ZSU Controller described a failed attempt to issue a descent clearance noting the frequency utilized has had problems for an extended amount of time.

Narrative

Shortly after I assumed the position I noticed that the previous controller's instruction for a B737 to expedite descent was not working as planned. I issued a turn to the descending B737 aircraft and called traffic with no reply from the B737. We have bad radios in the area; so I watch a second to see if he took the turn. I called a total of three times to the B737 before he took the turn. I had also called traffic to the C130 and the pilot reported traffic in sight. The projected path initially looked close but appeared separation would not be lost. I wanted the turn for extra comfort but as it played out it ended to be right at the minimums for separation as they passed. We have had frequency problems on 135.7 for an extended amount of time; but I'm not sure if I was blocked; pilot was not listening; or it was a blind spot. I believe if the B737 had taken the turn when I initialized it I would have had plenty of space. Proper frequencies would be great. We have been reporting those down for years. Maintenance seems to reset it; someone does a brief frequency check and it's called back up. The expedite descent clearance was not necessary in the first place. I should have just stopped descent vertically as soon as I sat down.