MDT HAS FREQ ASSIGNMENT WITH POOR; SCRATCHY; BROKEN COM. RPTED BY SEVERAL ACFT. CTLR TA RECEIVED AFTER TFC PASSED. TCASII HAD INDICATED PROX AND CREW HAD VISUAL CONTACT. HAS SECOND EXPERIENCE WITH SAME SCENARIO.

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OPERATING AS A SCHEDULED CARRIER FROM JFK TO BOS; EXECUTING THE KENNEDY 6 DEP TO BPT. COM HAD BEEN SWITCHED SEVERAL MINS PRIOR FROM JFK DEP ON 135.9 TO 133.1. RECEPTION ON 133.1 WAS SCRATCHY AND BROKEN AS RPTED BY OTHER ACFT AS WELL. WHILE PASSING 8800 FT ON THE BPT 228/19; CREW OBSERVED A POTENTIAL CONFLICTING TARGET ON TCASII AND VISUALLY LOCATED THE TFC. WITHIN APPROX 10 SECONDS; THE CREW RECEIVED AN RA COMMAND; BUT SINCE TFC WAS IN VISUAL CONTACT; DID NOT DEVIATE FROM CLB CLRNC. SHORTLY AFTER A 'MONITOR VERT SPD' COMMAND HAD BEEN GIVEN BY THE TCASII; THE VOICE OF THE ATC CTLR WAS HEARD CALLING OUT THE TFC WHICH HAD JUST PASSED. THE CTLR'S XMISSION WAS WEAK BUT READABLE. WITHIN 5 MINS; NOW LEVEL AT 9000 FT ON THE BPT 228/3 AND STILL COMMUNICATING WITH ATC ON 133.1; THE CREW VISUALLY LOCATED THE TFC. THE CREW RECEIVED ANOTHER RA COMMAND BUT DID NOT DEVIATE FROM ASSIGNED ALT BECAUSE THE TFC WAS IN VISUAL CONTACT. THE DEP CTLR WAS HINDERED IN HIS NORMAL DUTIES OF SEPARATING TFC BY A LIMITED ABILITY TO COM WITH THE PLTS INVOLVED. PARTIAL RADIO FAILURE APPEARS TO BE THE PRIMARY CAUSE OF A POTENTIALLY DANGEROUS SITUATION. PROPER OP AND USE OF TCASII ASSISTED AIRCREWS IN OVERCOMING THE COM LIMITATIONS OF THE CTLR. VMC WX ALLOWED ALL ACFT TO SEE AND AVOID WHILE UNDER ATC CTL.