Hardware & Technical Anybody knowledgeable about LCD Tvs?

Stealthie

Stealthie
It's turning into one of those weeks....

Next door neighbour is an old dear who only has a teensy little 19" TV.

She was around at our house, saw our 42" telly and said she'd like something similar.
I've found her one (a 2-year old "Digihome" model, sold via Tesco, I believe) but, upon testing it, the screen backlight is failing after 5-10 minutes use.
The picture IS still being displayed but, without the backlight, it's almost impossible to see.

I trained as an electrical engineer (many years ago) but most of my experience is with industrial electronics rather than domestic stuff so I don't have a lot of experience with this sort of thing.
I've heard that the inductors in LCD TV PSUs degrade and that's what causes the backlights to fail.

I've had a look on eBay and there seems to be a couple of places selling replacement power-supply boards for this model of TV dirt-cheap.
Thing is, if the PSU boards are available, that makes me wonder if that's because it's something else which tends to fail in these TVs - such as the backlight, itself, perhaps?


So, the question (for anybody who's knowledgeable about this stuff) is: IS it normally the PSU boards that fail or is it more likely it's the backlight, itself, that's worn-out?

Given that the PSU boards are only about £10, I suppose it's worth a punt but I don't want to waste my time/effort/money if it's unlikely to fix the problem.
 

Patrick_68000

Patrick_68000
Cmdr BrinBrown said:
I am/was a TV repairman:)
My advice is just buy a new TV, they are cheap.
BB
Click to expand...

Recently the HD LED TV of my mother, lost the HD tuner

She can still use the TV with the scart socket but not HD of course

The seller said, "no need to replace the HD tuner, it is better to buy a new TV"

The TV is a Samsung HD of three years of age and 2 years warranty

:rolleyes:
 

Shadowdancer

Shadowdancer
Stealthie said:
I trained as an electrical engineer (many years ago) but most of my experience is with industrial electronics rather than domestic stuff so I don't have a lot of experience with this sort of thing.
Click to expand...
As long as you know which end of a multimeter goes where, you should be able to do basic troubleshooting. See if the backlight still has supply voltage when it goes out, if it doesn't I'd try my luck checking for stuff getting hot in the backlight power supply. Just try not to shock yourself on the thing if it's a CCFL backlight (the high voltage stickers would be a giveaway).

(edit) Also try not to shock yourself on mains, I trust you know about the importance of an isolation transformer.

If the backlight itself is bad, it may be time to look for a new device unless you're willing to brave the seas of AliExpress :p
 
Last edited:

Stealthie

Stealthie
Shadowdancer said:
As long as you know which end of a multimeter goes where, you should be able to do basic troubleshooting. See if the backlight still has supply voltage when it goes out, if it doesn't I'd try my luck checking for stuff getting hot in the backlight power supply. Just try not to shock yourself on the thing if it's a CCFL backlight (the high voltage stickers would be a giveaway).

(edit) Also try not to shock yourself on mains, I trust you know about the importance of an isolation transformer.

If the backlight itself is bad, it may be time to look for a new device unless you're willing to brave the seas of AliExpress :p
Click to expand...

The results of a bit of basic fault-finding were... inconclusive.

The PSU is obviously clever enough to shut down power to the display driver, backlight and input circuit boards when they're not connected, which means you have no way of knowing, for sure, if it's the PSU that's faulty or the board it's trying to power.


I discovered something kind of interesting in the course of playing with the TV though...

The backlight has 3 settings (High/Medium/Low).
It was set to high and the backlight was shutting down after a few minutes.
I set it to Low and the TV works perfectly.

Presumably, what's happening there is that the backlight draws less current in the Low setting and the PSU can still manage to supply enough juice to keep it running.

So, if you've got a TV or monitor with a failing backlight, you might be able to squeeze a bit more life out of it (or have time to order a replacement PSU) by reducing the intensity of the backlight if that's possible.


Anyway, ordered a replacement (used) PSU off eBay on Friday for £5.28, it arrived today (great service), I stuck it in, set the backlight to High and... works perfectly. [up]

Given that the TV is only 3 years old and the PSU failed, I'm guessing this PSU won't last forever but at least it seems to be a commonly used model so another replacement should be easy to find.
There are brand-new replacements available on eBay too, for £15, so I guess I'd buy one of those next time.


The whole thing definitely seems to confirm what I'd heard about the induction coils in PSUs degrading and, effectively, down-rating the PSU over it's lifespan.
I guess if you buy a brand-name TV they fit an over-spec' PSU so that it'll be years/decades before it degrades enough to cause problems.
When you buy a budget TV, I guess they fit a PSU that's just good enough to ensure it outlasts the 12-month warranty.
 

Stealthie

Stealthie
Shadowdancer said:
I'd blame capacitors before inductors :p
Click to expand...

Maybe.

No idea why but the stuff I've heard seems to blame the inductors.

In practice, the new PSU is emitting an almost inaudible whistling sound from the inductor whereas the old one was emitting a much lower-pitched "buzz" from the inductor which was clearly audible.
I guess it certainly could be other components degrading which causes the induction coil to function differently but it definitely seems to be the coil, itself, which is acting differently.
 

Para Handy

Para Handy
I used to be plagued by the HT / flyback Coil whine in CRT days - nowadays it is mainly just faulty LED illumination circuits, most people cant seem to hear this as it is a higher frequency.
 

TorTorden_VR

T
Para Handy said:
I used to be plagued by the HT / flyback Coil whine in CRT days - nowadays it is mainly just faulty LED illumination circuits, most people cant seem to hear this as it is a higher frequency.
Click to expand...

God. that crt whine was half the reason I spent so much time wearing headphones in my misspent youth.

It's like that could pierce walls..
 
You must log in or register to reply here.
Back
Top Bottom