Transmitters

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Transmitters

Post by mikeSR on Sun Dec 19, 2010 3:06 pm

Hi. I'm new to the forum and glad to be part of it. I have a small transmitter I've had for years. It powerful enough for my small house. Trouble is, there is a hum if I move it out of the room away from the receiver. Any tips on removing the hum.

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Re: Transmitters

Post by Resistance is Futile on Sun Dec 19, 2010 3:17 pm

Check house wiring plugs. You may have to check that house is wired per code, or if hot and neutral are reversed. Also check if the grounding wire is actually grounded. Does the radio reciever have no hum on other real broadcast stations? Or just on your transmitter?
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Re: Transmitters

Post by mikeSR on Sun Dec 19, 2010 9:27 pm

R.I.F. Thanks for the reply! I've tested it on several radios, including new ones. My goal is to get it to play properly in my vintage tube radios of course. The radios tested on have no hum on other close stations. The transmitter loses its hum if put very close to the radio.

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Re: Transmitters

Post by Resistance is Futile on Mon Dec 20, 2010 2:42 pm

This is a difficult problem, not being there and seeing what you are doing. There may be ground loops. What I mean to say is the audio cable from your source may not be grounded or shielded adequately.

Some audio cables use foil as a shield and may be connected only on one of the jacks, or plugs, and have a poor connection From flexing, or corrosion. Do a resistance check on the cable alone, not hooked up. Then do a resistance check from the case of the transmitter to the shield on the audio cable. Make sure neither is connected to a power source for safety reasons.

Its funny that you get no hum whilst close to your receiver. Have you a battery radio that does the same thing? Try running it on just the batteries then alternating it to run on a power cord. Is there a difference? If it occurs only on AC power, then ground loops are a strong possibility.
Does the transmitter have a 3 wire plug on it?
If so then try reversing the power plug on your receiver, assuming that it is a two wire AC plug on tube radio. Does that make a difference?

What brand of Transmitter do you have? Was it a kit?
Does the hum make a difference when you move your receiver to another room?

Another possibility is Fluorescent lights, if they are running, they are a prime suspect, as they are closer than the transmitter and have a stronger field, but when you bring your transmitter into the room you then have a closer (stronger) signal from the transmitter, then the signal from the fluorescent lights.
Hope this help you to investigate and find a solution.
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Re: Transmitters

Post by mikeSR on Mon Dec 20, 2010 10:38 pm

I'm going to take these suggestions and work on them one at a time. In the meantime I uploaded two pics of the kit and you can view it here http://picasaweb.google.com/coatesaddress/TransmitterKits?feat=directlink

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Re: Transmitters

Post by Resistance is Futile on Mon Dec 20, 2010 11:03 pm

That is a nice looking layout. A schematic would help, however just looking at the picture, you should replace the audio input leads with shielded and the shielding should be grounded. I would also put a piece of steel sheet between the power supply and tube circuitry.

That is a very neat wiring project.
Keep the audio and RF leads away from the power leads and transformer. Both sets of leads should be shielded and separated from each other to reduce AC-RF interplay and induction. RCA audio cable is my suggestion inside the cabinet and connector to the audio source.

I'm still thinking that your hum source is close to the radio and overpowering the received transmitter signal. It could be a wall wart, phone power supply (mobile phone) or Cell phone power supply.
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Re: Transmitters

Post by mikeSR on Mon Dec 20, 2010 11:21 pm

R.I.F. Well, that gives me things to consider. I will rewire the audio input leads and ground the shield. I'll also steer away from transformer and put a steel sheet around transformer. As you say, the problem could be one of many things, so I'm going to go over the list of possibilities you suggested one at a time. Thanks very much for your input!!!

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Re: Transmitters

Post by Resistance is Futile on Tue Dec 21, 2010 2:11 am

Pleaselet us know what your results are, and what you did to address each. Wink
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Re: Transmitters

Post by Bill Cahill on Tue Dec 21, 2010 2:26 am

CFL's will also cause that problem along with television, and, computers.
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Re: Transmitters

Post by Bill Cahill on Fri Aug 17, 2012 4:55 am

Anything new on it? How did shielded wirring work? It should help.
Cliff was right.
Good luck.
I used to have a low power transmitting set up at my house. Still have oscilator, and, it still works.
It's a legal Motorola am oscilator from the '40's. Changer was toast, so, I rehoused it.
Haven't used it in years.
But, they sure are fun.
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Re: Transmitters

Post by mikeSR on Fri Aug 17, 2012 1:53 pm

Ended up buying an Sstran AMT-3000 which works well with no hum. The original is still down for a recap, but it's still low on the totem pole for now. Lots of other vintage radio work to do. Thanks for asking.

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Re: Transmitters

Post by Bill Cahill on Fri Aug 17, 2012 1:57 pm

No problem. Please keep in touch.
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