B737-300 CREW HAD A TRACK DEV IN ZLA CLASS A. THE ACFT IS ONLY EQUIPPED WITH A SINGLE FMS; AND HAS NO NAV DISPLAY.

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WE WERE ESTABLISHED AT FL350 APPROX 50 MI SE OF THE BTY VOR AND PROCEEDING DIRECT TO BTY. ZLA GAVE US A XING RESTR TO CROSS 10 MI NW OF BTY (THE OTHER SIDE) AT FL310. I HAD LEARNED A NEW TECHNIQUE SEVERAL WKS AGO ON HOW TO USE THE FIX PAGE TO INSTALL A XING RESTR SUCH AS THIS. I INPUT THE RESTR AND HAD THE CAPT VERIFY IT AND THEN EXECUTED IT AND THEN SWITCHED TO THE CRUISE PAGE ON THE FMC. SHORTLY AFTER; I STARTED A DSCNT FOR THE XING RESTR. DURING THE DSCNT; THE ACFT STARTED A L TURN. I WAS NOT EXPECTING A L TURN AND I CHKED THE CDI AND IT WAS 60 DEGS OFF CTR AND THE NEEDLE WAS FULLY DEFLECTED. I THEN DISCONNECTED THE LNAV AND TOLD THE CAPT WE WERE OFF COURSE. I STARTED A TURN TO THE R BACK TO BTY AND ZLA QUERIED US ON OUR HDG AND THE CAPT REPLIED WE WERE UNABLE RNAV. WE PICKED UP A VECTOR DIRECT TO LIDAT AND THE LNAV WORKED NORMALLY AFTER THAT POINT. LOOKING BACK; I BELIEVE THE PROB WAS THAT A DISCONTINUITY WAS INTRODUCED INTO THE RTE. WE BOTH VERIFIED THE ROUTING PRIOR TO EXECUTING; BUT AFTER EXECUTING; I IMMEDIATELY SWITCHED THE FMC TO THE CRUISE PAGE TO ESTABLISH MY CRUISE PWR SETTING. I SHOULD HAVE STAYED ON THE LEGS PAGE FOR A SHORT TIME TO ENSURE THAT A RTE DISCONTINUITY WAS NOT INTRODUCED. THIS ERROR WAS MY FAULT; BUT I FEEL THAT THE SINGLE FMC SYS IN THE 300 SERIES WAS A CONTRIBUTING FACTOR. HOWEVER; I WILL BE MORE OBSERVANT AFTER INPUTTING CHANGES IN THE RTE TO PREVENT THIS OCCURRENCE AGAIN. SUPPLEMENTAL INFO FOR ACN 616225: WHAT APPARENTLY HAPPENED; AFTER A DISCUSSION WITH MY FO; WAS THAT A FIX WAS INCORRECTLY ENTERED INTO THE FMC EARLIER IN THE FLT WITHOUT EITHER OF US CATCHING IT PRIOR TO EXECUTION. WE BOTH LEARNED TO REALLY PAY ATTN WHEN EITHER PLT CREATES A NEW WAYPOINT IN THE FMC.