Chec to see if you are getting 12vdc at the T-CON board power input pin first.
First, look at the screen in darkened room to see if you can see any slight glow from the screen, if it does, then the problem can be the T-CON (not getting the 12vdc) or the main logic board not feeding the signals to the T-CON board.
if not, next place to check will be the power supply module, make sure it has 24vdc which is required by the backlight inverter to work, the reason for not having 24vdc can be from bad DC filter caps (leaking/bulging top, high ESR) or short circuits in the inverter board which will cause the 24vdc supply to go into shutdown mode, so you may have to disconnect the power cable that feed the inverter board to see if the 24vdc will work without the inverter board hookup, you will need to apply about 3~4 amp of load to the 24vdc to make sure it can handle that. There are fuses on the backlight inverter boards also. So there is some troubleshooting you will have to do to find out the causes of the problem you are having.
If you are going to DIY and have proper tools, electronics knowledge, and know safety precaution then please read on:
Please see my basic troubleshooting guide here:
http://www.fixya.com/support/r5093881-lcd_flat_panel_tv_troubleshooting_guide
Failed TV and Monitors: http://s807.photobucket.com/home/budm/allalbums
http://en.wikipedia.org/wiki/Capacitor_plague
Parts: www.shopjimmy.com
Capacitors kit: http://lcdalternatives.auctivacommerce.com/Default.aspx he can make you a set of caps.
Testimonial: "So far so good! Almost got it figured out. Thanks!"
Here is the service manual:
http://www.my.electronicshelponline.com/...
Look at page 38, you will see MOSFET designator NV5 OUTPUT VCC_PANEL, it should have 12vdc output.
I get 2.2 VDC on each side of the P12V fuse on the TCON board to GND. So, from your help guide the TCON gets its 12V from a transistor on the main board.... the main board is bad? PS, the backlight does come on, and all the voltages from the power board look good.
TCON Board Info - LG Philips, Desc - LC370WUN-SAA1(B1), P/N - 6870C-0214A
MAIN Board Info - Silk Screen - 569HV0169B, Sticker - 6HV0216914*HP
If you look at pin 1 and pin 3 of N5, it should have 12v (if the panel uses 12, or 5v if it uses 5vdc) for powering the panel. Tell us what you read at pin 1 and 3 of NV5 first.
1 to 3: 0.0 VDC, 1 to GND: 4.5 VDC, 3 to GND: 4.5 VDC
Ok, this panel uses 5vdc power supply. Since you are getting 2.2vdc at the output of this transistor, next thing to do is to disconnct the cable to the T-CON Board and check the out put of the transistor at pin 5 ( 5, 6, 7, and 8 are all tied together) to ground and see if you get the 4.5~5vdc without load, and then use a 6v small bulb as a load, the output should stay at 4.5~5vdc. if it does then there may be short circuits in the T-CON board. Pin 2 (or 4) to ground of the transistor should also have 4.5~5v on it also, this is the bias voltage to turn on the transistor N5.
I didn't have a bulb with me, but I checked all pins to ground with and without the TCON board plugged into the MAIN board. With the plug in, I get 1=4.6VDC, 2=2.3VDC, 3=4.6VDC, 4=2.3VDC, 5=6=7=8=2.2VDC. WIth the plug out, I get 1=12.0VDC, 2=6.0VDC, 3=12.0VDC, 4=6.0VDC, 5=6=7=8=12.0VDC. (All on NV5 - main board, ref-gnd)
OK, that is strange that the voltage feeding the transistor goes up to 12vdc without the load, it is either the T-CON board has short circuits which it should have blown the fuse or burnt out this transistor and loaded down the 12vdc power supply or the 12vdc power supply is not regulating. We need to find out if this 12vdc feeding the the transistor at pin 1 and 3 ( pin 1 and 3 are tied together so you do not have to take the reading on both pins, reading on one of the pin either 1 or 3 will be good enough) is regulated and maintained at 12vdc by using a small 12v bulb rating at 1a (about 12w bulb will do) such as 1157 bulb low filament http://www.run-n-lites.com/bulbspec.asp) as a load connected to pin 1 of the transistor and measure th voltage across the bulb, it should be 12vdc. Did you verify that the 12vdc power supply of the power supply module is good?
There are 2 cables between the power supply board and the main board. On the power supply side, the silk screen says, 5,9,16,24, and 32 VDC (including all pins on the power supply board that are labeled with voltages). All of these voltages match the corresponding labels at the various points. On the main board, there are 2 pins on XV4 that are labeled 12V on the silk screen, but they are connected to the 16V pins on the power supply and do in fact measure 16 volts. I was digging through my bins and found an 1157, but I'll have to get some supplies tomorrow to rig it up as a test light safely.
OK, I will look at the diagram to find out where the 12vdc is generated from to feed this transistor. At this point we now know at least we are not getting 12vd to the T-CON board.
OK, I look at he sch and there two 12vdc source for the N5 transistor, you need to find out if R57 or R58 are installed, if R57 is installed then it is getting the VCC_5/12V from NV4 switching power supply circuits as shown on to the left side of NV5 on page 38, I believe that is where this TV gets its 12vdc for the T-CON from since the R57 is circled in the sch. Please check.
NV4 SWITCHING POWER SUPPLY circuits gets the 12vdc directly fro the 12vdc (VCC_12V) power supply module output.
I have been looking at the sch of the power supply and the main board, from what I see is this:
Pin 7 and 8 of X508 connector on the power supply board is labeled with 16v, and they go to the main board connector xv4 pin 7 and 8 on the main board and is labeled with VCC_12V (it should be 16vdc, so it may be an error on the sch. this 12vdc is fed to RV58 (0 Ohm resistor) according to you which is connected to NV5 (Source of the MOSFET pin). So it does not make sense if the TV is using RV58, then RV7 should not be on the board and either NV4. You should be able to Ohm out the connection between pin 7 of xv4 to pin 1 of NV4, it should be 0 Ohm and it must have 16vdc on that pin to ground, Please verify that for me.
By the way, at this point it does look like the problem is in the main board not supplying the voltage to the T-CON board for sure, but I am still not sure if it uses 5vdc or 12vdc for power source for sure since you found both RV57 and RV58, it should be using one or the other, not both for the power.
Well, I got this TV to fix today with the same symptom as your TV, it has backlight and sound but no pictures, justn glowing screen.
I check the LCD panel VCC, it is 12vdc not 5vdc (the 12vdc is generated by NV4 16 TO 12VDC STEP DOWN switching power supply circuits) , The 16vdc power supply does put out 16vdc, not 12vdc as you indicated, so you need to verify that again.
The cause of the no picture was in the T-CON board 3.2vdc section, the filter cap was shorted out. I was going to buy the used T-CON board from shopjimmy.com but I did not feel like spending $80~90 so I decided to find the cause of the problem.
See the pictures athttp://s807.photobucket.com/albums/yy352... NS-LCD37HD-09/
The TV isworking now and I am letting it burning in over night to make sure it will be OK. It was about three hours of works to ifnd the shorts, but it was worth it.
Interesting coincidince! The power supply board is outputting 16V, and I never found a RV57. I am a board troubleshooting novice. What was your indication that the cap was shorted? Any pointers for going through the board?
BTW---- thanks for the help!
At this point you are not getting the 12vdc at the LCD panel with the T-CON board disconnected, we need to get that 12v from the NV4 step down switching regulator (16vdc in 12vdc out) circuits.
I have 12vdc going to the T-CON board but the 3.2v stepdown switching supply was only putting out 0.5vvdc. I had to remove the caps one at a time until the short went away since there are about 10 caps total in parallel.
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1 to 3 - 0.0
1 to GND - 4.5
3 to GND 4.5
adding commas... 1 to 3: 0.0VDC, 1 to GND: 4.5VDC, 3 to GND: 4.5VDC
After much looking I couldn't find RV57 or RV58 anywhere. I finally took the board back off and looked on the back to find a square around two solder joints directly below NV5 labeled RV58. To be sure I found every other callout from page 38 for NV4 and NV5 with no issues. On a different section of the dwg, it showed the square around something as optional (RV37), but I found several others with squares around them like RV35 on the back, or RV21 in a box on the front with no real physical reference.
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