A Captain reports radio bleed interference between VHF communication radios on their DHC8-400 aircraft interferes with a Pilot's ability to hear instructions from ATC. Bleed over problems have not been remedied and continue since the first of two Service Letters DHC8-400-SL-23-005A dating back to 05 December 2006 was issued.

Narrative

We left ZZZ in a DHC8-400 aircraft en route to ZZZ1. When asked to contact Center on Comm #1 at 1XX.75; we noticed radio bleed over occurring when we transmitted on Comm # 2 on 1YY.00. This bleed over was crystal clear. I had written up this type of discrepancy before I knew that Maintenance Control would use Service Bulletin DH8-400-SL-23-008; dated 16 OCT 2009; to return the aircraft to service. The Service Bulletin says; 'A number of Operators have reported occasional interference between the two VHF communication radios. The interference can be unwanted audio noise (open squelch) or possibly garbled speech.' This problem has been known for many years judging from the date of another Service Bulletin that addresses bleed over (DH8-400-SL-23-005A Dec 5; 2006). When a professional aviator places a discrepancy in the Logbook it isn't returned to service until a licensed Mechanic addresses the problem. My safety is twofold. First; I see a large safety concern when a problem known to the radio and aircraft manufacturers is not remedied but; is allowed to continue to interrupt ATC communications. Second; I see an even larger concern when a Mechanic will sign off a discrepancy without even looking at the discrepancy. I witnessed this at another station when a Mechanic took the Logbook and went out to his vehicle and proceeded to sign-off the discrepancy. If we are going to continue to allow our aircraft to be flown with an issue that interferes with a pilot's ability to hear instructions from ATC then we are going to have problems (violations or accidents).