DINGMAN@RADC-TOPS20.ARPA (03/20/85)
I'm a non-ham, so I've come here to ask the experts for help. I recently
bought a wireless FM intercom for my house (two-station). It works fine
until I turn the TV on, then buzzes like mad. I tried a Radio Shack AC
line filter, but that didn't have any effect at all. It also makes the
noise no matter how far from the TV either station is.
So, what is causing the buzz? Is my TV xmitting through my wiring, or
might it be some other kind of interference? And how do I quiet the little
bugger down?
Any suggestions or info would be appreciated. Please send direct to me,
or post to this BB if easier.
Thanks in advance,
John (DINGMAN@RADC-20.ARPA)
-------jhs%Mitre-Bedford@d3unix.UUCP (03/20/85)
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
rpw3@redwood.UUCP (Rob Warnock) (03/22/85)
+--------------- | ...bought a wireless FM intercom for my house (two-station). It works fine | until I turn the TV on, then buzzes like mad. I tried a Radio Shack AC | line filter, but that didn't have any effect at all. It also makes the | noise no matter how far from the TV either station is. | So, what is causing the buzz? ... And how do I quiet the little | bugger down? | John (DINGMAN@RADC-20.ARPA) +--------------- You don't say what frequency the intercom works on, or anything about your T.V., so this is pure speculation: Try cleaning the inside of your television very well, particularly the high-voltage lead to the picture tube, and all around where it plugs into the picture tube (especially the glass between the connector and the conductive outer coating). There can be arcing or corona discharge through the accumulated dust (which is naturally attracted to the high- voltage D.C. areas) causing your RFI problem. The corona can also occur in other parts of the high-voltage system. ** BE VERY CAREFUL! ** If you do not understand the safety precautions yourself, don't attempt to do it. Get a hardware wizard or service person. (There is a trick to discharging the tube, which can store a nasty shock. While said technique is quite easy, I couldn't safely attempt a detailed decription here, beyond saying it basically involves slipping a grounded probe up under the insulated "nipple" of the high-voltage wire.) Another possibility is that the one of the horizontal or vertical output stages has a parasitic oscillation in the F.M. band (somewhat harder to diagnose and cure). Borrow a wide-band receiver to check this one out. Rob Warnock Systems Architecture Consultant UUCP: {ihnp4,ucbvax!dual}!fortune!redwood!rpw3 DDD: (415)572-2607 USPS: 510 Trinidad Lane, Foster City, CA 94404