WHILE EN ROUTE ON AN IFR FLT PLAN (1ST OFFICER AT CTLS), AMERICAN FLT 803 (A BOEING 727, N1965) BGN A DSCNT FM FL 190 TO9000'. THE SEAT BELT SIGN WAS TURNED ON AT 18,000'. WHILE DSCNDG BTN CLD LAYERS, AT APRX 13,000 TO 12,000', THE CAPT SAWA CESSNA 421C, N1200L, CONVERGING FM OPPOSITE DRCTN. HE TOOK CTL & 'EXECUTED A HARD NOSE OVER' TO AVOID A COLLISION. THECESSNA PLT RPRTD HE WAS CRUISING IN VFR CONDS AT 12,500' WHEN HE SAW THE BOEING DSCNDG OUT OF SCAT CLDS. HE BGN DSCNDG, THEN ENTERED AN ABRUPT CLBG TURN TO AVOID COLLIDING. RPRTDLY, THE ACFT PASSED EACH OTHER W/I 50 TO 100'. AN UNRESTRAINEDPAX & 1 FLT ATNDNT (FA) WERE SERIOUSLY INJURED; 2 FA'S RECEIVED MINOR INJURIES. AFTER LNDG, WRINKLES & SKIN DMG WERE FNDON THE CESSNA'S HORIZONTAL STABILIZERS. THE CESSNA PLT WAS USING HIS #2 TRANSPONDER (XP); ARTCC WAS RECEIVING ITS REPLY INTERMITTENTLY; IT WAS FND TRANSMITTING 3 MHZ (MAX LMT) ABV THE DESIRED FREQ. ON 4/12/84, XP'S WERE CHKD & WRITTEN UP AS'#2 OFF FREQ HIGH.' AFTER THE NR COLLISION, THE ARTCC CTLR NOTED THE 'ALL PRIMARY' FEATURE OF HIS RADAR WAS NOT IN USE.