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No Backlight on LED TV

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Turnip:
Hi Kenny,

Used to do screen cleans on Pye VT4's where the plastic screen attracted even more dust - Mind you, it slipped out from the bottom so could suss it weekly.

Even worse were Cossor 950s where the tube became invisible.

Ex 'Visionhire' man myself where thinking folks bought thousands of 'Ferguson' and 'Peto Scott' tellies as they were cheap but total rubbish.

Admit Visionhire bought quite a lot of Philips sets, The mono versions were the biz, but the G500 series led to a decline as being quite the worst dual standard colour set 'designed' for somewhere else.

The G8 got it together again, but nothing near as reliable as anything else due to power and LOPT failures.

Sensible folks bought Japanese sets - Gather 'Visionhire' is now 'resting' at companies house, and has been for some years - Goes to show - Chris.

Hi Bruce - Didn't realise you were into Rugby - Guess passes the time - Chris.

Southerner1959:
Thank you for the replies . . .

So are you suggesting that, although the screen looks fine for the split second it is on, it is detecting a fault in one of the LED strips and turning the voltage feed off?

BUT if there was a fault in one of the LED strips, why would it turn off BOTH of them? (each one is fed by a separate power rail, as mentioned above) - surely it would only cut the power to the faulty one?

PLUS - what could go wrong that would cause it to cut the power? Surely, if the LEDs fail, they go open circuit, and just don't work. (yet that can't be the case, as the whole screen looks fine for the split-second the Backlight flashes on)

I have checked all the voltages coming in and out of the PSU Board to the Main Board, and they are all correct.

I suspect there is just a fault in the Backlight dimming circuit on the PSU Board. If so, surely I can just by-pass the control circuit?

I gather that dimming of the backlight is achieved NOT by reducing the voltage, but by PWM, ie pulsing the feed and reducing the width of the pulses (similar to an ordinary room dimmer)

The Voltage going into each of the FETs is 75V . . . so presumably they are running at 100% at full brilliance, so I could just by-pass the FET on each feed and connect the 75V straight to each backlight strip?

But is there likely to be a constant-current driver circuit INSIDE the screen (attached to the LED Strings) . . . or is the power supply board feed likely to perform that function too? (I wouldn't have thought so)

fix2003:
if one strip has a fault the monitoring circuit would normally shut them all down

downunder:
Yes, fix2003 is correct. "2 seconds to black" is typically a backlighting problem. You have stated you have audio and can make out data on the screen. This means the TV is fully operational except for the backlights.

The earth return line for all the LED strips is monitored, and here any anomalies in the backlight are detected, an error signal is generated, and the backlights are extinguished. This cannot be confined to individual LED strips. One LED out, all LEDs out.

The duration of the brief flash you see is the time it takes for the protection to activate. If there is an ERROR
pin on the LED Driver Board, you'll note it sits at near 0V at switch on, then jumps up quickly as the error occurs.

Only sure way to know is to strip the panel and observe the LEDs at switch-on, to see which ones are inert.
You may not be comfortable with this as the display sheet is wafer-thin and easily cracked.

Bruce

Southerner1959:
What I don't understand is HOW the backlights could come on for a moment and work perfectly on switch on and switch off if there was a fault?

What kind of fault with the LED strip could cause that?

Having said that, I have tried measuring the resistance of each of the strips, and can see nothing, is that normal?

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