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.

Can you elaborate a little bit on RF reflection into the final stage? I would like to learn what that is.

BTW I'm using a run of RG6 about 15ft long run from the RF out on the modulator into a matching transformer set on 75ohms to a pair of 16ga wires approximately 43 inches long each. Could a part of my setup be causing RF reflection? I'd love to have an antenna analyzer to fine tune my dipole, but I don't have that in my bag of tricks. Maybe I could find a ham guy here in Pittsburgh to help me out.

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.

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.

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.

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

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

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

I'm at my wits end! I've swapped antennas, swapped sources, killed 100% of the power to my entire home except source, modulator, and monitor, I've removed my cable input to the house, I've replaced the 4 main filter capacitors on the modulator power supply. I don't know where to go next other than to hard wire the modulator to all of my vintage sets (no small feat at all!!!!). There has got to be something I'm missing.

Just so we are all thinking of the same thing, is the following correct?

1) coax output of modulator -> 75/300 ohm transformer -> TV monitor 300 ohm input terminals: picture OK
2) coax output of modulator -> any kind of antenna; some kind of antenna -> TV antenna input: hum bar

Is that right?

That is correct, except you can also add coax output of the modulator to coax splitter (main splitter for my house) to coax input on a modern flat panel in the "picture ok" camp.
I've uploaded a video to youtube to show what it looks like on my test monitor https://www.youtube.com/watch?v=bBi1uZ7j0ng.

Same hum bar on any other TVs using an antenna? How do you know it's not your TV?

Should have also added coax output of modulator -> any kind of antenna; some kind of antenna -> TV antenna input on modern flat panel: hum bar
Same problem on all TVs in the home, vintage and modern alike.

Hiya John--sorry to hear of the continued woes. My humbar was different than the one on your video link. Perhaps it is not a transmission issue, but a circuit issue in the B-T. It would be interesting to see whether or not a different B-T unit would have the same fault, in the same room and set-up. Could the interference be external? (doubtful I feel)

Just had a chance to look at the video - something in a power supply somewhere is oscillating on your channel frequency. probably radiated, but might also be carried on the power line.

1) can you see it in the "snow" when your transmitter is off?
2) if you have a small portable TV with rabbit ears and can see it on that, go snooping around your house for where it is strongest.
3) you said you tried turning things off - how about light dimmers, compact fluorescent lamps, anything on the AC line with a clock display that runs even though the main function is off (like a microwave oven?).

Edit: how about your furnace?

I actually killed power to my entire house at the breaker box with the exception of the dryer's 110 outlet. To this outlet I plugged my monitor, my modulator, and my source. If there is interference from another device it is either battery powered, or at my neighbors house.

This is the worst I can get the hum bar to look. I adjusted antenna placement in transmitter and receiver and stood between the two to get it to this point for the sake of showing it on video. In my frustration I bought another modulator last night. There happened to be one on eBay for $39 including shipping. We'll see if it exhibits the same symptoms.

Let me repeat,
1) can you see it in the "snow" when your transmitter is off?
This would prove it's from something else.

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

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

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?

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?

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

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.

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

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.

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...

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.