Ground Loop "Hum" Using BT Modulator

[jr_tech]

This is really a WAG, but I see diagonal lines in the "hum bars". Counting the lines as best I could, there are about 70 or so across the screen. 70 times 15750 (the horiz line rate) gives about 1.2 Mhz. Perhaps this is an indication of another strong signal, like a radio station contributing to this mess?
jr

[lnx64]

My same model Blondertongue also needs new caps. Mine are buldging actually.

[N8NM]

That looks a lot like an interfering over-the-air signal. I'm not familiar with the BT devices; is it possible to change it's channel and see if anything changes?

[vts1134]

I can't really tell if the problem persists with the transmitter off, I don't see the hum bars in the static, but I'm not sure I would see them if they were there. I did change the broadcast channel from 3 to 2 and the symptom disappeared. It will be interesting to see if the new modulator has the problem with channel 3 or not. Does any one know if you can get these modulators to broadcast on channel 1?

[N8NM]

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

[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.

[vts1134]

Originally Posted by N8NM

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

It is. No analog stations left though so I simply have it for interwebs.

[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...

[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.

[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.

[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