JVC LT-40C750 Backlight goes off after 5 or 10 minutes

[sickboy]

I have a nice JVC LT-40C750 smart TV with an annoying fault.
The TV works great for anything between 5 to 30 minutes then the backlights will suddenly go off. Sound still works and you can see image with a flashlight.
When it happens, you can switch to standby then back on again and it works great again for another while.

Power board is a 17IPS71 and main board is 17MB95M.
Heating or freezing the board doesn't have any effect, same with tapping the boards and wiggling the backlight connector.

There's 3 wires go to the backlights. When working there is 68v between black wire and chassis, 5v on brown and 7.8v on red. When the backlights go off I have 0v on black, 2v on brown and 2v on red.

Can anyone offer any guidance? I'll add pics of the boards shortly.

Many thanks.

[budm]

Re: JVC LT-40C750 Backlight goes off after 5 or 10 minutes

Try turning down backlights level to as low as possible to see if the backlights will stay on longer, you may have bad LED's problem.
Please also show pictures of the whole backside of the TV showing all the baords.

Vestel 17IPS71 schematic.

[sickboy]

Re: JVC LT-40C750 Backlight goes off after 5 or 10 minutes

Thanks Budm, it was on medium backlight, now on low and we're 22 minutes in and so far so good.
Curious as to how I'd identify which particular LED was the culprit though? I have one of these cheap chinese backlight testers but I don't think it would be much help in this situation, unless I hooked it up for 20 minutes or so?
Thanks again

[budm]

Re: JVC LT-40C750 Backlight goes off after 5 or 10 minutes

The tester only puts out about 30mA instead fo about 200 ~300mA, so tester may show fine but when the LEDS are driven hard then they fail if they are bad.

[sickboy]

Re: JVC LT-40C750 Backlight goes off after 5 or 10 minutes

So I might be better buying 1 complete strip, and fitting it in each position in turn until the fault no longer happens? I'll have a look around on ebay to see what's available, thank you for your help.
(now over 30 mins and still on)

[sickboy]

Re: JVC LT-40C750 Backlight goes off after 5 or 10 minutes

Quick update on this, I ordered a LED strip, as well as used power supply from ebay (as it was dirt cheap).
Still waiting on the LED strip arriving but the power supply came this morning. Fitted it around 2 hours ago and turned the backlights to maximum. So far it's been on for 2 hours with no problems. It never usually lasted anymore than 30 minutes (with backlights on medium) before so HOPEFULLY that's the problem cured. Will keep it on for the next 24 hours with max backlights and see w2hat happens.
Finger's crossed!

[Russjg23]

Re: JVC LT-40C750 Backlight goes off after 5 or 10 minutes

Hi, did this permanently fix the problem?

[sickboy]

Re: JVC LT-40C750 Backlight goes off after 5 or 10 minutes

Hi, did this permanently fix the problem?

No, I had to fit a full new backlight set in the end.

[colt2x]

Re: JVC LT-40C750 Backlight goes off after 5 or 10 minutes

Hi,

I have a 17IPS71 PSU which only blinks the backlight when TV is turned on. The TV is powering up and working, but the backlight not.
My question is only for a tip, what seems to be bad? The LED driver IC, the FET, or the strips? I think it should not flash the backlight if the IC or Q17 may be bad, and i don't measure these faulty too.

[neilc6]

Re: JVC LT-40C750 Backlight goes off after 5 or 10 minutes

Flash of backlights, backlights turning off or no backlights, the backlight are the main suspect. https://techhometravel.com/lg-ln-ser...ed-backlights/