Front Door Intercom Picks Up Local AM Station
Dave Shingledecker
house. It has the feature where any telephone in the house can talk to
the person at the front door by dialing 22. There is obviously a
speaker/microphone installed at the front door.
When I dial the front door, I hear a local AM radio station very clearly
and loud. This isn't heard at the front door. It makes communications
with the front door impossible.
I disconnected the wires at the phone system, and the radio station went
away. I took the speaker/microphone to the phone system closet and
installed it so that there is only 3 feet of wire between the
speaker/microphone and the system, everything worked fine, no radio IF.
Obviously the local radio station is getting into the system through the
cable run from the phone system to the front door. The cabling was built
into the walls and very very hard to replace with shielded cable.
It has a 2 wire run to the front door. I tried the following; I took a
1000pF ceramic capacitor and ran it from earth ground to each of the
SPKR/MIC connections in the phone system. From the "-" connection to
ground, there was no change. When I ran it from the "+" connector to
ground I got a change. The radio station was knocked down in volume, a
new approx 5Hz noise appeared, but I could at least carry on a
conversation now with the front door. The front door can hear me fine,
but I can barely hear the front door and the radio station is still there
as is the new 5Hz noise.
This can't be new or unique problem for you installers (and others). I
need some sort of filter to add to the the front door SPKR/MIC circuit to
kill the local radio station. I tried the capacitor because I beleived it
would shunt the RF energy to ground, I think I was partially correct.
I need help and suggestions as to what to do next. (Before the wife kills
me, she really wants the intercom to the front door).
Oh yea, the radio station is 1070 on the AM dial.
Thanks in advance for any help you can provide.
Goober
approx 150 foot run of cable outdoors along a fence getting to the remote
station. We replaced existing cabling and installed shielded cable. Grounded
at both ends. This cured the problem.
oshawasecurity.com
that has shielding will cut down on many of the troubles with this
type of problem.
As well the wire should be twisted not run like most alarm wiring. The
wire itself should be grounded as well to provide the best RF
protection.
I have a special wire I use for most installations of intercom systems
with the DSC 5800 Power 832 board. It has made the difference but the
wire runs are much more difficult to run.
You might find that the wire on the intercom is acting as an antenna
right now. Using shielded wire will cut this down.
On Sun, 24 Jan 1999 09:35:51 -0800, dks...@earthlink.net (Dave
Shingledecker) wrote:
]I need help and suggestions as to what to do next. (Before the wife
Brian Hess
Otherwise there's the potential for a ground loop?
The only EMI reason to ground both ends is if you think the screen has a
split in it somewhere. That's a reason to purchase screened with a "drain
wire" which is in contact with the screen, but is what you actually wire to
the ground, rather than playing with that screening foil.
--Brian
James Brown
(insulate it). Otherwise if it makes contact with the conduit, it could
act like an antenna.
You do have it in conduit don't you, if it's outside along a fence like
you mentioned?
My .02 cents.
JB
Dave Shingledecker
Don't know where you got the idea that it is outside along a fence. It is
inside the walls of my home. It isn't in conduit and it isn't shielded
wire. The installer used regular 3 pair telephone wire for the run.
For the mess I'm in, I'm looking for a solution that doesn't require me to
rewire the run. Looking for a filter or something to add to the line. If
I have to rewire it, I'll keep your post in mind.
Thanks,
Dave
In article <36ABA0BA...@Brownfamily.com>,
Dave Shingledecker
when he wired the front door. Not CAT 3 or CAT 5 and nothing shielded
The wire is definetly acting as an antenna. I've been trying to jerry rig
a low pass filter on the wire that will effectively block the RF signal.
Any suggestions besides rewiring the run??
Thanks,
Dave
James Brown
installation where wire was run along a fence outside, That is what I was
responding to.
If you're having that problem in a home, I'd rerun the wire, it make take a
couple of hours, but at least you'll rest assured knowing it was done right.
JB
Dave Shingledecker
Here is the original post. The run to the front door is 3 pair telephone cable. 1 pair for the intercom and the other 2 used
by the door bell (double up for the current). The frequency is 1070 on the AM dial? Got a schematic of a simple LC filter??
original post:
I recently installed a Nutone PTX-1200 residential telephone system in my house. It has the feature where any telephone in the
house can talk to the person at the front door by dialing 22. There is obviously a speaker/microphone installed at the front
door.
When I dial the front door, I hear a local AM radio station very clearly and loud. This isn't heard at the front door. It
makes communications with the front door impossible.
I disconnected the wires at the phone system, and the radio station went away. I took the speaker/microphone to the phone
system closet and installed it so that there is only 3 feet of wire between the speaker/microphone and the system, everything
worked fine, no radio IF.
Obviously the local radio station is getting into the system through the cable run from the phone system to the front door. The
cabling was built into the walls and very very hard to replace with shielded cable.
It has a 2 wire run to the front door. I tried the following; I took a 1000pF ceramic capacitor and ran it from earth ground to
each of the SPKR/MIC connections in the phone system. From the "-" connection to ground, there was no change. When I ran it
from the "+" connector to ground I got a change. The radio station was knocked down in volume, a new approx 5Hz noise appeared,
but I could at least carry on a conversation now with the front door. The front door can hear me fine, but I can barely hear
the front door and the radio station is still there as is the new 5Hz noise.
This can't be new or unique problem for you installers (and others). I need some sort of filter to add to the the front door
SPKR/MIC circuit to kill the local radio station. I tried the capacitor because I beleived it would shunt the RF energy to
ground, I think I was partially correct.
I need help and suggestions as to what to do next. (Before the wife kills me, she really wants the intercom to the front door).
Oh yea, the radio station is 1070 on the AM dial.
Thanks in advance for any help you can provide.
Bill Unruh wrote:
> In <dkshing-2401...@pool021-max5.ds18-ca-us.dialup.earthlink.net> dks...@earthlink.net (Dave Shingledecker) writes:
> >JB,
>
> >Don't know where you got the idea that it is outside along a fence. It is
> >inside the walls of my home. It isn't in conduit and it isn't shielded
> >wire. The installer used regular 3 pair telephone wire for the run.
>
> I didn;t see the original post so am not sure of the situation.
> How many wires are actually used by
> the intercom itself? Just 2 or more? Youcould try using one of the wires
> as a "shield", grounding it at one end (only) as otherwise it could form
> a loop antenna (in fact that may be what is happening now). Make sure
> that the signal and the signal ground are both on a single twisted pair.
> (Note, standard telephone wire is 2 pair, not 3. Does 3 exist? I thought
> it was usually either 2 or 4 pairs (4 or 8 wires)), or if not twisted (
> the stuff I have here is not twisted- it is 4 wires in a flatish cable)
> use adjacent wires for the signal and signal ground. Use the other two
> as ground wires (grounded at one end only). You could try wrapping the
> wire through a ferrite core at the inside end.
>
> You could find out the frequency of the station, and make a trap for
> that frequency-- Ie, and LC "short" tuned to that frequency from the
> live wire to ground.
Bill Unruh
James F. Cooper
Have you tried disconnecting the door bell circuit at the AC transformer?
You may be inducing the RF through the AC circuit. You can also rewire the
door bell to a single pair and interface the door bell transformer via a
low current relay. Use the new spare pair and split the audio circuit
between the existing pair and the new spare pair. Take the additional
conductor in each pair and tie same to ground at the master station side.
This will provide a "shield" via the twisted conductor to ground.
Additionally, you will isolate the audio pairs into separate twisted
circuits.
CONDUCTORS
1/2 Door Bell
3/4 Ground / Audio
5/6 Ground / Audio
John
Have the wife call the Cable TV Co out to put in a RF filter prior to
entering your house.
--
John Syme
Dave Shingledecker
Good suggestion! I like it. Will try it this week. Gotta take an hour
off work to go get the low current relay.
Thanks,
Dave
In article <01be483f$1e16c1e0$9b6a4e0c@jcooper>, "James F. Cooper"
Sarah Ann Thayer
had a similar problem with a phone system which picked up an AM station LOUD
when intercoming a certain station. Found that the problem went away when we
mounted the phone jack away from the metal conduit and box.
Gary McAuliffe
Suggest you upsize the grounds related to the problem area, by doubling up
on spare conductors on the earth side, check the integrity of the main
grounds for the whole house (power, tel, catv and your systems all
referenced to the same point) and also add a local ground at the offending
area (assuming you can tie it to the same potential as the main house ground
by copper water pipe, driven rod, plane or plate AND within NEC guidelines).
Check also integrity of power wiring grounds to the individual receptacles
that power the offending systems. All the consumer gear is unbalanced, and
therefore suceptible to ground loops, differences in "earth potential", and
actual diode action from loops and corroded connections - even on the power
receptacle ground pin. As the length of the wire (read capacitive
reactance) increases the problem will be more pronounced. I've seen RF
faults like yours ( I was a radio station CE who fielded these complaints
for community relations purposes). The capacitor / filter voodoo never
works, after all a tuned circuit has to have a theoretical zero Z at some
point in the network to work.
In summary, don't forget mother earth (or neutral , or hot)
Dave Shingledecker wrote in message ...
Dave Shingledecker
Solution was:
1) Of the 3 pair wire to the intercom, I used 1 pair for the door bell, 1
pair for the mic/spkr, and the 3rd pair for a ground plane. This helped
only a little, but it did help. It knocked the AM station level down a
couple of dB.
2) Unplugging the door bellpower supply did nothing.
3) The real fixer was a 1000pF ceramic capacitor. I placed it between
the + and - terminals of the intercom speaker connection. I had
previously placed it between the + terminal and ground. Whereas it helped
when I placed it between the + and ground, it totally eliminated the
interference when I placed it between the + and - terminals.
Noise gone, wife very happy.
Thanks to all who offered help. I appreciate your responses.
Dave