Interference on Intercom

[ayed]

My home Nutone intercom works fine but as we "Go Green" with the new replacement flourescent bulbs, I now get a faint but annoying Buzz on the intercom. Nutone doesn't offer a solution ... anyone have an idea? Seems like there ought to be a filter or something I could use somewhere.
Ed

[HayZee518]

true incandescent bulbs didn't need a transformer to operate. these new fangled bulbs have built in transformers. besides they're manufactured in taiwan and china - answer your question?? I'm also thinking that nutone uses a simple power supply in their units - they have to change AC to DC to operate and the filters are just a capacitor type called a pi filter.

[ayed]

Thanks

Sounds like there's not much I can do. Nutone says even their newer units have the same problem, that the nature of the interference makes it impossible to deal with except at the source. Guess I need to go to China?

Thanks
Ed

[HayZee518]

You could use an oscilloscope to measure the ac hum - i.e. figure out the frequency and make a filter for that frequency. But that is not to say the interference frequency is gonna stay there - it probably is continually varying. I'l go look into ac hum filters and get back to you.

[LazyPup]

There is a very cheap and easy solution to this problem.

The audio signal produced by the intercom is a pure DC signal while the interference is a low frequency AC signal.

A capacitor can pass AC but it will not pass DC, therefore to filter the interference out you shunt (intentionally short) across the two speaker terminals with a 220pf capacitor. In this manner the offending AC signal will be passed across the capacitor to ground before it enters the speaker, while the desired DC audio signal cannot pass through the capacitor and it will continue to the speaker as desired.

You would need to install a capacitor on the speaker at each terminal that is being interfered with.

A 220pf capacitor is about 3/8" diameter and 1/16" thick with two protruding wire leads. The leads are not polarized so it doesn't matter which lead you get on which terminal, just so long as you attach one lead to each of the speaker terminals. Once the connection is made there is no other action that needs to be taken.

Any local Radio & TV repair shop should be able to supply you 220pf capacitors for about $.10 ea.

You can either simply wrap the leads around a speaker screw terminal or for a more permanent solution you can solder them to the speaker terminals. Just be sure that you use "Rosen Core Solder" not "acid core".

Most high end electronics and entertainment equipment will already have the capacitors installed from the factory.