Ground Loop "Hum" Using BT Modulator

[vts1134]

I've searched this a bit and found a few threads here and there where other folks report having the same problem, but no one reported a solution.
I am using a Blonder Tongue AM60-450 modulator with a dipole antenna to broadcast analog channel 3 throughout my home. The setup has plenty of range for my needs and both audio and video are quite clear. My problem is that I am experiencing a "ground loop", that is I have a white line that "rolls" up the screen from the bottom to the top. I have tried both an off air ATSC tuner, and a BLuray disc player for my sources and they both exhibit the same symptom. I have removed the ground from the modulator's three prong plug with an adapter with no results. I have also removed any copper connection to my cable company, also with no results (this is the culprit 99% of the time in my professional experience with "new" systems). Does any one have any ideas on what could be causing this, and what might solve it?

[jr_tech]

If you connect the output of the B-T directly to the TV, do you still see the "hum bar"?
jr

Edit add: I have observed occasional "hum" on strong shortwave signals, due to "hum modulation", perhaps something similar is occuring :

http://home.computer.net/~pritch/shortwav.htm

[Phil Nelson]

My AM60-550A modulator developed the slow-moving horizontal bar after a couple of years of continuous use. Replacing the electrolytics in the power supply cured it:

http://antiqueradio.org/HomeTVTransmitter.htm

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[vts1134]

Originally Posted by Phil Nelson

My AM60-550A modulator developed the slow-moving horizontal bar after a couple of years of continuous use. Replacing the electrolytics in the power supply cured it:

http://antiqueradio.org/HomeTVTransmitter.htm

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

This is where I was leaning, but I thought I'd ask first. How did you get that board out? It seems like it is adhered to the side of the chassis with something in addition to the three screws.

[Phil Nelson]

Originally Posted by vts1134

How did you get that board out? It seems like it is adhered to the side of the chassis with something in addition to the three screws.

I don't recall doing anything tricky to remove that board. In my photo showing the new caps the board looks like it may have four empty screw holes.

Phil Nelson

[vts1134]

The adhesive holding the board to the chassis was pretty soft. I was able to coax the it out. Now I just wait on the replacement caps.

[vts1134]

Originally Posted by jr_tech

If you connect the output of the B-T directly to the TV, do you still see the "hum bar"?
jr

I do.

[jr_tech]

If so, then I would suspect power supply caps.

jr

[vts1134]

Out of curiosity, what would you theorize if the symptom went away with a direct connection?

[jr_tech]

"Out of curiosity, what would you theorize if the symptom went away with a direct connection? "

I would suspect that something like the "hum modulation" (see post 2) was occuring... somehow the transmitted signal was being combined with 60 Hz after it leaves the transmitter. I hear this once in a while on strong Shortwave signals.

jr

[old_tv_nut]

See the section on "feeding a dipole antenna"
http://en.wikipedia.org/wiki/Dipole_antenna

I presume your matching transformer is 75 ohms (your coax side) to 300 ohms (your dipole side)? According to the above you should not be using a 75 to 300 ohm matching transformer unless you use a folded dipole.

[vts1134]

From that article it looks as if I should be using a current balun for my antenna for best results, although it states "Many people have had success in feeding a dipole directly with a coaxial cable" so maybe I'll try coupling my RG6 line directly to my dipole leads and see what I get.

[zenith2134]

Regarding RF reflection back into the transmitter: When using an impedance-matching balun( which is essentially a transmission-line transformer), not all emitted energy gets relayed to the antenna. Some gets sent backward into the final stage. This causes less output power and heats up the rf amp more. That is my understanding of it, anyway.
Remember, these agile modulators, as originally used, fed coax cable distribution networks which were a solid 75-z load.

[vts1134]

Tried a direct connection from coax to dipole leads, as well as increasing the coax feed length to 70+ feet, both to no avail.

Originally Posted by zenith2134

On one of my B-T AM-60-550 units, I had a rolling bar which was antenna-dependent. upon installation of a proper coax run and dipole antenna, the issue went away 100 percent of the time. Prior to the antenna switch, I was using an F-connector-to-300 ohm balun and I figure that's what caused it.

Can you tell me exactly what antenna setup you have? Maybe there's something small that I'm overlooking.

[zenith2134]

8 feet of RG-6 cable terminated into a Yagi antenna, with the shield mounted right the base element. It is hidden in the attic. The others have simple dipole antennae. I run 3 channels all VHF