LG 42LH50 - no video, backlights on, no USB voltage

[ngth82]

Hi all -

I picked up an LG 42LH50 last week. As far as I can tell, the TV has backlights ON, but there is no video, and no onscreen display when i press the menu (or any of the buttons on the side of the unit or via the remote). Pictures are attached.

I see that someone named "Shinju" asked questions about the same model TV in June of 2014, but there was no resolution and the thread just died. Unfortunately, there isn't much information about others trying to fix this particular TV model number.

I found the service manual for the TV, which included the schematics. ()

Initial Observations:

• No burnt looking parts on the board
• TV powers on - backlights are on
• No USB voltage is observed (+5V) - i was going to use my Chromecast as a video source
• No on screen display when changing inputs/volume/channel/menu
• Fuses are good - continuity on the power board main fuse as well as the T-CON
• All voltage rails appear to be present between the power board and main board (+5V, +12V, +24V).
• The TCON has a LED on it. It is blue while the unit is starting up, then turns off (LD1)
• The main board has a constant amber/red LED (LD2400). The schematics show this as a dual color LED, but only the green LED part is hooked up, the red is not used... weird.
• The main board processor comes out of reset; you can "see" the TV reset when you press the button SW101.

Power board to main board connector
The following is a list of pins + expected voltages/name and actual voltages on the 24 pin connector between the power board and the main board. Things seem to be okay from the power/main board interaction... voltages below.

PIN -- PIN NAME -- VOLTAGE

• Pin 01: no connect, n/a
• Pin 02: PS_ON, 4.45V
• Pin 03-06: GND, 0V
• Pin 07-10: 5V (on silkscreen), 5.15V (actual)
• Pin 11-12: GND, 0V
• Pin 13-14: 12V (on silkscreen), 12.4V (actual)
• Pin 15-16: GND, 0V
• Pin 17-18: 21.4V (on silkscreen), 24V per schematics,, 20.84V (actual)
• Pin 19: no connect
• Pin 20: INV.ON, 4.65V (actual)
• Pin 21: A.DIM, 2.8V (actual)
• Pin 22: ERR OUT, 0V (actual)
• Pin 23: no connect, n/a
• Pin 24: PDIM, 3.35V (actual) - value dependent on the OSD backlight setting.

Main board switching voltage regulator (SVR) and linear voltage regulator (LVR) measurements
I went through the service manual and found all of the SVR and LVRs. The measurements at the SVR output inductor or the LVR output pins are as follows. Note this list does not include the rails that are individually filtered by ferrite beads or with over current monitoring circuits... see the next section below for that info.

LOCATION - RAIL NAME - Measured (actual)

• General 12V -- i see 12V on the input side of most of the big caps
• General 21.4V -- I see ~21V around the audio amp portion of the main board
  
• IC2300: +1.8V_AMP (audio amp), 1.8V (actual)
• IC2403: +3.3V (general, and DDR3 controller), 3.3V (actual)
• IC2400: +1.2V_CORE (BCM processor core voltage), 1.2V (actual)
• IC2402: +1.8V_MEMC (DDR3 voltage), 1.8V (actual)
• IC2401: +5.0V_USB_FAN (USB), 5V (actual)
• IC2408: DDR_VTT (DDR termination voltage), 0.9V (1/2 of the DDR voltage which is 1.8V)
• IC2407: +2.5V, 2.5V (actual)
• IC2406: +1.26V core, 1.26V (actual)
• IC2405: +1.8V_DDR (for DDR3), 1.8V (actual)
• IC3400: +5V USB current limit, 0V (actual)
  - Interestingly, the +5V output on the USB current limiter (MIC2009YM6) is 0V. Vin=+5.0V_USB_FAN is sitting at 5V.
  - The enable pin on this device is low, and that is controlled by the BCM processor. Not sure why that is the case yet.

Cables/panel testing
I've tried disconnecting and reconnecting each cable to make sure it is seated in place. Here are some observations from that:

• There is no panel test jumper like other LG TVs.
• I reseated the cable between the power board and the main board - no change.
• Disconnecting/reconnecting the LVDS left/right cable between the main board and the TCON doesn't do anything different when the cables are connected or disconnected.
• However, disconnecting the LVDS cable between the PANEL and the TCON makes the respective side of the screen display white - at least it looks like the display is okay (or at least capable of displaying white which means all the pixels turned on).

Things I want to try, but I haven't yet:

• Probe the SPI flash chip (IC1100) that hangs off of the BCM processor and check for data fetch from the SPI flash on boot.
• Probe the voltages on the TCON

This is what i have for now... does anyone have a good suggestion on what to check next other than checking instructions are fetched from the SPI flash? As far as am concerned, the Broadcom processor is out of reset and should be running.

[budm]

Re: LG 42LH50 - no video, backlights on, no USB voltage

What do you see on the screen if you have one ribbon cable between the T-CON and the LCD panel connected one at a time?

[ngth82]

Re: LG 42LH50 - no video, backlights on, no USB voltage

What do you see on the screen if you have one ribbon cable between the T-CON and the LCD panel connected one at a time?

Hi budm!

I just finished my post and wrote about that:


Cables/panel testing
I've tried disconnecting and reconnecting each cable to make sure it is seated in place. Here are some observations from that:

• There is no panel test jumper like other LG TVs.
• I reseated the cable between the power board and the main board - no change.
• Disconnecting/reconnecting the LVDS left/right cable between the main board and the TCON doesn't do anything different when the cables are connected or disconnected.
• However, disconnecting the LVDS cable between the PANEL and the TCON makes the respective side of the screen display white - at least it looks like the display is okay (or at least capable of displaying white which means all the pixels turned on).

[dick_barton]

Re: LG 42LH50 - no video, backlights on, no USB voltage

Checking the EEPROM/SPI data lines to see if they are buzzing at switch on seems like a good idea and also can you 'scope the LVDS cable mainboard/tcon to see if there is data on the lines.

[ngth82]

Re: LG 42LH50 - no video, backlights on, no USB voltage

I'll check on this when i get home this evening. It's been one heck of a work week.

[ngth82]

Re: LG 42LH50 - no video, backlights on, no USB voltage

Not much of an update but I was able to spend a few minutes at the unit today.

• The differential pairs between the main board and the T-Con are sitting at ~100mV (single ended), ~200mv peak-peak (differential). However, there is no data being transmitted across the pairs that i looked at.
• The SPI flash is on the backside of the main board. It is covered by an RFI cage that is soldered in place. I'll try and remove it down the line. I just didn't have enough time to look. I was not yet able to ascertain if the SPI flash is being read. I'll probably try and hook up my Total Phase Beagle SPI/protocol analyzer when I get a chance to do this.

[ngth82]

Re: LG 42LH50 - no video, backlights on, no USB voltage

I think this looks largely like a main board issue, purely because the LVDS lines are quiet.

• I forgot to mention that i checked the 12MHZ oscillator (X1100) that is associated with the same chip that has the SPI flash. The oscillator reads 12MHz dead on.
• I'll probe the SPI CLK and chip select lines to see if anything is even occuring. No need to decode data unless the SPI flash is being selected in the first place and with a valid clock. ---- just need to spend some quality time with my soldering iron to remove the RFI cage.

Question for the community:

• What's the RS-232 port used for? Is it for debug and panel updates?
  -- if so, i'll try and scope the RS-232 TX line to see if anything comes out. If yes, that can help to determine the baud rate and see if something useful can be ascertained.

[dick_barton]

Re: LG 42LH50 - no video, backlights on, no USB voltage

I believe that if you run a terminal program it will display the various bootup sections of the files being read to initialise the TV so will give you some indication of what it is possibly failing on.
Never tried it but you can do a search of the forum for more information since it was discussed not long ago by Tom66

[ngth82]

Re: LG 42LH50 - no video, backlights on, no USB voltage

I believe that if you run a terminal program it will display the various bootup sections of the files being read to initialise the TV so will give you some indication of what it is possibly failing on.
Never tried it but you can do a search of the forum for more information since it was discussed not long ago by Tom66

I looked at the TX RS-232 line. When the TV is off, the RS-232 TX line is at 0V, when the TV is on, the only trigger is having it go to -6V, this is the MARK (e.g. idle) condition on RS-232. No output stream is given at all.

The RFI cage is heavily soldered onto the board. There is also a thick thermal pad that covers most of the inside of the cage. I left it alone.

I think i'm just going to leave this TV alone unless someone expresses interest in it.

Thanks!
-Thomas

[ngth82]

Re: LG 42LH50 - no video, backlights on, no USB voltage

The TV is gone... but I hope someone finds this thread useful in the future. Until next time!!