Hi Tim,
The doorbell module interface may not be the actual RFI victim in
this case. But it may very well be the thing that improves the
coupling of the RF to the real victim enough to generate the
problem. In a case like this I would suggest common mode chokes
on all interface wiring to the Nutone Main Unit. These chokes
should be on _ALL_ of the wires going into and coming out of the
unit. Power, speakers, aux inputs, etc. 0.01 or 0.005 uF caps
across all the remote speaker I/O terminals may help but will
probably not be necessary if you can stop the unit from being a
high current RF node. A 0.01 uF to chassis from each AC power
lead (right at or inside the unit) in conjunction with the common
mode powerline choke may also be helpful. Be sure to use caps
rated for use across the AC line.
73, Eric N7CL
>From: "Tim Taylor" <tim-taylor@home.com>
To: <rfi@contesting.com>
>Date: Fri, 7 Dec 2001 19:11:43 -0500
>
>I need some help. I've been trying to solve an interference
>problem with my neighbors Nutone Intercom for some time now. He
>has an extensive system with 19 speakers. At first, I thought
>the RF was coming in through the speaker leads. I investigated
>what it might take to eliminate that. By chance, during a test,
>I removed the doorbell module, and the RFI stopped. The
>doorbell module connects to the doorbell with a two conductor
>cable. One side is ground and the other side is the input to
>the module. I have tried the following:
>
>.01 uF cap across doorbell - no effect
>
>.01 uF cap across input terminals to doorbell module - no effect
>
>put 10 ferrite beads on each lead at doorbell module - some
>reduction, but still audible
>
>I also noticed that both the ground and input leads must be
>disconnected to eliminate the RFI. If either wire is left
>connected, the RFI is present. I tried a clip lead between the
>doorbell module ground wire and the intercom chassis - no
>effect.
>
>I have hope in that it appears that the RF is not coming in
>through the speakers. I have isolated it down to two wire.
>This should be simple.
>
>Can anybody help?
>
>Thanks,
>
>Tim K8XS
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