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SMATV Headend buzzing/interference/hum

2937 Views 6 Replies 3 Participants Last post by  AntAltMike
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 :
2 racks
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.
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I've had the Blonder Tongue power supply cause that symptom. Since modern power suppliers are nearly all "switching" supplies, the power supply failure characteristics are no longer 60 Hertz hums. If you don't have a spare supply, you can at least swap the two supplies you are using to see if the symptom moves from one frame to the other.

If you are guessing ground loop, you don't use a filter for that, you use a ground loop isolatorr. These can be bought for about $10 or so each and are put on input and output RF coaxes.

I've never popped the cover off the BAVM 860SAWs, but in other BT products of that vintage, the filter capacitors had "inline" leads so you could just lap solder just about any size capacitor on top of it to see if it mitigates the symptoms.
That should have simultaneously caused graininess, as the signals got weaker and should have been detectable with a signal strength meter.

The weirdest similar problem I had was an outside coax that made BETTER contact when it got cold and overloaded the downstream amplifier. It would happen when the outside temperature got to maybe 20 degrees or colder but unfortunately, this was a 70 foot wire running down the outside of a 7 story building and it was on the southeast side, so the sunlight would start hitting it immediately upon sunrise and it was always back to "normal" by the time I was called out to service it.

The way we diagnosed it was that I actually stayed in the hotel and had them wake me up at 3:00 AM when a guest called in to complain.
1 - 3 of 7 Posts
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