WorldwideDX Amateur Radio Forums

With many radios, if you put one of the slow color-changing LEDs in the meter the LED actually makes an awful noise that sounds like motor gears grinding away as it changes colors.

I have thought of putting a small capacitor across the leads to see if that would filter out the noise but I don't have any extra or junk radios around to try it on and don't want to tear into either of my good radios just to make this experiment.

Has anyone here come across this and tried or done a fix for the noise?

It's worse in some radios and makes no noise in others. The noise is picked up and passed along in the audio circuits. You can both hear the noise through your speaker and the noise is transmitted out too and is very annoying.

They are never a problem when putting them in outboard power/swr meters for meter lighting and looks neat in them.

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Seems to me that the solution would depend on finding which specific circuit inside the radio is picking up the interference. Those little color-changing LEDs work by way of PWM, which means they are fed a rapidly oscillating signal, the duty cycle of which determines the brightness of each LED. That signal is what is being picked up by some part of the radio's internal circuitry. Since the source of the interference is inside the radio chassis it will likely be very difficult to get rid of. One possibility is to enclose the LED assembly itself in a metal shield.

Shielding the power lead before the circuit or a parallel of caps (.1,.01,.001 pf) on that power lead may be the solution, a cap across the leads of a diode doesn't sound like a good idea.

IDK if that would work since the PWM signal is being generated inside the LED device itself. Intercepting any AC on the device's power leads shouldn't impact the internal PWM signal, and if the interference is being radiated from the LED device's PWM circuit directly then conditioning the power wires might not have any effect.

Then the metal housing might be the answer if the led's are generating a field.

I don't think the units are spewing RF. The unwanted signals are going back into the radio through the wiring.

I think it's similar to windshield wiper motors making noise in a mobile radio which can be cured by placing a cap across the line at the wipers.

You can power one of these bulbs with a remote source and place it in the radio and they'll be no noise so that seems to rule out RF.

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Welcome to the WWRF. Plenty of Free Parking and with 5,000 posts you get egg roll

I'd agree with that logic.. the frequencies involved in PWMing a couple LED chips are not RF, so it's unlikely that they are radiating, but I've seen interference be caused by AF sources before. I think you're right though.. try Mack's suggestion and let us know how it goes.

Have you tried powering them directly off the power input socket? if you don't see the issue there then perhaps a small switching transistor is the solution.