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Curing Interference to Non-Radio Equipment

The ARRL Laboratory staff fields phone calls and letters about interference problems. Those calls about “non-radio” equipment can be complex and the solutions are sometimes elusive. In addition to the personal diplomacy involved, you and your neighbors must understand the regulatory and technical aspects of the problem and work together towards a solution.  The Audio Interference FAQ Page should help by answering many of your questions.

The Audio Interference FAQ Page

Q: My neighbor bought a new stereo system which “hears” me on 20-meters.  My transmitter is picked up by her new stereo. She says it's my fault because the stereo system is brand new.  I want to help, but need to understand more. Can you help?  

A: This is a classic case.  Since you are interfering with a non-radio device, your neighbor’s stereo is acting as a radio receiver through no fault of yours.  Telephones, stereos, computers, and other in-home devices can receive interference from nearby radio transmitters. When this happens, the device can improperly function as a radio receiver. Proper shielding or filtering can eliminate such interference.  However, if your transmitter is not putting out illegal spurious signals, the problem is almost certainly not caused by your harmonics.

Q: I guess the law has let me off the hook?  

A: As far as the technical and regulatory issues are concerned, you’re in the clear. But, in the interest of diplomacy, offering a helping hand as a public service by applying your technical skills, or working with the ARRL to help solve the problem is always encouraged.

Q: I browsed the ARRL RFI page with information for neighbors of hams.  My neighbor is ready to work with me toward a solution. Where do we begin?

A: First, do not attempt to repair your neighbor's equipment. While this problem can often be fixed with proper internal filtering and shielding, this is something best done by the equipment manufacturer. If you dismantle a new stereo system, you’re setting yourself up for liabilities. You will probably void the warranty and assume responsibility for anything that goes wrong with that system. You may also be breaking the law. Most states have laws that prohibit you from working on your neighbor's electronic equipment (even for free) unless you hold a valid state service license. Your ham license is not a substitute for an otherwise required license.

However, there are things you can do for your neighbor.  For example, you can help your neighbor contact the manufacturer of the stereo system. You may discover that the manufacturer already has fixes available or will do the work under warranty. You can help the manufacturer's representative understand the technical issues involved. Manufacturers are usually willing to assume their responsibilities fairly, but they do not always understand the fundamental causes of the problem, nor all the solutions.

Q: The manufacturer has agreed to send a service person to investigate the problem. I’d like to be helpful. I've read the "ARRL RFI Web Pages" and learned some troubleshooting techniques. What's the best way to start?  

A: First, you should make your station available for testing when the service person arrives. You need to be there to put the radio on the air, and to make sure the “cures” actually worked. Be helpful, and try to simplify the problem. For example, the service technician can disconnect all inputs to the stereo system, one by one, and see if the interference suddenly goes away.  Try disconnecting portable units from line power as well.  These simple tests might be key in learning where to apply any cure.

Q: We disconnected everything, including the long audio cable from the stereo TV. The interference is still there.  

A: Long cables can be suspect since they make a good long-wire antenna and pick up lots of RF. Components don't pick up RF energy—their wires do! Take a look at the wires that are connected to the amplifier. 

Speaker wires are often 8 to 16 feet long. When you put two of them together, you make an efficient receiving antenna. Try bundling the speaker wires to reduce their effectiveness as an antenna. This procedure has been known to eliminate the interference all by itself.

Q: We moved the speakers and tried bundling the wire. It helped a little, but not enough. "CQ DX" is still interspersed with my neighbor's favorite songs. What's next? 

A: You're on the right track. Disconnect the speaker wires altogether and plug in a set of headphones. What happens?

Q: The interference is gone! What now? 

A: Well, you've just learned that the RF is being picked up on the speaker wires and being conducted into the amplifier. (The output transistors are possibly rectifying the RF into audio, and the amplifier's internal negative feedback circuitry is conducting it back to the high-gain stages of the amplifier). Let's start with common-mode chokes on each speaker wire pair.

Q: What is a common-mode choke? 

A: For a detailed explanation, you can refer to the ARRL Handbook's section on controlling RFI, and the section of the ARRL web site on EMI/RFI Products and Other Resources.  You can build two of them, one for each speaker output. Wrap ten to fifteen turns of speaker wire onto an FT-140-43 ferrite core.  Use an FT-240-43 if the speaker wires are large, and use "73" material for interference from 80 or 160-meter signals. Alternately, type "31" material is an excellent general purpose choice for HF.  Install them right at the amplifier. If the system uses amplified speakers, you should install one at each speaker, too.

Q: I read an old QST article that recommended placing a 0.01 microfarad capacitor across the speaker terminals. Wouldn't that be a lot easier? 

A: Don't do it! That was good advice when amplifiers used tubes. Many modern solid-state amplifiers don't work well into capacitive loads and may break into a full-power (sometimes ultrasonic) oscillation. This can result in the near-instantaneous destruction of the output module or transistors. And you thought you had a diplomacy problem before you blow up her stereo!

Q: I installed the speaker-lead chokes, and they worked! However, as soon as I hook up the long cable to the TV set, the interference returned. Should I put chokes on that cable, too?

A: Yes. Many interference problems have multiple causes. This is why you want to break the problem down into smaller pieces. Try a ferrite common-mode choke at one or both ends of the long cable.

Q: The chokes helped somewhat. What else can I try?

A: Now we are getting into the more complicated cures. You may need to use an L/C filter in the input lines or purchase a commercially available ac line filter, a couple sources are located below:

These companies sell a broad line of interference-reduction products, you can contact them for more information.

Q: We're going to order the filter. In the meantime, my neighbors can’t use their stereo TV connection. Am I going to have similar problems if one of my neighbors installs an intercom or alarm system?

A: You might. Many consumer devices are not designed with EMC in mind. You can apply what you’ve learned to any non-radio interference problem. You can even use those 0.01 microfarad capacitors, putting them across terminals (or from both terminals to ground) of simple alarm systems, or across input connections to intercoms. You can also install some common-mode chokes on long wire runs. Some think a single choke for both wires in a twisted pair works best; others have reported that separate chokes for each wire cured the interference.


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