Ground Loop "Hum" Using BT Modulator

[cbenham]

Originally Posted by N8NM

That definitely sounds like a local interference problem. By any chance, is your house connected to cable TV?

AND, by chance is there a local *digital* channel 3 broadcasting within 15 miles of your house? If I missed this in someone else's previous post I'm sorry. I read the entire thread.

[Electronic M]

With the FCC selling of TV spectrum I wonder how long it will be until it becomes hard to find an unoccupied former analog TV channel to run our home transmitters on?

I need to try a different channel on my BT as I was also getting a hum bar that was not caused by caps...Or at least not caused by the big 2200uF as putting another in parallel did nothing.

[N8NM]

You might try disconnecting it where it enters the house and see if you get any improvement. Loose/bad cable connections can allow signals to leak, and these wouldn't have gone away when you cut power at your breaker panel.

It's kind of a long shot, but I've seen it before.

[zenith2134]

Why are you attempting to broadcast on the lowest channel possible (channel 1)? AFAIK, the B.T.s cannot do broadcast at over the air 1...Even if they could, I would choose the highest channel allocation for the band..since any power-supply oscillations/anamolies would be furthest away from modulated signal space at a higher channel. I like 12 and 10 personally.
I do not care about other channel's digital transmit realm at all.......We are dealing with low power. it is an FCC "intentional RF radiator", true. But so is my garage door opener and my 802.11 N router...

[vts1134]

Originally Posted by N8NM

You might try disconnecting it where it enters the house and see if you get any improvement. Loose/bad cable connections can allow signals to leak, and these wouldn't have gone away when you cut power at your breaker panel.

It's kind of a long shot, but I've seen it before.

That was the first thing I tried. I run into this ground loop problem all the time at work, so is it was the first thing I thought of.

Originally Posted by zenith2134

Why are you attempting to broadcast on the lowest channel possible (channel 1)? AFAIK, the B.T.s cannot do broadcast at over the air 1...Even if they could, I would choose the highest channel allocation for the band..since any power-supply oscillations/anamolies would be furthest away from modulated signal space at a higher channel. I like 12 and 10 personally.
I do not care about other channel's digital transmit realm at all.......We are dealing with low power. it is an FCC "intentional RF radiator", true. But so is my garage door opener and my 802.11 N router...

Some of my sets only have three channel tuners so I have to use 1, 2, or 3.

[zenith2134]

Electronic M, do you have any factual evidence to confirm the statement regarding 'better DX ability' of lower frequency channels? It seems to me that the opposite would be true.

[ChrisW6ATV]

I did not see this specifically mentioned in this topic, so here is one thing that may have worked with this problem for me: Try plugging the modulator into one of those "grounding adapters" that have only two prongs, to eliminate the ground connection from the modulator to the building. Then, use a DVD player or VCR (also with its two-prong cord) as the signal source. (Do not have the VCR connected to any cable box or antenna system, just standing by itself.) Let me know if you tried this already.

[vts1134]

Originally Posted by ChrisW6ATV

I did not see this specifically mentioned in this topic, so here is one thing that may have worked with this problem for me: Try plugging the modulator into one of those "grounding adapters" that have only two prongs, to eliminate the ground connection from the modulator to the building. Then, use a DVD player or VCR (also with its two-prong cord) as the signal source. (Do not have the VCR connected to any cable box or antenna system, just standing by itself.) Let me know if you tried this already.

I tried this as well.

[Electronic M]

The lower channels have better DX abilities given fixed output power on a broadcast scale so logic would state that they preform better on a low power scale too.

[Electronic M]

I've read it on TV DXing sites that the lower channels/frequencies tend to want to skip like Shortwave radio signals where as high band VHF and UHF are more of line of sight propagation. My sets are all over the house so something that is more line of sight would probably be attenuated by walls and floors.

[zenith2134]

Interesting! I never had much luck with TV DX, but have received FM DX in a big way.
When I set up my first modulator, I tried finding a 'signal propagation by frequency' chart on the net and couldn't find much info on the subject.

[vts1134]

I received my new modulator and it too has the same symptom when broadcasting on channel 3, so I gave up the dream and hardwired all of my sets. On the plus side they look FANTASTIC with a hardwired signal. I have most of them completed now, and will hopefully wire the remaining sets this weekend. I am not quite done with my modulators though . I started a new thread that might be interesting here http://videokarma.org/showthread.php?t=260846

[old_tv_nut]

Originally Posted by vts1134

I received my new modulator and it too has the same symptom when broadcasting on channel 3, so I gave up the dream and hardwired all of my sets. On the plus side they look FANTASTIC with a hardwired signal. I have most of them completed now, and will hopefully wire the remaining sets this weekend. I am not quite done with my modulators though . I started a new thread that might be interesting here http://videokarma.org/showthread.php?t=260846

This still seems to be something like a switch-mode power supply that radiates in your channel. Really, you should take a portable TV and go around your place to see where it is strongest. You may be able to locate it and fix it. Worth the effort if you don't have to hardwire more sets in the future.

[vts1134]

Originally Posted by old_tv_nut

This still seems to be something like a switch-mode power supply that radiates in your channel. Really, you should take a portable TV and go around your place to see where it is strongest. You may be able to locate it and fix it. Worth the effort if you don't have to hardwire more sets in the future.

This would only be possible if it were A: battery powered, or B: in a neighbor's home as I have killed power to every last thing in my house except modulator, source, and monitor with no change.

[jr_tech]

Have you tried any ferrite filters on the line cord or the coax output of the B-T? They work well for keeping undesired signals out of equipment.
http://www.amazon.com/Ferrite-Core-C...ferrite+filter
not affiliated,
jr