I am having a problem with a 24 channel headend system. I am getting a buzzing/humming noise on a couple of channels. The noise is not constant. Usually everything is fine from sunrise until around 3:00 our time. It is currently happening on moduted channels 6 and 8. Over the past 6 months or so this issue has popped up on other channels. In the past I just replaced the modulators when I couldn't really find a source for the noise. The new modulators would then work fine. However, after 3 weeks or so, a new channel would start to get the noise. I just can't believe that one by one these modulators are just going bad. When the noise starts out there are not any "hum bars". As time passes, however, hum bars to roll up the screen on these channels rather faintly.
The buzzing is occurring at the actual modulator, not just somewhere down the distribution line. It is also audible/visible throughout the distribution, but is happening right at the source. If I connect a test TV to the test port of the modulator I can hear the buzzing.
If I disconnect the audio cable, the buzzing is still there. If I isolate a problem modulator from the distributed signal, I can still get the buzzing when my TV is connected directly to the TV.
Disconnecting the video cable gets rid of the buzzing.
I'm assuming it's some sort of ground loop or a hum but I can't seem to find it or figure out where to insert some sort of filter. I don't know if maybe I'm just picking up some RF interference from a poorly shielded cable or what is going on. I'm just out of ideas. Any help would be greatly appreciated.
The equipment is :
Dish Network 311 receivers x24
Blonder Tongue BAVM-860SAW channelized modulators x24
Two Tripp Lite PS6020 20 outlet power strips (one on each rack for the receivers)
One Tripp Lite surge protector power strip.