[RFI] LCD TV wipes out AM BC band - UPDATE Report

Tue Feb 22 11:25:09 PST 2011

Ken,

Thanks again for more info and details.  OK, your set is used exclusively as a monitor.  Point noted.

After my post below, I went and grabbed my other working portable AM BC band radio: my 1958 (yes!) Motorola 6 transistor set with ferrite bar antenna, 4" speaker, and power from 6 "C" cells.  (All of my true cheapie sets have either died or been scrapped.  This set cost a whopping $40 back when I was a kid and was the first on my block with one of these!)  Yes, it was made in Chicago, USA, has plug-in transistors, and all of them are 2N, 2-digit germanium.  I use the set daily and it also has never been in the computer room.

My test:  Check for presence of the same noise I heard yesterday, cycle the Toshiba on and off several times to verify it as the source, test with the computer on and functioning, then again with the computer fully off, and do "RF sniffing" on cords and cables using the radio as the sniffer.  I did not disconnect the antenna connection to the HDTV.  Here are the results:

1.  For one thing, this test proved how very "hot" the Super Radio III is compared to "standard" sets.  As I reported in my first posting, the Super Radio was interfered with to a distance of at 10 to 15 feet.  The Motorola did not detect the noise until I had it within 2 to 3 feet of the TV set.

2.  Cycling power to the TV still proved that it was the source of the noise.  Operation of the computer (on or off mode) did NOT affect the noise level or characteristics.  Again, the computer is connected via the SVGA port of the Toshiba.

3.  The noise level on the Motorola was equal to that heard on the Super Radio when I placed the Mototola approximately 6 inches in front of the diplay screen.  When I was using the Super Radio yesterday, it was on my lap at a distance of about 2 to 2.5 feet from the front of the display screen.

4.  The radiated noise level did not change with the "audio out" cables disconnected from the set.  With the cables connected. I could detect the hash noise all along the cables (which are 6 feet long) and it was present at the input of the audio amp that I use to drive a set of book shelf-size speakers.  (The audio amplifier is totally linear in design - no switching power supply.)  I held the radio about 3 to 4 inches away from the audio cable when "sniffing".

5.  The sounds heard from the Motorola's speaker were the same as I heard yesterday, including the start-up sweep.  

6.  Unlike yesterday, I did tune the Motorola radio across the entire AM BC band and found that the noise level falls off a bit above roughly 1100 kHz, but never dies out completely.  I can not say if what I am hearing is at the fundamental frequency of the source, or is from low order harmonics.  More analysis would be needed, and I don't have time for that right now. 

So, Ken, you have been very helpful and I appreciate your feedback comments.  If you have time, I sure would like to know what happens if you place your AM/FM set (in the AM mode) in front of the display and at a distance of roughly 1 to 2 feet in front of the display screen.  Thanks in advance.

My take on all of this:  the RFI is real, I am sure of that.  "Hot" radios are more affected than "standard" ones - Ken's radio and my Motorola are probably indicative of most consumer grade AM BCST sets on the market insofar as sensitivity is concerned.  (That's the reason that the Super Radio has endured as a product for some 30 years - it does what most other sets can not do: pull in real DX.)  It is not likely that most 19AV600U owners will ever have an RFI issue.  It is also possible that my particular set is an anomaly (just my luck!) in being so noisy.  Again, any info on these, or other, LCD sets will be appreciated.    

73, Dale
WA9ENA     

----- Original Message ----- 
From: Ken Winterling 
To: svetanoff at earthlink.net
Cc: rfi at contesting.com
Sent: 2/22/2011 12:35:29 
Subject: Re: [RFI] LCD TV wipes out AM BC band

Dale,

I do not have the set connected to a cable or antenna; I do not use it for watching TV programs. Â The set is used as a second monitor (expanded desktop) for a laptop so there is that video cable between the set and the laptop. Â I also use the PC audio input to provide external speakers for a second laptop adjacent to the first. Â Both laptops are within 18" or less of the TV.

Ken, WA2LBI

On Tue, Feb 22, 2011 at 13:15, Dale Svetanoff <svetanoff at earthlink.net> wrote:

Ken,

Thank you for the prompt feedback. Â That is very interesting and brings up 3 questions in my mind: Â 1) Does my set have a problem that is specific to it or which was caught by Toshiba and then fixed in production? Â 2) I suspect that your radio has lower sensitivity than does the Super Radio III. Â To verify the concept, I'll have to drag out one of my cheapie sets and try the same test. Â I'll report my test results here. Â 3) Could there be some sort of interaction between my set and the computer system?

A question for you: Â Do you have anything else connected to the Toshiba? Â You would have the AC power cord, of course, but other than a coax to the Antenna port, anything else? Â I have Audio Output (line level) going to an external amp, and the SVGA cable from the computer (going into Input 7). Â The signal on the coax is from my over-the-air antenna.

Again, thank you for doing the test.

73, Dale
WA9ENA

----- Original Message -----

From: Ken Winterling
To: svetanoff at earthlink.net
Cc: K8RI; rfi at contesting.com
Sent: 2/22/2011 11:48:45
Subject: Re: [RFI] LCD TV wipes out AM BC band

Dale,

When I saw you have theÃ‚ Toshiba 19AV600U I checked my Toshiba TV in the shack and we have the same model. Ã‚ I just tried listening to AM on the cheap AM/FM radio that is about two feet from the TV. Ã‚ I turned off the TV, tuned in a weak AM station, then turned on the TV. Ã‚ I didn't notice any change in the received AM signal. Ã‚ Also, there weren't any "sweeping" sounds.

Ken, WA2LBI

On Tue, Feb 22, 2011 at 12:34, Dale Svetanoff <svetanoff at earthlink.net> wrote:

Roger,

An excellent question, plus I should have included the Toshiba model info
in my posting.

I can think of at least two systems within the set that could be the
noise-makers: 1) The switching power supply that operates the whole set.
These days, it is not uncommon to have switching freqs as high as 3 (or

more!) MHz. Ã‚ I made no analysis of the noise or its pattern, except that it
is broadband in nature and strong across the AM BC band. Ã‚ 2) The other

likely candidate could be the drivers for the CFLs that provide the

backlight. Ã‚ I did notice an upward "sweep" in frequency (as heard on the
speaker of the radio) each time I switched on the Toshiba set. Ã‚ That sweep
lasted for perhaps 2 to 3 seconds, then stopped. Ã‚ My thoughts are that

maybe the CFL drivers are "ramping" up the lamps over that period of time,
or they are adjusting to a pre-set drive level to maintain a given
background intensity.

My set is the Toshiba Model 19AV600U, which seems to be a popular model. Ã‚ I

suspect that it has been superceded by something "better" by now, but I
have seen these in hospitals, motels/hotels, commercial establishments, and

so forth. Ã‚ Remember, any set that is going to cause a problem in the region

of 1 MHz or so will do so when in close proximity to the antenna of a

victim radio. Ã‚ Since most AM BC band radios have integral antennas (usually

ferrite bar or rod types - the one in the Super Radio III is a whopping

200mm long [nearly 8 inches]). Ã‚ If anyone does note such interference from

a TV set or monitor, please post to the reflector with info about the

offender. Ã‚ Thanks.

73, Dale
WA9ENA
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