LG W2234S Repair Help...

[docksteaderluke]

Re: LG W2234S Repair Help...

I'm sourcing some caps right now. I'll give some of your other suggestions a try when I get home from work.

When rigging a resistance load (to the inverter outputs I'm assuming) what sort of resistance is needed?

[PlainBill]

Re: LG W2234S Repair Help...

Here's my hookup:

One thought. I'm not clear on the usual voltage range on the brightness control. A few ideas.

1. On the brightness voltage divider, remove the resistor going to + 5V, this should pull the input down close to 0 V, which I think is maximum brightness.

2. Vary the voltage to the brightness control input.

3. Check the voltage at the brightness control input of a working monitor.

4. It's time to rig a resistance load and see if the power supply is stable.

5. Just go ahead and replace all the caps in the power supply.

PlainBill

[docksteaderluke]

Re: LG W2234S Repair Help...

Here's my hookup:

[PlainBill]

Re: LG W2234S Repair Help...

Another idea - check the voltages with this rig hooked up and see if they are stable.

PlainBill

[docksteaderluke]

Re: LG W2234S Repair Help...

Yes... the CCFLs are connected to the inverter outputs.

I'll check the grounding when I get home from work... but like you said... it seems like a long shot.

I'll even snap you a photo of the setup once I'm home so you can see for yourself how I've got it hooked up.

[PlainBill]

Re: LG W2234S Repair Help...

The LDVS cable IS disconnected from the LCD panel.

Excuse me for asking this, but there has been a lot of stupidity going on here lately (not all of it on the other side of my keyboard) , but are the CCFLS connected to the inverter outputs? And as a long shot, make sure the power supply ground is connected to the metal of the LCD panel.

PlainBill

[docksteaderluke]

Re: LG W2234S Repair Help...

The LDVS cable IS disconnected from the LCD panel.

[docksteaderluke]

Re: LG W2234S Repair Help...

Those voltages seem about right, yet the backlight still isn't on.

[alexanna]

Re: LG W2234S Repair Help...

(I should mention the LCD panel gets it's power through the ribbon cable from the signal card). The signal card itself draws relatively little power.
PlainBill

PlainBill, when you mention about the LCD panel getting its power through the LDVS cable, it reminds me of a nightmare I had.
Not knowing what else to call it, a “trace” on the LDVS flat cable had come loose and the 5V power was touching a ground “trace” on the flat cable.
I seriously doubt that what's wrong with this monitor, but if the cable was disconnected from the logic board it would rule it out.

[docksteaderluke]

Re: LG W2234S Repair Help...

That's what I understood, but I just wanted to be sure. With both resistor networks hooked up I still get a black screen and no power LED (obviously since it's not hooked up). The measurements I get are:

Brightness - 1.04V
On/Off - 3.09V

I understand what you're trying to do and I appreciate your help very much. I have an electronics background but just no experience with fixing monitors specifically. Again... thanks for your help!

Luke

[PlainBill]

Re: LG W2234S Repair Help...

Should I have both resistor networks hooked up at the same time?

Yes. AGAIN, the object is to simulate the control signals from a signal card. Pin 5 at about 3.3 volts should turn on the inverter, pin 6 at about 1 volt should set the brightness.

As a simple explanation, when the power supply comes up, the processor on the signal card initially is held in a reset state until the power supply is stable. It then initializes itself and sets the LCD panel and backlights to the state they were in when power was lost. If the monitor was on when someone pulled the plug, the backlights and LCD panel will be turned on. My working theory was the power supply would shut down momentarily when the load of the panel and the inverter was applied. This (of course) would cause the processor to reset, and you get the on-off pattern you described.

USUALLY if the load is reduced the pattern will be broken. The two biggest loads on the power supply are the inverter and the LCD panel (I should mention the LCD panel gets it's power through the ribbon cable from the signal card). The signal card itself draws relatively little power. Removing those loads still resulted in the same flashing problem.

One of the fundamental concepts of troubleshooting is you must identify the cause of a problem before you can fix it. I COULD have had you replace all the caps on the power supply, but I would rather have you do that only if I am confident it will fix the problem. It seems to me temporarily using four resistors to force the inverter on is faster and less expensive than replacing more than half a dozen caps.

PlainBill

[docksteaderluke]

Re: LG W2234S Repair Help...

Should I have both resistor networks hooked up at the same time?

[PlainBill]

Re: LG W2234S Repair Help...

Should the signal card be plugged back in for this?

I plugged it back in. Soldered up the resistor network and plugged both a VGA cable in and the power cable in.

What I got was a white screen for about a second, then the LG logo for a second or so and then off for a second or two. Repeat.

If I didn't have the signal card connected I couldn't see anything on the screen... it was just black and with no backlight (or power LED) either.

No, I didn't want the signal board hooked up. The resistor network was to turn the inverter on. Perhaps it would be a good idea if I also had you feed a voltage into the brightness input - pin 6 of CN801. The idea was to see if the inverter would work properly if it were fed appropriate voltages without the influence of the signal card.

Make up a voltage divider similar to the one driving pin 5. But this one should have a 3.9K resistor going to 5V and a 1K going to GND. The common point would go to pin 6 of CN801. And while power is applied, measure the voltages at pins 5 and 6, if you would.

PlainBill

[docksteaderluke]

Re: LG W2234S Repair Help...

Should the signal card be plugged back in for this?

I plugged it back in. Soldered up the resistor network and plugged both a VGA cable in and the power cable in.

What I got was a white screen for about a second, then the LG logo for a second or so and then off for a second or two. Repeat.

If I didn't have the signal card connected I couldn't see anything on the screen... it was just black and with no backlight (or power LED) either.

[PlainBill]

Re: LG W2234S Repair Help...

Ok... gotcha. Did it and still no change.

I'm getting very close to recommending a total recap, but let's try this first.

Work from the back of the power supply / inverter.

1. Take the two resistors and twist one lead of each together, but staggered so one lead extends beyond the other.

2. On the power supply / inverter lay a strip of tape - electrical, duct, or surgical between the pins of CN801 - the output connector - and the edge of the board.. You want to avoid accidental shorts between the leads of the resistors and the ground bus.

3. Solder the two twisted leads of the resistor together.

4. Solder the longer lead of the twist to pin 5 of CN801.

5. Solder the free end of the 3.3K resistor to pin 3 or 4.

6. Solder the free end of the 2.2 K resistor to pin 1 or 2.

7. Check your work; make sure you don't have any shorts.

8. Lay the power supply / inverter on an insulating surface - a piece of dry cardboard is fine.

9. Plug the 4 CCFL leads into their sockets.

10. Take a deep breath, cross your fingers, and plug the power cord into the power supply.

At this point the CCFLs should come on. If they are flashing at about the same rate as when the signal card was plugged in, the problem is the power supply / inverter card. If they are on steadily, the problem is the signal card.

Disconnect the power cord, unsolder the resistor network, and report the results.

PlainBill

[docksteaderluke]

Re: LG W2234S Repair Help...

Ok... gotcha. Did it and still no change.

[PlainBill]

Re: LG W2234S Repair Help...

Yeah, what Alexanna said.

PlainBill

[alexanna]

Re: LG W2234S Repair Help...

Disconnect the LDVS cable from the logic board to the LCD panel

[docksteaderluke]

Re: LG W2234S Repair Help...

When you say to try it with the LCD panel disconnected do you mean the VGA cable or the cables going from the inverter side of the power supply to the lcd?

Yes I have a 3.3k and a 2.2k right here.

[PlainBill]

Re: LG W2234S Repair Help...

Decided to wait on soldering L802 back on. Soldered the second cap in parallel with the first and no difference (on and off in 2-3 second intervals). Still measuring about 160.5V across them.

With the second cap wired in I could hear a slight charging then tick (discharge) of the caps.

I'm trying to think of something else I can use as a load across c819. What kind of resistance are we talking about here (for the load)? I've got tons of resistors lying around that might do the trick.

One idea. Obviously you have the signal card connected. Do you also have the cable from the signal card to the LCD panel connected? SOMETHING is overloading the power supply or a power rail. I've been assuming (I know, I KNOW!!) it's the inverter, or a bad filter cap. If you haven't already done so, try this with the LCD panel disconnected.

Do you have a pair of resistors in roughly a 3:2 ratio? For example, a 3.3K and a 2.2K would be ideal.

PlainBill