RPTR IS UNABLE AFTER NUMEROUS CALLS TO ESTABLISH CONTACT WITH ZLA. PSP SENT THE RPTR BACK TO TRY SOME MORE WITH STILL NO CONTACT. PSP TOLD RPTR TO MAINTAIN 13000 FT INSTEAD OF 17000 FT. A C172 WAS AT 13000 FT. THE RPTR CLBED AND TURNED TO AVOID HIM.

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HAD FREQ PROB COMMUNICATING WITH ZLA -- 4 TRIES; NO COM. WENT BACK TO PREVIOUS FREQ. PSP DEP SENT US BACK TO 128.15 FREQ. TRIED 4 MORE TIMES AND THEN BACK TO PSP DEP FREQ. OUR CLB CLRNC TO 17000 FT WAS THEN CANCELED AND TOLD TO MAINTAIN 13000 FT. TOLD TO TRY AGAIN ON 128.15 FREQ. INFORMED ATC OF PROB WITH FREQ AS RPTED YESTERDAY ON INBOUND FLT. AT 13000 FT SAW A SMALL CESSNA SINGLE ENG C172 TYPE; AT SAME ALT. TCASII CONFIRMED 13000 FT FOR BOTH ACFT. NO TA OR RA SOUNDED. WE TOOK ACTION AND CLBED TO 13300 FT AND TURNED R TO AVOID ACFT AND ALLOWED IT TO PASS CLR UNDER L WING. THE CESSNA APPEARED TO DSND AT THE SAME TIME. PASSING DISTANCE WAS LESS THAN 700 FT VERTLY AND VERY CLOSE HORIZLY -- LESS THAN 1/2 MI. INFORMED ATC ON FREQ 128.6 OF PROB. THEY TRIED TO SEND US AGAIN TO BAD FREQ OF 128.15. INFORMED THEM WE WERE STAYING WITH HIM AND NO LONGER WERE GOING TO TRY ON 128.15; SINCE WE NOW HAD CONTACT WITH ATC. FLT CONTINUED WITH NO FURTHER INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS THE CAPT AND THE PF. HE HAS ABOUT 15000 HRS TOTAL TIME; 200 HRS IN THE PAST 90 DAYS AND ABOUT 230 HRS TOTAL TIME FLYING THE MD80. THE OCCURRENCE WAS IN THE MORNING. WHEN LEVEL AT 13000 FT THEY WERE MOVING ALONG AT ABOUT 320 KIAS OR ABOUT 400 KTS TRUE. THE CESSNA 172 CAME UP ON THE TCASII SCOPE AT THE SAME ALT. THE RPTR CLBED 300 FT AND THE CESSNA DSNDED 200 FT. THE MAX MISS WAS 700 FT. THE TCASII RA AND TA NEVER WENT OFF AND WAS WRITTEN UP. THE MAINT TESTED THE TCASII AND CHANGED THE COMPUTER. THE RPTR STATED WHEN THEY WERE AT ABOUT FL260; 128.15 WAS RECEIVABLE. THE RPTR HAVING AN AVIONICS KNOWLEDGE SUGGESTED THAT A REPEATER WAS NOT WORKING TO BEAM THE FREQ INTO THE PALM SPRINGS AREA. HE TALKED TO ZLA AND THEY DON'T BELIEVE ANYTHING IS WRONG WITH THEIR FREQ. THE RADIOS ON THE RPTR'S ACFT WERE CHKED TO ASSURE 128.15 WAS WORKING. THE RPTR SUSPECTS FROM HIS EXPERIENCE THE DAY BEFORE AND THIS DAY THE FREQ WOULD BE UNUSABLE BELOW FL200.