LG 39LN5700, TV shuts off immediately after being turned on, no picture

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

What resistance do you have between pin 3 and GND?

I think I may have made a mistake in my resistance measurements in the last post, I'll have to double check those.

For the IC202 pins:

Between IC202 pin 3 SW (red) and Chassis GND (black): 131k
Between IC202 pin 3 SW (black) and Chassis GND (red): 116k

Between IC202 pin 3 SW (red) and IC202 pin 4 GND (black): 135k
Between IC202 pin 3 SW (black) and IC202 pin 4 GND (red): 120k

Between IC202 pin 4 GND (red) and Chassis GND (black): 2.2k
Between IC202 pin 4 GND (black) and Chassis GND (red): 2.2k

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

Yeah, I made a mistake on the resistance measurements in post #19, the zero measure was actually from pin 5 instead of pin 4 (I had the sides of IC202 reversed).

Corrected:

Between IC202 pin 5 FB (red) and Chassis GND (black): 0
Between IC202 pin 5 FB (black) and Chassis GND (red): 0

[budm]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

That Gnd reading is not right, it should be less than one Ohm if the board is mounted to the metal chassis because the circuit Gnd of the cold side is tied to the chassis.

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

Triple checked, these should all be correct:

Voltage:

IC202 pin 7 to Chassis GND: 4.1 v
IC202 pin 2 to Chassis GND: 23.6 v


Resistance (Ohms):

Between IC202 pin 3 SW (red) and Chassis GND (black): 3.46 k
Between IC202 pin 3 SW (black) and Chassis GND (red): 2.13 k

Between IC202 pin 4 SW (red) and Chassis GND (black): 0
Between IC202 pin 4 SW (black) and Chassis GND (red): 0

Between IC202 pin 5 SW (red) and Chassis GND (black): 2.31 k
Between IC202 pin 5 SW (black) and Chassis GND (red): 2.31 k

I can't believe I made the same mistake twice in a row with IC202. Think I better call it a day...

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

Update! After a lot more searching, I think I found a better schematic for the Power Supply Board. This is for the LGP42-13PL1 PCB and not my TV's LGP3739-13PL1, but it should be very close to (if not the actual) schematic.

The important thing to note is the difference from the last schematic I posted (for a 32" LG). From the looks of it, the 12V supply may actually originate before IC202. Unlike the last schematic, this one has 12V going into IC202 instead of 24V (it also has C225 on the far right, which was missing from the last one).

I've attached a new picture of the IC202 part of the schematic for reference, the full schematic this was taken from is located here.

[budm]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

If you check the resistance between pin 2 of the IC and the 24V pin of the connector, you will find that it should have <1 Ohms between the two point.
The bottom line is that your measurement in post 24 also confirm that the IC is fed by 24VDC and the circuit is not working to produce the 12V output.
i suspect that you have bad IC.

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

Would it just be IC202 that's bad in this circuit? I did verify a resistance under 1 Ohm between pin 2 and the 24V pin 9 on P201 (18 pin connector), but I also wasn't seeing voltages anywhere close to 12V at R238 as shown on the left side of the schematic.

[budm]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

Would it just be IC202 that's bad in this circuit? I did verify a resistance under 1 Ohm between pin 2 and the 24V pin 9 on P201 (18 pin connector), but I also wasn't seeing voltages anywhere close to 12V at R238 as shown on the left side of the schematic.

Yes, more likely.

'but I also wasn't seeing voltages anywhere close to 12V at R238 as shown on the left side of the schematic'
Are you talking about the DCV on pin 7 (EN) PIN? That pin only requires 1.4V (minimum is 0.70V) to turn on the IC per spec sheet on page 44, you have 4.10VDC per post 24.

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

Are you talking about the DCV on pin 7 (EN) PIN? That pin only requires 1.4V (minimum is 0.70V) to turn on the IC per spec sheet on page 44, you have 4.10VDC per post 24.

Yes, that and R238, but I forgot I had a 23.7V reading on J44 (which I think directly connects to either R238 or C217).

[budm]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

As long as EN (ENABLE) PIN gets minimum Voltage of 0.70V (0.7~2.1V, 2.1V is worst case) the IC should operate, that pin is connected to the Voltage divider resistor to drop the Voltage feeding to the EN pin.

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

As long as EN (ENABLE) PIN gets minimum Voltage of 0.70V (0.7~2.1V, 2.1V is worst case) the IC should operate

2.1V is worst case? I'm reading 4.1V at IC202's pin 7 using the TV's chassis as a ground when the TV is plugged in.

On a different note, could NR887D be used as a substitute for NR891D? I'm not seeing any reputable vendors selling NR891D, but NR887D is widely available.

[budm]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

'2.1V is worst case?' That means some batches of the IC may require at least to 2.1V for it to turn on, so 4.1V is more than enough.

'NR887D be used as a substitute for NR891D' the problem with that spec sheet is that the Vin MAX is shown as 20V but the spec sheet does not show what the different between the 887 and the 891 is so I would the same P/N as the original.

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

Very delayed update. I did some more research and it seems NR887D cannot be used as a substitute for NR891D (different voltages), so I went ahead and ordered NR891D from a Chinese supplier. There was some issues with the order and it took well over a month to finally get the part, but I did eventually receive it.

Well, I just installed it today and it seem like NR891D was the sole source of my problems. The TV (sans backlight) now appears to be working again and an image was visible when I held a flashlight up to the screen. Success!

I tested the old LEDs and I think only 1 of the 8 strips actually lit up, I might have damaged them when removing them though. Hopefully all that's left is ordering some new backlight strips and maybe some kapton tape.

Anyone know if the strips sold on ShopJimmy come with adhesive?

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

Well, just installed new backlights and put the TV back together just enough to get a pitcture and... well... yeah...



Vertical lines on half the screen.

I'm guessing it's either a problem with one of the ribbon cables or one of the tab ICs. So frustrating, the panel seemed to be working just fine when I did the flashlight test a few hours ago.

edit: Looks like it's the left ribbon cable. Some pins seem to have gotten chipped near the top:



Anyone know a "simple fix" that's applicable here or does the cable and/or something else need to be replaced?

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

Okay, so I thought I would try an experiment, I cut the cable just below the damaged pins and clipped off the 2 side tabs (see the red lines in the first photo below). After a very careful re-insertion.... No more lines!!!

Anyone know if there would be any problems / side effects with this repair I should worry about or am I in the clear?

[jiroy]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

Okay, so I thought I would try an experiment, I cut the cable just below the damaged pins and clipped off the 2 side tabs (see the red lines in the first photo below). After a very careful re-insertion.... No more lines!!!

Anyone know if there would be any problems / side effects with this repair I should worry about or am I in the clear?

This is a normal procedure in such cases . you're fine .

[pgram]

Re: LG 39LN5700, TV shuts off immediately after being turned on, no picture

This is a normal procedure in such cases . you're fine .

Thanks for the info. I didn't think it would be a problem, but wanted to make sure.