LCD-TV repair

[PowerAmpFreak]

Hello.
I've worked most on Plasma sets and have some questions about LCD technology. I've got a LAVA 32" LCD which doesn't start the backlight.
The powersupply seems to operate correct. The output voltages is within limits. I've gone through various voltage regulators on the mainboard, without finding anything abnormal. The inverter board gets an ON signal from the mainboard.
But then I don't know what goes wrong.
I don't know the level of high voltage generated by the inverter board, I can only measure up to 1500V with my oscilloscope.
I'm looking for general troubleshooting advice.

[Scenic]

Re: LCD-TV repair

first thought: bad fuse(s) on the inverter board..?

[dobber]

Re: LCD-TV repair

I just did a Sharp 26" LCD, and I got all the voltage to the transformers, but no high output. I checked the fuses, and they were good. I later found a bad fuse on the 3 volt signal that tells it it is ok to make the high voltage. It was a very small fuse about 2 millimeters, and after that it worked great. So I say fuse too.

I do have some questions for you if you are good with the Plasma tvs. I have a need to repair a power board on a sony, if you search my posts, you will see what I need.

[crtfool]

Re: LCD-TV repair

PM sent to dobber

[PowerAmpFreak]

Re: LCD-TV repair

The only fuse found on inverter board is OK. Resistance measurements across the two high voltage transformers indicate similair results, so I would expect them to be good. How often is there a problem with the lamp/lamps?

[Tvbob]

Re: LCD-TV repair

does the power led stay on, do you have audio, can you see a faint image with a flashlight, do the scaler/formatter ic's get warm?

[PowerAmpFreak]

Re: LCD-TV repair

The powerled stays green. The rest of your questions will be answered on Tuesday, next time I will be in the workshop. Thanks!

[Dgtech]

Re: LCD-TV repair

Sometimes, whenever you have a voltage on the secondary going towards the lamps and dont have any lamps on, you'll hear arching sounds with a distinct smell from the back. You might also have an open connection - disconnected wire.

[PowerAmpFreak]

Re: LCD-TV repair

I can see no faint picture when using a flashlight. The powerled stays on, all buttons seems to operate, for example when increase volume, the noise increase in the speakers. Though I haven't been able to actually hear audio from a connected DVD player. This might be because I cannot see what I'm doing on the screen (of course) and it's hard to switch over to correct source then. It doesn't automatically switch over when a scart is inserted either.
The set consumes about 50W continously when on. Scaler / processing IC's appears to have "normal" heat, the mainboard voltage regulators seems all to regulate at correct voltages.
So this story might end up in an inverter failure or lamp. There is no heat generated from the inverter board. I haven't done any measurements either.
Except from verifying it recieve proper voltage levels from the powersupply.
The set is a LAVA LD-32, it gives kind of a "vestel" feeling.

[daimoku]

Re: LCD-TV repair

Make sure you check the tcon too. It also can cause the symptoms you're experiencing.

[PowerAmpFreak]

Re: LCD-TV repair

Make sure you check the tcon too. It also can cause the symptoms you're experiencing.

Could the tcon make the backlight not come on?

[daimoku]

Re: LCD-TV repair

I can see no faint picture when using a flashlight.

If the tcon is faulty, the backlight will usually power on and you will get audio, but no picture. Since the backlight isn't illuminating in your case, the tcon probably isn't the issue.

When you get a chance, I'd focus on troubleshooting the inverter (since it's not generating any heat), the lamp or wiring to the lamp.

[PowerAmpFreak]

Re: LCD-TV repair

I measure resistance across the two highvoltage outputs (across the lamp wiring) and it reads 65 ohms. Is this a normal measure?
Guess this is the high voltage transformers secondary dc-resistance.
Is the lamp any resistive and could be measured at all?

[Dgtech]

Re: LCD-TV repair

The difference in primary to secondary resistance on transformers is due to the design and the function of what the transformer does. It either will step up the voltage while lowering the current or step up the current while lowering the voltage. If you find 65 ohms on the secondary and find for example 650 on the primary, we would say that this is a 10:1 transformer.

The lamps should show no resistance - infinite on meters. The high voltage from the secondary of the transformer is needed to force its way to the other side of the vacuum within the CCFL.

[PowerAmpFreak]

Re: LCD-TV repair

Yeah, of course - what I wanted to know was if 65 ohms is a reasonable dc-resistance of a inverter transformer, or not.
I will try a new inverter controller IC for this buddy and see what results this will make, keep fingers crossed.

[Th3_uN1Qu3]

Re: LCD-TV repair

High voltage outputs of inverters usually measure in the 1kOhm range but that's on small LCDs, a higher power inverter is likely to have thicker wire, lower resistance. Measure each transformer secondary, with the lamps unplugged. Remove those high voltage caps and measure again. One of them might have gone bad. If there are any resistance differences more than 3% between the transformers, they could be bad.

[PowerAmpFreak]

Re: LCD-TV repair

Got this beast up on the bench again. Latest findings:
The set starts the inverter oscillator. The high voltage transformers gets proper drive from the power switching circuit. If I keep my probe close to the secondary high voltage output I can see that HV exsists.
But the lamp doesn't turn on. Wire seems to be fitted properly.
What else could be made? Does this mean that lamp is out?

[PlainBill]

Re: LCD-TV repair

Got this beast up on the bench again. Latest findings:
The set starts the inverter oscillator. The high voltage transformers gets proper drive from the power switching circuit. If I keep my probe close to the secondary high voltage output I can see that HV exsists.
But the lamp doesn't turn on. Wire seems to be fitted properly.
What else could be made? Does this mean that lamp is out?

I really haven't dug into this much, but here are my observations.

There are two different designs used for backlight inverters. One is an upgrade of the design used on LCD monitors. I have seen them with a dozen transformers. Usually each transformer drives a single CCFL.

The second divides the output from a single source (or master / slave pair) to provide power to a 'balancer' that drives all the tubes and prevents current hogging by a single tube.

PlainBill

[PowerAmpFreak]

Re: LCD-TV repair

This inverter design uses two transformers on the same board. Each transformer has one single output. Each output is connected to the panel.
Plainbill, how do you verify the level of high voltage? Do you have a high voltage probe?
Is lamp failure a common problem, really?
I don't know what it could be else, if the inverter provide proper HV.

[Th3_uN1Qu3]

Re: LCD-TV repair

Is lamp failure a common problem, really?

I've never had one but quite a few members here have replaced failed CCFLs on several occasions. So yes, it isn't uncommon for the lamps to fail.