  Preventive Maintenance Checks and Services for Electrical Power Systems(Continued) B -  Before D -  During A -  After H -  Hourly Interval INTERVAL ITEM PROCEDURES EQUIPMENT ITEM TO BE CHECK FOR AND HAVE IS NOT READY/ NO. INSPECTED REPAIRED OR ADJUSTED AVAILABLE IF B   D   A H AS NECESSARY 250 hrs 9) When coolant is at proper level and level stabilizes, replace radiator cap. 250hrs n.     Check tubes in heat exchanger expan- sion tank for scaling. 1) If obstructed or scaled, clean tubes by passing a rod slightly smaller than the internal tube bore thru the tubes.  Do not use excessive force. 2) If expansion tank tubes are so clogged that a rod cannot be passed thru them, completely remove the core and clean by boiling in a caustic soda solution or with a commercial cooling system cleaner. 3) Reassemble the heat exchanger and activate. o.     Remove red tag from generator switch- board. 500 hrs a.     Service primary and final fuel filters by performing the following: 1) Redtag affected SSG on switchboard panel indicating, "WARNING-DO NOT ACTIVATE.  REPAIRS BEING MADE." 2) Shut off fuel tank supply valve on engine. 3) Loosen nut on top center of primary filter body.  Lower filter case. 4) Remove filter element.  Wash both ele- ment and filter case in clean, non- flammable solvent. 5) Reinstall element in reverse order. 6) Remove and discard final filter ele- ment by turning from left to right (right- hand thread). 7) Remove all old gasket material.  Clean gasket sealing surface in filter mount. 9-19