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From: jhs%Mitre-Bedford@d3unix.UUCP
Newsgroups: net.ham-radio
Subject: Re: FM Intercom/TV Interference
Message-ID: <9365@brl-tgr.ARPA>
Date: Wed, 20-Mar-85 12:11:29 EST
Article-I.D.: brl-tgr.9365
Posted: Wed Mar 20 12:11:29 1985
Date-Received: Sat, 23-Mar-85 02:27:19 EST
Sender: news@brl-tgr.ARPA
Lines: 58

I dunno if we qualify as "experts" but of course flattering us will at least
get us to try to help you!

I would have thought a line filter might help.  Did you place it at the
TV set or at the intercom?  If you haven't tried both, you might do so.
If it works at the TV set end, that's best because then you only need one
line filter.

Does anybody know what frequency those FM carrier-current intercoms work on?
I have some at home but have never bothered to look into this question, as
mine have worked fine.  It might be that the line filters supplied by RS
aren't effective at this frequency.  If that is the case, a more effective
line filter (boughten or designed and built for the purpose) would help.

Does your intercom have an adjustable squelch setting buried on the bottom
somewhere?  You might cure the problem just by tightening up the squelch.
This raises the question: does the buzz come on as soon as you turn on the
TV even if the intercom is idle or only when the push to talk switch is
actuated?  My "cure" would only work if the former is the case, I think.

If line filters simply don't fix it, even with one on each of the three line
cords involved, then the set is probably radiating junk pretty strongly at the
intercom frequency.  Then you might have to go in and modify the TV set in
some way to fix it.  My suspicions would run to an oscillation or ringing in
some wiring in the TV set, excited by the vertical retrace signal - the very
sharp waveform that makes the CRT beam zap up from the end of the frame to the
beginning of the next one.  This occurs at about a 60-Hz rate and has LOTS of
harmonic energy in it.  If a piece of wiring in the TV set's vertical sweep
circuit is resonant near the carrier frequency of your intercom, this could
radiate RF pulses at the vertical sweep rate of the TV, which could indeed be
picked up as a buzz.  Changing the length of this wire or "dressing" it close
to the chassis or maybe replacing it with shielded cable could be solutions.

Another possibility is that some stage of the vertical sweep circuit is
breaking into oscillation at the intercom frequency during some part of its
cycle.  I would think you would see something funny in your TV picture if this
were the case, but "only the electrons know for sure"!

If you can get hold of an oscilloscope for a weekend, you may be able to
track the problem down more systematically:  Look at the AC line where it
connects to your intercom that is hearing the buzz and see if you see spikes
on it when you turn on the TV set.  If so, try to speed up the sweep and see
if you can see the basic frequency of oscillation or "ringing" in the pulses.
If you don't see anything on the AC line, tear your TV set apart and see if
you see anything funny in the vertical sweep waveshape anywhere along the
chain from the sweep oscillator to the CRT itself.

Anybody else got any ideas on this one?

Good luck, and get back to the net with info on where you put the line filters
and such like if you need more hairbrained ideas.

						"73" (old ham/telegrapher
						   abbreviation for "best
						     regards"),

						John Sangster, W3IKG
						jhs at MITTRE-Bedford.arpa