Ground Loop "Hum" Using BT Modulator
 

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?

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

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.

I do.

If so, then I would suspect power supply caps.

jr

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

"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

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

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.

Swapped 4 new filter capacitors into the modulator today, no dice :dammit:. I'm going to go looking for some external causes in the environment, dimmers, motors, etc. I was also thinking of taking the modulator to work with me and seeing if the problem persists.

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.

Did you try unplugging (not just turning off), one by one, every appliance and electronic device in your domicile? Often this can identify the source of the hum.
But if you live in an apartment with adjoining units, this won't help if the source is in another unit.

I did unplug the other items on the same circuit as the modulator, source unit, and test monitor to no avail. This is a really great suggestion though. I remember going on a service call for a CD player and amplifier that had static noise in the background. I never heard static quite like this before and after swapping out the CD player and getting the same results I started unplugging things. It turns out the problem was this machine that "created ozone for therapeutic purposes". It looked like a quack device to me (this was a boutique beauty salon). The machine created this interference even when it was turned off! It was a great learning experience for me and I remember it every time I have a problem (work or hobby).

This has to be the problem. When I connected the modulator directly to the TV before I was simply paralleling off of the antenna directly to the TV, which as I said before didn't solve the symptom. This morning I took the antenna off of the modulator all together and did a straight connection from the RF output on the modulator into a matching transformer to my test monitor. Absolutely clean signal with no hum bar :banana:. I will experiment with different antenna setups to see what configuration gives me hum free operation.

I haven't had my agile modulators apart, but I reckon that the hum-bar is some type of RF reflection back into the final stage, which certainly should be avoided. If these were higher power transmitters, any reflection could theoretically smoke the final RF stage.
However I am not certain that this is what's happening.