Whistling TV

[Speedy Builder5Participant @speedybuilder5]

Eager to see the news from last night this morning, switched the TV on and SAT box and there was a persistent soft pulsating whistle from the TV.  So, decided to Re-boot TV and SAT box.  Whistle continued.  Did a hard re-set by unplugging everything from the mains – TV, SAT box and fiber internet router and for good measure, the digital telephone from the router. Whistle continued, so I thought it was still caused by some residual power in capacitors etc and left it for about 5 mins still whistling. Now the only connection to the TV was the Ariel which has a mains amplifier about 6 meters away from the TV cabinet where the down lead enters the house. Unplugged the Ariel and hey presto, whistling stopped. Plugged it back in and no whistle! (No mains connected to the Tv and other equipment).  Re-connected all mains equipment and still no whistle.

So, what is going on ??

Bob

[Mark RandParticipant @markrand96270]

It used to be a loose core in the flyback transformer that generated the high voltage for the cathode ray tube. All the children in the house would hear it, but the older adults often couldn’t. I wasn’t aware that satellite television sets still needed them. 😀

[Mark Rand]

On Mark Rand Said:

It used to be a loose core in the flyback transformer that generated the high voltage for the cathode ray tube. All the children in the house would hear it, but the older adults often couldn’t. I wasn’t aware that satellite television sets still needed them. 😀

Had that when I was a youngster at home.  The guy from the TV rental company said exactly that, but with a little less technical detail.  I seem to remember him also saying that it didn’t help having the TV in a corner.

[Mark Rand]

On Mark Rand Said:

It used to be a loose core in the flyback transformer that generated the high voltage for the cathode ray tube. All the children in the house would hear it, but the older adults often couldn’t. I wasn’t aware that satellite television sets still needed them. 😀

Though flyback transformers aren’t needed in modern TVs, something very like them is very common!   Almost all modern power-supplies are switch-mode.

Rather than have a heavy iron-cored transformer at 50Hz, the mains is rectified and used to power an oscillator running at about 100kHz.    A transformer running at that frequency is tiny, and doesn’t need an iron-core.  Switch-mode supplies save power.  An idling 50Hz transformer wastes about 10% of it’s rating, whereas the electronic version wastes less than 1%, your mileage may vary!

Nothing is perfect.   The transformer in a switch-mode PSU can vibrate audibly if it or the windings come loose, and the oscillator might drop temporarily into the wrong mode, making it whistle.  The box and circuit board have resonances that the transformer might reinforce.  Turning the power off and on disturbs the resonance, and the whistle stops.

Maybe!

Dave

[John HaineParticipant @johnhaine32865]

I suspect it was vibration in the scan coils rather than the LOPT. The horizontal scan excitation frequency was 15.625 kHz for 625 line TV and the coils were excited with a sawtooth current waveform at that frequency. The collapse of the current at the flyback generated a high voltage that was stepped up to produce the EHT. From my memories of taking TVs apart the transformer cores were firmly clamped together to optimise the magnetic circuit.

[Mark RandParticipant @markrand96270]

^^^ true.

[Speedy Builder5Participant @speedybuilder5]

Dave,

“Did a hard re-set by unplugging everything from the mains – TV, SAT box and fiber internet router and for good measure, the digital telephone from the router. Whistle continued ”   – I may add for several minutes

Bob

[Trevor GaleParticipant @trevorgale]

I’m not sure what kind of television set (that’s what we used to call them back when I was a boy!) you have, but if it’s one with a cathode ray tube then there is a line output transformer (LOPT) that drives the horizontal deflection coils on the neck of the tube, and also provides the EHT (15 … 25kV) voltage for the tube with an overwinding.

In models with the old 405-line VHF channels (BBC1, ITV) that ran at 10.125KHz, well within human hearing range, whereas for the 625-line UHF channels (BBC2 and other colour broadcast) channels it ran at 15.625KHz.

If the antenna was connected and a programme with a good signal was being watched then the LOPT would run consistently at the required frequency and sometimes create such a ‘whistle’ which could be heard, usually by the younger members of the family! With no signal the line oscillator would ‘free-run’ as there were no ‘synchronisation pulses’ to keep it locked, so it wasn’t so easy to hear.

It reminds me of the days when (being a lot younger!) I could walk around in the village and tell if someone in the street was watching 405-line T.V.! Now, at 70 years old I can still hear (just) a 10KHz sinewave, as I write this using a Hewlett-Packard signal generator, which I consider to be not too bad!!!

[Speedy Builder5Participant @speedybuilder5]

TV with CRT ??? – I thought everyone had a LED type “thin” Tv these days.  I assume its all thin film semiconductor stuff inside our Tv – BUT I hear what you say about the possibility of the Ariel driving some components.

Bob

[duncan webster 1Participant @duncanwebster1]

Anything above about 6kHz I can’t hear. I reckon it’s self defense, as women’s voices tend to be higher pitch.

[Robert Atkinson 2Participant @robertatkinson2]

The only thing I can think of that would cause these symptoms would be two “issues” First is a power supply fault (either in the power supply or it’s load in either the TV or antenna amplifier power supply) causing the power supply switching frequency to drop AND some kind of earth loop issue involving the antenna cable.
There are too many variables to determine exactly what was going on.

Robert.

[SillyOldDufferModerator @sillyoldduffer]

As Robert says too many variables and unknowns.

Bob said ‘Now the only connection to the TV was the Ariel which has a mains amplifier about 6 meters away from the TV cabinet where the down lead enters the house. Unplugged the Ariel and hey presto, whistling stopped. Plugged it back in and no whistle! (No mains connected to the Tv and other equipment). Re-connected all mains equipment and still no whistle‘.

Although Bob disconnected everything obvious, there must still be a power source in the system, and it’s likely connected to the aerial.   Maybe a pre-amp in the loft, or one (best) mounted on the antenna.   Either way, another power supply, probably switch-mode , able to misbehave like flyback transformers and such as discussed above.   Or, although unlikely, maybe the aerial coax is coupling to a power line, or a strong local transmitter.  Nearby cell-phone or utility tower?

My guess is this was a double problem: a errant power source wandered to a frequency that coincided with a mechanical resonance inside the TV set.   Like soldiers marching in step over a bridge, or a violin bow scraping a saw, something physical in the TV set vibrates like a primitive loudspeaker.  Resonant structures store energy, so tiny inputs can build up a dangerous physical movement.

In Bill’s example, could be anything: transformers, chokes, and even some capacitors are piezoelectric.   When resonance occurs, the physical and electromagnetic source couple and lock the frequency.   Precision oscillators are often based on piezoelectric quartz crystals, cut to a particular size and shape to resonate accurately and strongly at a design frequency.  Then the electronics lock to this frequency because it takes less energy to vibrate in resonance with the crystal.   Exactly the same can happen by accident, except the resonant ‘Q’, will be much lower, making the whistle less likely to reappear if conditions change.

When Bill broke the coupling,  I guess the source frequency moved away from the physical resonance, and because the TV had stopped vibrating in tune with it, the resonance was unable to reassert control when power was reapplied.

Quite a few bridges and buildings have collapsed due to resonance.   Even in modern times when computers can be used at the design stage to identify significant resonant frequencies in structures.   Remember the Millennium Bridge?  Open two days before closing for two years to dampen out the resonance that caused it to sway.

Or let’s blame Gremlins!

Dave

 

[duncan webster 1Participant @duncanwebster1]

Or let’s blame the architects, lateral frequency of suspension bridges is a known problem.

[Chris GillParticipant @chrisgill22114]

Ah, CRT televisions – I doubt I could lift our old one now. It suffered endless dry joints.

But I reckon the whistle is the Little Mermaid trying to draw attention. Ariel was only 6m away, after all.

Sorry, 😁😁

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