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