LG W2242T inverter

[avr]

Hello to all!

I have a small problem with my LG W2242T monitor.

Symptom:

The backlight lids on for a couple of seconds and then shuts down. Picture shows up ok.

What has been tested:

- The CCFL-tubes are ok. I tested them with another inverter, and also tried to cross the upper and lower tubes.
And seems that another side of inverter is dead (never lids up, even tested with another ccfl-tubes).
- Caps seems to be ok, at least from the outside.
- Transistors aren't at least short circuited (are they ok otherwise, I don't know?)
- Transformer winding resistances: primary 0.7 ohm and high voltage sides 950 ohms and 805 ohms (dead side).
- operating voltage main connector is 22V when light on and 27V when off (should be 22V). But on driver Vin only shows 10V? (driver is FAN7314A, Datasheet -> )
I'll try to figure out why that is only 10v (just measurement fault?)
- I have resoldered the dead side, and it had no effect.
- The dead side is the lower two in picture.


So any ideas what to test or to do next? Or is it simply the transformers fault, because the other side is 950 ohms and other 805 ohms? Or could that low voltage to driver cause this?


ps. Thank you for this great forum, and sorry for my bad english

[PlainBill]

Re: LG W2242T inverter

Hello to all!

I have a small problem with my LG W2242T monitor.

Symptom:

The backlight lids on for a couple of seconds and then shuts down. Picture shows up ok.

What has been tested:

- The CCFL-tubes are ok. I tested them with another inverter, and also tried to cross the upper and lower tubes.
And seems that another side of inverter is dead (never lids up, even tested with another ccfl-tubes).
- Caps seems to be ok, at least from the outside.
- Transistors aren't at least short circuited (are they ok otherwise, I don't know?)
- Transformer winding resistances: primary 0.7 ohm and high voltage sides 950 ohms and 805 ohms (dead side).
- operating voltage main connector is 22V when light on and 27V when off (should be 22V). But on driver Vin only shows 10V? (driver is FAN7314A, Datasheet -> )
I'll try to figure out why that is only 10v (just measurement fault?)
- I have resoldered the dead side, and it had no effect.
- The dead side is the lower two in picture.


So any ideas what to test or to do next? Or is it simply the transformers fault, because the other side is 950 ohms and other 805 ohms? Or could that low voltage to driver cause this?


ps. Thank you for this great forum, and sorry for my bad english

Great first post. While your English isn't perfect, you certainly gave enough information.

The key is the mismatch in the resistances of the secondaries. The fact that the dead side has a lower resistance indicates shorted turns. That will reduce the output to the point the CCFLs won't ignite.

PlainBill

[ecking767]

Re: LG W2242T inverter

I will check the gates of both of the output transistor to see if there is a signal coming from the FAN7314A pin OUT. the transformer maybe ok because its not open or making any noises right?

[kc8adu]

Re: LG W2242T inverter

shorted turns also cause high current to the driver on the primary side which trips the ocp.

[avr]

Re: LG W2242T inverter

I will check the gates of both of the output transistor to see if there is a signal coming from the FAN7314A pin OUT. the transformer maybe ok because its not open or making any noises right?

The transformer doesn't make any sounds.

I just tested that gate1 (OutB) shows 2V, and gate2 (OutA) 22V (backlight on).
And 0V and 27V when off.

[ecking767]

Re: LG W2242T inverter

I just tested that gate1 (OutB) shows 2V, and gate2 (OutA) 22V (backlight on).
And 0V and 27V when off.[/QUOTE]


Do you have an oscilloscope or a frequency meter ? The oscilloscope will be best to see if there is any signal being outputted on the OUTA and OUTB. 22volts indicate an open.

[PlainBill]

Re: LG W2242T inverter

I will check the gates of both of the output transistor to see if there is a signal coming from the FAN7314A pin OUT. the transformer maybe ok because its not open or making any noises right?

The resistance of the secondaries should match to within a few percent - closer than that, actually. A defective component on the input will not effect the output resistance. While there may be other defective components, the transformer is bad.

PlainBill

[ecking767]

Re: LG W2242T inverter

The resistance of the secondaries should match to within a few percent - closer than that, actually. A defective component on the input will not effect the output resistance. While there may be other defective components, the transformer is bad.

PlainBill

with out specification on the transformer we are guessing. A little over 100ohm different between the two shouldn't make that transformer output be zero volts.There should be a flashing of the lamps. Then 22 volts on gate2 (OutA) the means either that MOSFET isn't on or the inverter chip OUTA open

?? avr

What are the part number for the output transistors?
did you measure the transformer in circuit or out of the circuit?

[PlainBill]

Re: LG W2242T inverter

with out specification on the transformer we are guessing. A little over 100ohm different between the two shouldn't make that transformer output be zero volts.There should be a flashing of the lamps. Then 22 volts on gate2 (OutA) the means either that MOSFET isn't on or the inverter chip OUTA open

?? avr

What are the part number for the output transistors?
did you measure the transformer in circuit or out of the circuit?

You may be guessing about the transformer, I am certain.

The most likely scenario is shorted turns in the transformer overloaded the drivers. Unless the transformer is replaced, the replacement transistors will be destroyed.

PlainBill

[avr]

Re: LG W2242T inverter

Yep, PlainBill was right. It was the transformer.

I fixed my oscilloscope (power button), and checked the driver outputs and transistors. Seemed that everything besides the transformer were ok. I soldered the transformer out and measured it again. Now secondaries showed almost 1k and the dead side below 800 ohms.

Anyway, I modded universal lcd inverter in there for now, and it works now

[PlainBill]

Re: LG W2242T inverter

Yep, PlainBill was right. It was the transformer.

I fixed my oscilloscope (power button), and checked the driver outputs and transistors. Seemed that everything besides the transformer were ok. I soldered the transformer out and measured it again. Now secondaries showed almost 1k and the dead side below 800 ohms.

Anyway, I modded universal lcd inverter in there for now, and it works now

Glad to hear you have it working.

For the novice, comparing the resistance of two transformers is not like comparing the resistance of two resistors, or even two transistors in circuit.

A transformer has hundreds (thousands) of turns of wire. The ratio of turns in the primary to the number of turns in the secondary is important. Typically the number of turns is controlled to within one or two turns per winding. If there is a difference between the secondary resistances of two identical transformers the most likely cause is a short between turns. Not only does this reduce the effective number of turns, but it acts as a shunt to the magnetic field, further reducing the output of the transformer.

PlainBill

[Behemot]

Re: LG W2242T inverter

I have this display with the same symptoms. Secondary windings are OK so I went ahead and replaced caps, no change. I am thinking, since there is discoloration under the transformer's primary windings, is it possible one of them went bad?

[Behemot]

Re: LG W2242T inverter

Found the problem - one CCFL is blown/cracked. I have one bad panel from the 22" Acer I got here a thread about, will tear it apart and take from there…