LG W2234S Repair Help...

[docksteaderluke]

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

[PlainBill]

Originally Posted by docksteaderluke

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]

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

[alexanna]

Originally Posted by PlainBill

(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]

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

[docksteaderluke]

The LDVS cable IS disconnected from the LCD panel.

[PlainBill]

Originally Posted by docksteaderluke

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]

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]

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

PlainBill

[docksteaderluke]

Here's my hookup:

[PlainBill]

Originally Posted by docksteaderluke

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]

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]

Originally Posted by docksteaderluke

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?

I was referring to the power supply outputs. A 2 - 3 amp load in the 5V supply and optionally 1 amp on the 12V would be my suggestion.

PlainBill

[docksteaderluke]

Varying the brightness voltage from ~0V to ~5V does not seem to have had any effect. Waiting on the caps to ship. When they arrive I'll swap 'em out and get back to you. So, does it seem like the consensus is that it's the power supply. I can probably order a new one for $35ish (ebay!) if the recap doesn't work.

[docksteaderluke]

Hey guys. My replacement caps finally arrived! I swapped out all of the electrolytic caps and tried hooking the monitor up again and it does the same thing! Looks like it wasn't any of those caps after all. Could it be the EEPROM chip on the logic board? I had read in another thread that it can sometimes get corrupted.

Help!

[alexanna]

Originally Posted by docksteaderluke

Hey guys. My replacement caps finally arrived! I swapped out all of the electrolytic caps and tried hooking the monitor up again and it does the same thing! Looks like it wasn't any of those caps after all. Could it be the EEPROM chip on the logic board? I had read in another thread that it can sometimes get corrupted.

Help!

I wonder if you would satisfy my curiosity, on some things.
Could you verify the voltages you see on the 6 pin connector from the power supply to the logic board?
Have the voltmeter hooked up and then power the monitor on, and measure when the backlights or on and again when they turn off.

[docksteaderluke]

Here are the measurements I get:

Pin (Label) - Backlight ON, Backlight Off

1/2 (Power) - 4.98V, 5.09V
5 (On/Off) - ~2.9V, ~0.4V
6 (Brightness) - <0.2V, 3.3V

[alexanna]

Originally Posted by docksteaderluke

Here are the measurements I get:

Pin (Label) - Backlight ON, Backlight Off

1/2 (Power) - 4.98V, 5.09V
5 (On/Off) - ~2.9V, ~0.4V
6 (Brightness) - <0.2V, 3.3V

Something strange is going on.
As always I have some more questions.
Are pins 3 and 4 ground pins?
You could verify this by touching the negative lead of the DMM to pins 3 and 4 and touching the positive lead to pin 1 if they really grounds you should see 5 volts.
Check these when you see the lights on and again when off.

[docksteaderluke]

Yes... 3 and 4 are ground pins and the measurements I gave you were with respect to those pins. So... the first measurement was between pins 3/4 and 1/2 which yielded 5V as expected. And you can see that when the backlight is on the load is slightly higher and the voltage drops a wee bit (4.98V when the backlight is on as opposed to 5.09V when the backlight is off) which is expected. That much seems right.

[alexanna]

I found this manual in another thread,See if it looks like yours