Lenny -

The RF is being demodulated somewhere in the house, and I suspect it is
the demodulated audio that is interfering with the modem. One thing you
might do (though it's time-consuming if done on a service call) is run
around the house and disconnect every device on the telephone circuit,
one by one, and see when the interference quits. Or disconnect 'em all
and then put 'em back one by one. You may be able to identify which
specific device is causing the interference.

BTW, if the audio level is high enough to interfere with a modem, it is
probably high enough to hear on a telephone handset. Do you have one of
those lineman's test sets so you can listen to the line even when you're
on hook? That would enable you to determine instantly what effect you
are having.

If it turns out that the modem itself is reacting to the presence of RF,
you might be able to construct a series-tuned LC filter and put it
directly across the line as it enters her modem. It should be a much
sharper (and deeper) filter than the Radio Shack jobbie.

Where did she put the Radio Shack filter, by the way? As close to the
modem as possible?

Or, as Isaac suggested, just put the modem near the service entrance, if
that really makes a difference (which would surprise me, by the way, but
the lady thinks it does, so ...).

Bill Jeffrey
===========================

wrote:

I'm going on a service call next week to address an issue of AM radio
interference on a telephone line. The lady has her computer connected
to this line and it seems to keep getting bumped off especially
throughout the day. There is a 5KW AM transmitter operating on 610KHZ
about a quarter of a mile from the customers home. At night they drop
their power to 1.0KW and she tells me the interference decreases. She
has tried filters purchased from Radio Shlock which were marked ".50MHZ
to 3.0MHZ but apparently they did not help.
The other day while discussing this with her on the phone, (she called
me from her work so I have never heard the interference), she told me
that someone had suggested that she plug her modem dierectly into the
interface on the side of the house. The length of cord used was
approximately 15 feet. When she did this she found that there was
apparently no interference problem. I'm suspecting then that if the
line is clean up to the interface, then the telephone company has
perhaps already addressed the close proximity transmitter issue in her
neighborhood using sheilded wire and perhaps filters on the street and
that her problem is the straight untwisted wiring throughout her home
which may be acting as an antenna.
I've considered running a new sheilded wire from the interface,
grounded to the telephone company's ground, directly to her computer.
I would try that first with no other house wiring connected.
In theory if that were the only line connected then that should work.
However, I would suspect that as soon as I would connect her existing
house wiring back up to the interface, (essentially reconnecting the
"antenna") the problem I would suspect would return.
Should I plan on completely rewiring the house with sheilded or does
anyone have any another ideas they might suggest? Thanks very much for
any assistance. Lenny Stein, Barlen Electronics.