I have a stand for a 50LN5750 that was missing screws from the base to the upper support. The manual describes them as P4 x 25mm. They are actually pan head Phillips self tapping screws (4mm diameter). LG part number appears to be FAB31740201 and a part site wanted a whopping $5 each with $10 shipping. Found some good suspects on Ebay shipped from China (search M4x25mm self tapping) but ended up finding some locally from a LG dealer. In a pinch, the closest imperial screw is a #10 – 1″ which is 4.8mm diameter and the thread pitch is a little wider. The screws to mount the stand to the TV are standard metric machine screws and the part numbers can usually be found in the user manual.

Model numbers LS27D590 LS27D590P S27D590P

Got this monitor in which didn’t turn on.  It uses an external AC power supply as most LED monitors.  The TV has only one main board inside and I was able to find a used on on ebay.  Part number BN94-07215A.

When the replacement arrived, the monitor behaved exactly the same and didn’t turn on.  I then noticed the connector to the LED backlight had broken off and was not making contact to the edge lit strip.  I disassembled the panel down to the strip but improperly mounted the connector back and overworked the solder pads.  During testing, something shorted on the main board as well and I had burning component smell from it.

There is a small blue led in the bottom right corner and it can be configured to be on with the monitor on or in standby. If you don’t know the current setting, it can be confusing to determine if the monitor is on or off if the backlight doesn’t come on.

Since I had the second main board, I was able to find a replacement edge lit strip on Aliexpress.

Once installed, the monitor worked with everything put back together and I am able to upgrade from my reliable 22″ Samsung LED monitor.  It would be nice to have a 1440p or 4K monitor since the 27″ just gives bigger pixels at 1920×1080 resolution.

Model UN55ES6100 UN55ES6100F UN55ES6100FXZC

This TV turned on but had no backlight.  The owner indicated the TV was set to dynamic mode which cranks the backlight level to maximum.

With the TV opened up, visible damage was found on several LEDs on the left strip.  The right strip tested good with an LED tester.  Searching the part number 2012SVS55 7032NNB LEFT76 2D (hidden beneath a covering label) found a similar part with a 3D suffix and a 10 pin connector.  The 2D version uses a 6 pin connector and I wasn’t able to find this version online.  I did have 3D strips salvaged from another TV.  With the 6 pin connector, only 3 pins are used and the entire strip can be lit by applying power across pins 1 or 2 and pin 5.  The strip has 76 LEDs wired as 4 separate banks of 19 LEDs.  Two banks are wired in series and these in turn are wired in parallel giving approximately 114V (38 x 3V) drop across the entire strip.

The 10 pin header had the following pinout

1 – Bank1-
2 – Bank1-
3 – Bank1+
4 – Bank2-
5 – Bank2+
6 – Bank3-
7 – Bank3+
8 – Bank4-
9 – Bank4+
10 – Bank4+

Wires going to the left strip were black (-), black (-) and white (+).  To create a single strip, I shorted pin 3 to pin 4, shorted pin 7 to pin 8 and wired black wire to pin 1, black wire to pin 6 and white wire to pin 10.  If I kept the original cable to the 10 pin LED strip, it would have made wiring easier.

Another option would have been to replace the LEDs themselves but my reflow station was melting them and a hot plate may be needed to replace them.  They are surface mount and appear to be part TS731A (lots of sellers on Aliexpress).

With the TV assembled, I did notice some mistakes I made during reassembly.  I left some finger prints on the clear plastic panel which were visible on the screen if the image was clear and light.  I also had tore edges of the reflector white backing paper behind the plastic panel.  I had taped the torn pieces back up but it created some bright spots on the edge of the screen.

For disassembly instruction on Samsung edge-lit TV’s, see this post.

This TV had no power and the service manual can be found here

I checked fuse F602 and it was open.  The main switching transistor Q601 was shorted gate to drain.  I checked the diodes in the secondary output and one of the SB3A0H diodes was shorted as well.  With these three  parts replaced, there was still no output on the AMP+13V and P-ON+21V line.

With Q601 shorted, there would have been unexpected high voltage at the gate and components connected to it may be damaged.  I found a repair kit on ebay which gave clues to which components to check.  Probing the circuit, I found D607A (27V 500mW Zener) questionable, Q602 (2SC5334S NPN 30V 0.8A) questionable, D608 (36V Zener) shorted and R611 (0.3ohm 2W) open.  D609A (schottky diode) tested good.

Here is a parts list with Digikey substitutes.

F602 Fuse 2.5A 250V 507-1180-ND‎
Q601 TK10A50D N-CH 500V 10A insulated
D608 Zener 36V 1N4753A-TPCT-ND
R611 Res 0.3 ohm 3W PPC3W.30CT-ND
Q602 NPN 125V 0.8A BCX41TADICT-ND‎ or 45V 0.8A BCW66GLT1GOSCT-ND
D653, D655 SB3100  SB3100FSCT-ND
D607A Zener 27V SM TFZVTR27BCT-ND (can substitute leaded axial part and place in D607)

With all these parts replaced, the TV powered up.  The heatsinking for Q602 is inadequate and probably caused the initial failure.

Update – Got another one of these models in using the same power board.  Model 39PFL2708/F7.  Fuse was blown and AMP+13V measured a short.  D655/D653 was shorted, Q601 shorted (D-S short, G-D and G-S meaured 60 ohms), R611 high resistance and D608 shorted.  D607A and Q602 appeared ok.

Update 2 – Repaired a 39PFL2908/F7 with almost identical failures as above.  With all the parts replaced, there was still no output on both AMP+13V and P-ON+21V.  After replacing D607A and Q602 (might have been ok), I finally got it working.

This TV wouldn’t turn on but the front LED would blink once when the power button was pressed.  Typically if you have a front LED (indicating you have standby voltage) and the TV doesn’t power up, it is a main board issue.  I probed the PS_ON and DRV_ON signals on the main connector and didn’t detect a DRV_ON signal so I ordered a replacement main board.  When I installed the replacement board, it behaved exactly the same way.  I ordered a used power supply board next and it was pretty inexpensive at under $20 USD shipped.  When I plugged it in, I noticed LEDs light up on the main board which weren’t on before and the TV powered up.

I should have done a more comprehensive job of checking power supply voltages before concluding the main board was bad.  Luckily the seller takes returns so I will ship it back and take the hit on shipping charges.

Model UN55C6400 UN55C6400RF UN55C6400RFXZC

This TV did not turn on and would click every few seconds.

This youtube video indicates to check for shorts on transistors for the LED drivers on the power supply board.

Power supply board is BN44-00358B.  Schematic is available online here

There are 4 transistors Q9101, Q9104, Q9201 & Q9204 and I found one shorted and previously replaced. I ordered replacements on Digikey part#FQU5N40TU-ND (Mosfet N-CH 400V 3.4A Ipak) and replaced all four proactively.

This board may also have an issue with blown fuses which means you should check QP801, QP802 and CP808 for shorts.

After replacing the transistors, the TV powered up but one side was dim.  This was traced to loose connections on the LED connector.

Update:  The TV came back with a dim side again which I suspected was a power supply fault.  I ended up buying a compatible substitute board from the Chinese company buyqual.com which ended up fixing the problem.

This TV was flashing a 2 blink code which is a power supply error.

A service and training manual can be found here.

Power supply board part number is APS-299 (1-474-303-11 147430311).  Searching on ebay found repair services and repair kits for this board.

Posts on badcaps indicated potential shorted transistors Q6101, Q6102 and open resistor R6540.  When i tested these parts, all three had failed.

Q6101 and Q6102 are shown in the schematic as TK5A50D.  These are 500V, 5A N-Channel MOSFETS.  I had FDPF7N50U (Digikey #FDPF7N50U-G-ND) spares from a previous repair.  R6540 is listed as a 0.1 ohm 1/2W resistor and I temporarily substituted the part with a 0.47 ohm 1/2W fusible resistor I also had.  Digikey substitute would be 0.1DMCT-ND (0.1 ohm, 2W fusible).

With these 3 components replaced, the TV powered up.  This failure is similar to a previous repair on a Sony KDL-46EX400  here.

This video describes repair of a 2 blink code on a KDL-46EX523.  The boards are very similar but the designators and transistors are different.

Update: I had another TV with a 2 blink error but the owner tried to repair it himself and ended up blowing R6001 (15 ohm 5W flameproof cement resistor). He had tried two other boards but they were both dead.

The fault was eventually traced to shorted FETs Q6602 and Q6603 (STP9NK50ZFP, N-CH, 500V, 7.2A, TO-220FP, Digikey 497-12057-5-ND) on the G8 board which is a separate power supply board for the LED backlight. It is powered by the PFC output on the main board. The input to the G8 board was shorted, R6540 was open and Q6101 and Q6102 tested good. Both R6540 and R6001 protect the PFC 400V output so either can blow with a shorted G8 board. It appears if R6540 blows, you will get 2 blinks but if R6001 blows, the TV will be dead.

The other boards the owner purchased were a slightly different part number 147430411 (APS-299W) vs the original 147430311 (APS-299). The difference was two additional connectors by the AC plug that are populated. I believe they are for an external hardwired switch and jumpers JS6000 and JS6002 need to be installed to work in the 55EX620 or else there will be no AC on the rectifier.

The front standby LED was on and would turn off and on when the power button was pressed but there was no backlight and no sound.

Probing the power supply connector, PWR_ON and DRV_ON signals were being sent from the main board but there were no voltages on the LED connector or secondary voltages like 12V.  Checking the big main filter capacitor across the leads, the PFC circuit was not boosting up to the expected 390VDC.  Power supply part number is EAY62512701.

Posts on badcaps suggested checking D607, C603, Q601 and Q602.  I found D607 shorted while the other parts testing good.  I ordered a replacement for MUR460AX (600V, 4A axial leaded diode) from Digkey part #MUR460GOS-ND

Normally, the diode in this application is a TO-220 package which can be mounted on a heatsink for better heat dissipation.  When I installed the new part, I floated it above the board to allow for more airflow around it.

With D607 replaced, the TV powered up with backlight coming on.  The interior of the TV had black dust on the power supply board and back of the TV and I confirmed that the owner was a heavy smoker.  There was also some smoke residue on the bottom corners of the TV.

This TV attempted to turn on but there was no backlight.  It was probably flashing a diagnostic blink code (possibly 10 blinks) but the front LED was out.

A service manual with all schematics is downloadable from here.  Power supply part number is TNPA5123CA.

Checking the voltages at the power supply connector, standby voltages at TUNER6V and VSB were present and stable at 6VDC.  With the power button pressed, 6V appears at RELAY and 2.3V on TV_SUB_ON which turns on the PFC circuit supplying DTV12V and 17V and 32V.  These voltages drop when turned on indicating an error condition was causing a shutdown.

Checking the main filter capacitor, I measured 390VDC on it when the TV was briefly on.   To force the power supply to stay on, I disconnected the connector to the main board and applied 6V to the RELAY pin and used a 10K external resistor from VSB to TV_SUB_ON.  With the PS on, the 390VDC stayed on but DTV12V and 17V was still shutting down.  This lead me to suspect IC7301 (SSC9512).  I measured  R7303, R7305 and R7306 by lifting one leg up.  R7306 measured a bit high – 1.3M instead of 1M so I replaced it and the TV powered up.

I still had the problem of no front LED and this was traced to a broken connector on the cable connecting to the main board.  I had to test and wire all the lines from the IR/LED board to test points on the main board  to bypass any bad connections.