2 sec to off, 22" LG Flatron, W2252QT

[Kwolff]

Hi guys,

First post, and first time trying to fix one of these.

Last night, my screen went dead during a movie i was watching, doing the whole on for 2 seconds, then off for several minutes, and after searching around the internet, i came upon this website.

Being at least a little knowledgeable, and always wanting to learn new things, i took it upon myself to start pulling this guy apart to take a look at the board.

The first thing i noticed is the burn marks on the Q305 Transistor, and checking the ohm value, it came up as 126 and 72. Shouldn't they be the same, or depending on what type they are, will the NPN junction show two different values?

Checking the capacitors, they all look like they are in good shape, and all built by reputable companies, Rubycon and Nichicon. No identifiable expansion or leaking, and no signs of heat or electrolyte.

When I checked the high voltage transformer, there was a huge difference between the two secondaries, with one being 1798 ohms, and the other being 971 ohms. Is this a clear indication that the transformer is cooked?

I checked the other transistors on the board, and none of them showed a short, and the ohmic value seems consistent, so i don't think there is an issue there. The diodes seem OK, except for one, D105, which shows ohms both ways, but may be due to the resistor in parallel to it.. R113?

Hope this is a good start. Any more ideas before i pull the transformer and get a new one?

[Th3_uN1Qu3]

Re: 2 sec to off, 22" LG Flatron, W2252QT

If the diode is in parallel with the resistor it's normal to have a value both ways. But do lift one side of the diode up to make double sure.

As for the transistor, please mention the exact way that you checked it. A transistor shouldn't show anything on the ohms scale (except for certain Darlington ones). Test it using the diode scale. If it shows values both ways on any two legs, and there are no resistors in parallel, the transistor is shot. Do try replacing it before doing anything to the transformer.

[Kwolff]

Re: 2 sec to off, 22" LG Flatron, W2252QT

Forgot to post pictures.. lol

[retiredcaps]

Re: 2 sec to off, 22" LG Flatron, W2252QT

When I checked the high voltage transformer, there was a huge difference between the two secondaries, with one being 1798 ohms, and the other being 971 ohms. Is this a clear indication that the transformer is cooked?

I beleive it should be within 3% of each other. You may be able to find a replacement at

http://lcdparts.net/T.aspx

If you find the correct replacement, it will show the "correct" ohm value for the secondaries.

The diodes seem OK, except for one, D105, which shows ohms both ways, but may be due to the resistor in parallel to it.. R113?

Yes, the other components around D105 could be affecting the readings.

[retiredcaps]

Re: 2 sec to off, 22" LG Flatron, W2252QT

Forgot to post pictures.. lol

Which, at the moment, i can't figure out how to re-size them to a smaller file size. Have to go out for the evening, any help?

I believe this site allows 2000x2000 pics with up to 1.7MB per pic.

I use

http://www.lview.com/download.htm

to resize. It is free.

[Wrog]

Re: 2 sec to off, 22" LG Flatron, W2252QT

Forgot to post pictures.. lol

Which, at the moment, i can't figure out how to re-size them to a smaller file size. Have to go out for the evening, any help?

Use Microsoft Office Picture Manager (if you have Office 2003) or download Paint.NET (it's free) and use the resize function.

[Kwolff]

Re: 2 sec to off, 22" LG Flatron, W2252QT

Done, thanks for quick reply, ill check everything when i get home later.

[jetadm123]

Re: 2 sec to off, 22" LG Flatron, W2252QT

Hi guys,


The first thing i noticed is the burn marks on the Q305 Transistor, and checking the ohm value, it came up as 126 and 72. Shouldn't they be the same, or depending on what type they are, will the NPN junction show two different values?



When I checked the high voltage transformer, there was a huge difference between the two secondaries, with one being 1798 ohms, and the other being 971 ohms. Is this a clear indication that the transformer is cooked?

I checked the other transistors on the board, and none of them showed a short, and the ohmic value seems consistent, so i don't think there is an issue there. The diodes seem OK, except for one, D105, which shows ohms both ways, but may be due to the resistor in parallel to it.. R113?

Hope this is a good start. Any more ideas before i pull the transformer and get a new one?

From the looks of it, Q305 and Q308 are possibly mosfets that are probably used to generate the AC sine wave voltage required by the inverter transformer to generate the high voltage required by the CCFL tubes. They DO run hot, hence, the discoloration. However, it's the way you checked Q305, can you describe how you placed the probes when you did the testing? The 2 values you listed are meaningless unless you tell us how you placed the probes. There are 3 leads on Q305, right? Q308 also needs to be tested, since it works in conjunction with Q305. The same logic also applies when you test the transformer secondaries.

[Kwolff]

Re: 2 sec to off, 22" LG Flatron, W2252QT

Th3_uN1Qu3 = I went back, and I had not checked the transistors with the diode check, something I should have picked up on. After diode checking them all, they all measured out fine. I did check the diodes with the diode setting though. The high voltage transformer I checked on a 2000 ohm scale, from EL26 to 1 and getting a reading of 1798 ohms, and from EL27 to 4 with a reading of 971 ohms.
Looking into the transformer manufacture, they either don't carry it anymore, or don't list the specs on their website, and the only one for sale is on E-Bay. Any place i can look up the specifications of the high voltage transformer?

[Toasty]

Re: 2 sec to off, 22" LG Flatron, W2252QT

A bad or failing tube will cause this too. Will cause the over current protection (OCP) to kick in. How old is the set? Have you opened the panel to inspect the tubes to see if they look burnt? As in regular size fluorescent tubes, if the ends are darkened, they are likely bad.

[Kwolff]

Re: 2 sec to off, 22" LG Flatron, W2252QT

A bad or failing tube will cause this too. Will cause the over current protection (OCP) to kick in. How old is the set? Have you opened the panel to inspect the tubes to see if they look burnt? As in regular size fluorescent tubes, if the ends are darkened, they are likely bad.

The panel is only 2 years old, but has some pretty hard usage, since I game a lot. The ends of the bulbs do look burnt, just like a regular compact florescent, and might be the problem. I'll replace them and see what happens.

When I first tried testing the CCFL's, I plugged them into the power circuit, but didn't hook up any of the image circuits. Since the CCFL's didn't turn on at all, does this mean that you need a valid input for them to turn on? Or even flicker? To me, if you have your power circuit cycling your tubes, they should try turning on regardless if your hooked up to a computer tower or not. Must be doing something wrong.

[Toasty]

Re: 2 sec to off, 22" LG Flatron, W2252QT

How burnt? A little bit is normal. A blackened 1/4" end is not. Pic?
Can you connect the whole thing together with the panel opened and see if one tube doesn't fire as well, or fires TOO well? Check the solder connections to them as well.

Please show what your mean by "burn marks on Q305". Do you mean on the transistor package surface or discoloration of the board it's mounted to, or...?

The fact that the unit works for a few seconds and shuts down is what is pointing me towards an over-current (or voltage) condition on the inverter. Can you swap connections to the tubes? If so, does anything change?

It very well could be the transformer. Without another unit for comparison it would be impossible to verify that it is good/bad/weak. Check to see if you get any resistance readings from one secondary to the other, and any from secondary to primary. You will have to pull the transformer to get the readings. Do you have a ring tester or can you perform such test? Oscilloscope?

Yes, there is a signal that comes from the video board to turn on the inverter. It actually goes from the inverter to the video and back. When you press the on button, the video board sends a signal to the inverter. I think it's typically a 5v line. One of them is also a lower volt line that controls the brightness.

Toast

[901THECAT]

Re: 2 sec to off, 22" LG Flatron, W2252QT

I have this exact same model(lg w2252qt) and I disassembled it and got to the power supply board and noticed it had 4 blown caps
So I could get the right specs off of them I removed all 4 at 1 time not taking in account the order I took them out
There is 2 1000uf 16 volt and 2 470uf 35 volt and I was wondering if it matters the order I solder them back onto the board
I know that the - and the + have to be correct but what about the position of the 2 different types of caps

[PlainBill]

Re: 2 sec to off, 22" LG Flatron, W2252QT

I have this exact same model(lg w2252qt) and I disassembled it and got to the power supply board and noticed it had 4 blown caps
So I could get the right specs off of them I removed all 4 at 1 time not taking in account the order I took them out
There is 2 1000uf 16 volt and 2 470uf 35 volt and I was wondering if it matters the order I solder them back onto the board
I know that the - and the + have to be correct but what about the position of the 2 different types of caps

They have to go into the proper positions. The 16 volt caps are for the 5V supply, the 35 V caps for the 12 volt supply.

Attach a sharp, well lit picture of the bottom side of the supply; it should be possible to determine which is which. Do not post inline; use 'Manage Attachments' - below the text entry area.

PlainBill

[Rtech]

Re: 2 sec to off, 22" LG Flatron, W2252QT

Sometimes on Boards, circles are shown on the top where the caps go,and they usually vary in size dependant on the Cap value...ie 1000Uf will have a larger circumference then the 470uF.......but in your case due to the Cap Voltages being so different 16v and 35v, it may not apply !!!!

[901THECAT]

Re: 2 sec to off, 22" LG Flatron, W2252QT

I got a really great closeup of the ones I;m going to replace but I'm a newbee and don't know how to do it
I read the tutorial but I don't have anything saying manage attachments

[Rtech]

Re: 2 sec to off, 22" LG Flatron, W2252QT

Click the 'Go Advanced' below, and then click Manage Attachments.

[901THECAT]

Re: 2 sec to off, 22" LG Flatron, W2252QT

There is nothing listed as 'manage attachments'
Is it because I never had an original post of my own and only replied to other members post?
Here are my choices when I hit 'go advanced'
Miscellaneous Options
Automatically parse links in text
Disable smilies in text
Attach Files
Valid file extensions: bmp doc gif jpe jpeg jpg pdf png psd rar txt xls zip
Thread Subscription
Notification Type:
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If you like, you can add a score for this thread.
It won't let me click on any icons above the letter head either

i

[alexanna]

Re: 2 sec to off, 22" LG Flatron, W2252QT

see if this helps.
YOU will need to resize your photos to under 2000x2000 Pix. and click on manage attatchments

[901THECAT]

I finally figured it out
the ones circled are the ones I need to replace
Two of them are 1000uf 16 volt and the other 2 are 47OUF 35 VOLT
Is there anyway to know which go where by the numbers on the board?