Bad T-Con replacement board?

[mikew67]

Folks,
See my earlier thread:

https://www.badcaps.net/forum/showth...787#post259787

on chasing down a no-video problem with my Vizio 42" LCD. Chased the problem down to some component on the T-Con board that was dragging down the internal 3.3v LDO output (from 12v input to the board). Started removing discretes without finding the problem. Looks like it might be a blown main IC (Philips TL2412MC). At this point I ordered a replacement T-Con board:

Desc: LC420WUN-SAA1 CONTROL PCB 2L
P/N: 6870C-4200C

Being a cheapskate I ordered a used pull from a working set. Just got it. Exactly the same silkscreened white Desc and P/N, same bottom line "4200 C1" rubber-stamped on white block, different top line ("HA924V" on the original). Put in the replacement and tried it out.
With power-on a blue LED lights after a few seconds on the T-Con PCB and fades out awhile later. Fluorescent backlight works as before. Now I DO get pixels on the screen, but the screen is almost entirely white, with a couple black lines. (Note: I did remove the connector and 3 ribbon cables from the board and put them in again, making sure everything was properly seated and latching bars were set on the two wide ones.)

A thin (single pixel) horizontal full-width black line at mid height.
A narrow (4 pixel?) vertical full-height black line at center width.
Another narrow (4 pixel?) vertical full-height black line about 4" to the right of the first.

OK, what's the diagnosis:

1. Do I have a bad or incompatible T-Con PCB?
2. Is there some firmware/parameters that need to be loaded?
3. Is there an internal reset button/factory reset?
4. What's the diagnostic USB port for anyway? Can I use it for anything useful at this point?

Thanks for all your help to get me this far.

Mike

[budm]

Re: Bad T-Con replacement board?

There should number on the barcode, those are the number you need to match with your new board. Is there barcode sticker or white sticker with number?
For example:
http://www.shopjimmy.com/lg-philipss...-con-board.htm
This is 6871l-1881a. (PCB assembly (PCBA) number)

Desc: LC420WUN-SAA1 CONTROL PCB 2L
P/N: 6870C-4200C
These are P/N of the blank PCB, they can use the same blank board and stuff them differently for LCD panel version.
Did you also check any voltage reading on the board?

[mikew67]

Re: Bad T-Con replacement board?

OK, here are pics of the original T-Con PCB and the replacement. No barcodes seen anywhere on them, but the second line of "rubber-stamped" text on the white block is the same, "4200C1" (as is the PCB silkscreen labeling), but not the first line. Is that significant?
WRT voltages, the replacement measures:

At the F1 fuse input: 11.94v
At the VCC test point: 3.35v
At the VCC1.8 test point: 1.84v

What now?

Mike

PS: Note the browning around the VCC supply on the original, probably by whatever shorted out the VCC1.8 bus (as noted by you in the earlier thread).

[mikew67]

Re: Bad T-Con replacement board?

BTW, the bottom connector with "squeeze release" buttons on the bottom is problematic. The plastic buttons popped off in one or another of my attempts to remove the plug and reinsert. Is it easy to mangle the pins on this plug or socket? (I know enough to be gentle and careful about alignment when inserting the plug :-)

Now that the plastic buttons are missing, how can I release the plug? Insert a small flat-blade screwdriver from above to release the metal-hook catches?

Mike

[budm]

Re: Bad T-Con replacement board?

1.94V for the B+ is way too low, panel will use either 5V or 12V to run the T-CON board. Are you sure you are getting 1.94V feeding it?. You need to trace out the wires that are connected to the fuse and then trace them back to the main board, then disconnect the cable and then read the voltage on the main board side to see what voltage you are getting without having the T-CON board connected, we are trying to find out if the voltage source on the main board is bad or the T-CON is loading down the main board.
What is the LCD panel model number and the Rev number as printed on the sticker?
Yes, it is real easy to damage the socket/connector.

[mikew67]

Re: Bad T-Con replacement board?

Sorry, typo (corrected later). The supply voltage is 11.94v.

Figured out how to actuate the connector latches. Removed it and examined both plug and socket. All OK (a couple plug pins aren't populated). Re-inserted. Same result. Plain white screen with black lines.

Mike

[budm]

Re: Bad T-Con replacement board?

OK, what do you get at test points VCC by L2, VGH, VGL (they are around the right side of the board)?

[mikew67]

Re: Bad T-Con replacement board?

Any idea where the panel ID sticker is? (Might be under the sheet metal that the power board and main video board are mounted to. Shining a flashlight in the 1/4" gap between that and the panel sheet metal back, I "think" I see a sticker. If so there are about a dozen screws to remove to check

Mike

[mikew67]

Re: Bad T-Con replacement board?

Measured T-Con voltages:

V(F1)   11.88v
VCC    3.347V
VCC1.8  1.840V
VGL    -3.527V
VGH   25.07V

Are these OK?

Mike

[tom66]

Re: Bad T-Con replacement board?

VGL looks not low enough, should be around -10V. However, it should simply cause a poor picture - not all white.

[budm]

Re: Bad T-Con replacement board?

I agree with Tom on what you are reading. At this point we do not know if this USED board is good.

[mikew67]

Re: Bad T-Con replacement board?

Thanks, tom66 and budm, for the advice.
So if I disconnect the 2 LCD panel ribbon cables and power-up, will that harm anything? If I then check VGL without a load and it's now ~-10v, does that then point to a bad panel? (Sigh....)
BTW, is VGL a bias voltage, i.e., very low current requirement?
Thanks, as always,
Mike

[budm]

Re: Bad T-Con replacement board?

It sounds like the you may have bad Row/Column TFT driver IC's which are on the flexible PCB on the top and on the side of the LCD Panel that load down the VGL.

[mikew67]

Re: Bad T-Con replacement board?

And, where can I find tech info on T-Con boards in general, and this Philips board in particular?
TIA,
Mike

[mikew67]

Re: Bad T-Con replacement board?

It sounds like the you may have bad Row/Column TFT driver IC's which are on the flexible PCB on the top and on the side of the LCD Panel that load down the VGL.

Thanks, budm. If that's the case, am I "dead in the water", i.e., beyond a practical repair?
Mike

[mikew67]

Re: Bad T-Con replacement board?

OK, did another diagnostic. Remember that Tom66 said that VGL should be around -10v, when it's actually around -3.5v. Budm suggested that a (downstream, on the flex cable) row/column driver IC might be bad.
So I measured VGL and VGH again, with both panel flex cables connected (as before), with first one, then the other disconnected, and finally with both disconnected. Here are the results:

• VGH stayed at approx. 25v in all cases.

• With both flex cables connected, VGL = -3.5v, as noted before.

• With left flex disconnected, VGL = -3.346v, about the same as before. Display is now gray on left half, white on right half. The vertical black bars are gone!

• With right flex disconnected, VGL = -3.390v, about the same as the last case. Display is now a mirror image of the last case, white on left, gray on right BUT the black vertical bars at the middle and on the right are back.

• Final case, both flex cables disconnected, VGL = -5.90v(!) and, of course, the display is blank (black).

Result: I'm discounting a bad row/column driver IC and strongly suspecting a bad T-Con card, in particular, where VGL is generated. Without a schematic, looks like I'll have to trace out that section. (Presume it's some sort of diode/cap driven charge pump.)
Hold the presses!
Sigh... Looks like the VGL behavior I see is correct (at least as far as the T-Con board is concerned). If you look at TI document for the TPS65162 chip eval board, specifically the schematic on page 7, you'll see that the rated output of the VGL charge pump is -5v at 50 ma. The actual T-Con circuit is consistent with the TI schematic, with the series cap consisting of two 840 nf caps in series = 420 nf vs 470 nf, and the addition of R190, a 100 ohm series resistor. The voltage across R190 is ~2v, which directly accounts for the VGL behavior noted above and implies a current of 20 ma., well within the IC's limits.
So it looks like chasing VGL is a dead end. Any suggestions now? Any idea what the many other test points on the PCB, e.g., VDD, should measure?
Thanks,
Mike

[tom66]

Re: Bad T-Con replacement board?

Have you checked the outputs of the two switching converters - near the inductors (L1, L2), test the brown caps.

[mikew67]

Re: Bad T-Con replacement board?

Tom, see the voltages in #9 msg in this thread. I presume those measurements are what you're referring to.

Mike

[tom66]

Re: Bad T-Con replacement board?

It seems everything is working well but you're still not getting a picture. I suspect the main IC may also be toast on this one too, but I can't be sure...

[stretch.0790]

Re: Bad T-Con replacement board?

i there i am new to the forume is there anybody that can help me i have not long bought a lg 37lh5000 lcd tv when i first bought it had a black screen with about a cm of the screen showing at the top i have read all the forums and they all said tcon board but i have replaced the tcon board, ribbons and the lvd cabels no all i get is a white screen even if i put the old board back in still white screen i have even checked all the caps on psu all ok can any body out there help me and tell me what the problem is as i bought this tv for my sons room