pn50c7000yf issue w/ cable box

hi all,
I recently bought a 50" samsung hd tv 7 series. I have comcast and I already called them to add hd service. since then I've gone through 3 hd cable boxes in the last 2 weeks and none of them seem to work- ranging from a dysfunctional hdmi to a box that constantly makes a screeching noise and channels cannot be changed. currently I have one box that doesn't make the noise but I can't seem to get the hd channels to work! When I plug the cable box in via HDMI, and I switch the source to the HDMI, it says no signal /device detected. I know the hdmi ports on the tv work becuz I've tried plugging my ps3 or blu ray player and those work.
Then, I tried plugging in the cable box with A/V cables and I get the regular non-HD channels but when i switch to the HD channels it says no HD service but I've called them to double check that the service is on on their end. Does anybody know what's the problem here and wat i can do to get this to work? thanks for your help!