CHANGING TO THE WRONG DEP CTL FREQ AFTER TKOF ALMOST CAUSES LOSS OF SEPARATION.

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FLT XYZA WAS GIVEN A CLRNC OF '...R TURN IMMEDIATELY AFTER TKOF DIRECT TO VNY VOR; CLB AND MAINTAIN 13000 FT; CONTACT DEP ON 124.6.' DURING ENG START; BUR GND CALLED WITH AMENDED CLRNC THAT READ '...DEP RADAR IS BACK UP; NOW CLRED VNY-7 DEP GAVE TRANSITION MAINTAIN 13000 FT; CONTACT DEP ON 124.6.' AS I WAS READING BACK THE CLRNC; THE CAPT WAS ENTERING THE NEW FREQ. THE VNY-7 DEP WAS THEN PROGRAMMED INTO THE FMS. AFTER TKOF; TWR CLRED THE FLT TO 'SWITCH TO DEP.' THE SWITCH WAS MADE; THE FO CALLED 'ACR XYZA PASSING 2XXX FT FOR 13000 FT.' A GARBLED RESPONSE WAS HEARD THAT INCLUDED THE NUMBERS XYZA. I ASSUMED THIS WAS DEP CTL AND TURNED UP THE RADIO VOLUME BECAUSE OF THE WEAK XMISSION. AFTER AN UNCOMFORTABLE DELAY WITH NO VECTORS TOWARDS VNY AND PASSING 8000 FT ON A HDG OF 210 DEGS; I DOUBLE-CHKED THE FREQ AND DISCOVERED THE CAPT HAD TUNED IN 120.6 VERSUS 124.6. (I HAD FAILED TO DOUBLE-CHK HIM ON THE GND.) WE SAW 2 ACFT ON OUR NOSE (LAX SADDE4 ARRS) ABOUT 8-10 NM AND KNEW WE HAD A CONFLICT GENERATING. AS THE CORRECT FREQ WAS TUNED IN; DEP CTL WAS CALLING FOR AN IMMEDIATE TURN TO 360 DEGS; LEVEL OFF AT 9000 FT. WE COMPLIED; BUT SAW THAT THE WDB AT 10000 FT WAS BEING TURNED AWAY FROM US. CLOSEST POINT OF APCH WAS ESTIMATED 6-7 NM OR GREATER. WE KNEW SOMETHING WAS AMISS BUT IT TOOK 30 SECONDS OR SO TO FIGURE OUT WE WERE ON THE WRONG FREQ. THE GARBLED RESPONSE WE GOT ON 120.6 WAS MISLEADING. WE HAD THE CONFLICTING TFC IN SIGHT BUT STILL CAUSED LAX APCH TO TURN THEM AWAY. BUR DEP HAD WARNED LAX APCH OF 'THE RUNAWAY' CONDITION. THE VNY-7 DEP CALLS FOR AN INITIAL HDG OF 210 DEGS AFTER TKOF; EXPECT RADAR VECTORS TO VNY.