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

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