A B737-300 RETURNED TO THE FIELD DUE TO EXCESSIVE RUDDER TRIM INPUT TO STABILIZE THE ACFT AND ON INSPECTION IT WAS DISCOVERED THAT THE R AILERON BUS CABLE WAS FRAYED AND BROKEN.

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APPROX 30 MINS AFTER DEP; CREW CALLED AND SAID THEY NEEDED 5 UNITS L RUDDER TRIM TO STABILIZE ACFT; AND THAT R AILERON WAS UP AND NOT FAIRED. THIS WAS CAUSING THE RUDDER TRIM PROB. UPON ARR ON GATE I NOTICED R AILERON BUS CABLE BADLY FRAYED AND BROKEN IN TWO. IT HAD WORN AWAY ON THE BACK SIDE OF THE PULLEY JUST INBOARD OF R MAIN LNDG GEAR. THIS CABLE IS APPROX 3/16 INCH IN DIAMETER. THERE WAS MUCH DIRT AND GRIME ON THE CABLE AND PULLEYS DUE TO ITS LOCATION IN GEAR WELL. L SIDE WAS ALSO INSPECTED AND FOUND TO BE OK. THIS RUDDER TRIM PROB HAD BEEN RPTED PREVIOUSLY (2 TIMES OR MORE) BUT RPTS MADE NO MENTION OF ANY FLT CTL POS NOR DID THEY GIVE CONFIGN DETAILS SUCH AS FLAP SETTING; AIRSPD; WIND DIRECTION; ETC. UPON OUR INSPECTION OR SVC CHK DONE PRIOR TO THIS DEP; AND PRIOR CREW RPT OF RUDDER TRIM PROB; WE COULD ONLY FIND A SPACER BLOCK MISSING FROM THE R OUTBOARD FORE FLAP CARRIAGE ASSEMBLY WHICH WAS REPLACED. ACFT WAS BROUGHT TO GATE FOR DEP AT XA00. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE ACFT WAS A B737-300 WITH GE CFM 56-3 ENGS. THE RPTR DESCRIBED THE CABLE AS BEING FRAYED AND WORN AND EXTREMELY SATURATED WITH DIRT; SAND; AND GRIT. THE RPTR ADVISES THE CABLE PULLEYS AND LEADS WERE SVCABLE AND NO INTERFERENCE TO THE CABLE WAS NOTED. THE RPTR BELIEVES THE AREA WHERE THE CABLE WAS BROKEN AND FRAYED WAS NEVER CLEANED AND BEING JUST OUTSIDE OF THE WHEEL WELL COULD ACCUMULATE THE DIRT AND SAND THROWN UP BY THE LNDG GEAR. THE RPTR STATED THE L SIDE CABLE AREA WAS CLEAN AND NO WEAR NOTED.