Saturday, June 25, 2016

Tech Troubleshooting - Samsung D7000 Series LED TV backlight troubleshooting – How to test LED failure - How to troubleshooting LED TV backlight inverter faults – LED LCD TV repair and service

Category: LED LCD TV Repair and Service 

Contents of this article 

  • How to test the backlight power supply
  • How to check the backlight LED 
  • Troubleshooting failures 

Samsung D7000 Series

D7000 Series LED TV Power supply and Backlight Failures
Troubleshooting the Samsung D7000 LED Series TVs has been a challenge for many techs, especially when trying to determine whether a problem is caused by the Power Supply, a Panel LED Backlight failure, or a Main Board failure. This has led to delays in repair, incorrect board and panel replacements, and REDO.
The Power On Sequence:
To test the Power On sequence, follow these steps:
1. Locate the Power Connectors on the SMPS Power Supply. In this example, that's CNM803 to the Main Board and CNL802A and CNL802B to the Panel‘s LED Backlights. Their pin designations should also be printed and visible on the board.
2. With the TV in Powered Standby Mode, verify Power Standby ―A5V‖ voltage on the Power Supply‘s CNM803 - pin 18 in this example. The ―A‖ signifies ―Always On‖. It should read approx 5.3V with respect to chassis ground, supplying important standby voltage to the Main Board.
3. Activate Power On while observing the Power on/off command from the Main Board, pin 20 on the same connector. It should change from approx 0V to 3.3VDC with Power On.
All the switched voltage supplies, usually designated with a ―B‖, will be turned on. This is a good time to quickly check those voltages. In this example, check B13VDC on pins 7 and 9 and B5VDC on pins 17 and 19 on CNM803.
4. Locate the Back Light On, BLU ON/OFF command coming from the Main Board, Pin 12 CNM803. It should change from approx 0V to approx 5VDV after a short delay of around 1 second from Power On, activating the back lights. The Samsung Smart TV Logo should then appear on screen for a few seconds, and then the TV displays the source selected. The backlights will momentarily turn back off after the Boot Logo is displayed and then back on with the input video displayed. This can be observed on the multi meter BLU on/off and through the rear of the panel and the front screen display.
5. Four P-DIM Control Signals, P-DIM 1 thru P-DIM 4, are sent to the Power Supply from the Main Board controlling independent sets of LED Backlights to the panel. They are TTL type control signals that can be tested with a standard multi meter. To verify these signals are working correctly, activate the TV‘s menu and select Back Light from the Picture Menu.
6. Check the voltage at P-DIM 1 (With respect to chassis ground) while changing the Backlight Menu from the 0 minimum thru 20 max setting. The voltage should change from approx. 0.2V min to 3.3V at max Backlight setting. You can repeat the test for P-DIM2 thru P-DIM4. This verifies the operation of the Main Board Back Light control circuit.

Back Light Power Supply Operation and Testing
The LED D7000 Power supply consists of 4 independent supplies. One supply for each of the four sets of LEDs. The DC Operating Voltage can be checked at the four Electrolytic capacitors C9110, C9210, C9310 & C9410 in this example. The voltage at each capacitor will be approximately 150V for 46 inch D7000 models. This is based on the number of LEDs used in each set. The larger the size TV, the more LEDs required and the higher the voltage supplied. Now carefully check the voltage at each capacitor‘s plus side. They should each read approx the same voltage (149VDC in this example), verifying that each of the four backlight supplies are OK.
Back Light LED Configuration
The SMPS Power Supply Board outputs the Back Light Drive voltages at CNL802B to the right half Panel LEDs, R1 thru R8, and at CNL802A to the left half Panel LEDs, L1 through L8.
The left and right side Panel LEDs are combined at the power supply and grouped into four distinct sets of LEDs to allow separate top to bottom backlight Screen control.
The R1 set of LEDs are connected in series with R2, L2, and L1, and are Edge Positioned in the Panel to light across the Top Quarter of the Screen.
The R3 set of LEDs are connected in series with R4, L4, and L3, and are Edge Positioned in the Panel to light the 2nd Quarter of the Screen.
R5 LEDs are connected in series with R6, L6, and L5, and are Edge positioned to light the 3rd Quarter of the screen. R7 LEDs are connected in series with R8, L8, and L7, and are Edge positioned to light the bottom quarter of the screen.
Since Right Side LEDs and Left side LEDs are connected in series, disconnecting either CNL802A or B will open each string of LEDs and create a no Back Light condition.
Looking at the Drive Voltages in each string:
The four Backlight DC Supply Voltages, approx 150V each, connect directly to the beginning of each of the four string sets of LEDs at R1+, R3+, R5+, and R7+ (Figure 1, left side).
The four P-DIM Control signals from the Main Board are processed by the Power Supply and produce important Back Light Control Voltages for each return line: L1-, L3-, L5-, and L7- located at the end of each string set of LEDs (Figure 1, right side).
The 150 VDC Supply voltages will remain constant while the Backlight control voltages will vary from approximately 0.7V at the maximum Backlight setting to approx 34V at the minimum Backlight setting. This can be observed by adjusting Backlight from 20 thru 0 in the on screen menu setting. The voltages across each individual string of LEDs will also vary from approx 37VDC to 27VDC with the Max to Min Backlight changes as shown on the bottom of the Figure 1 diagram. The corresponding voltage changes at each junction, with reference to chassis ground, are also shown and can be verified at the CNL802A and CNL802B panel connectors.
Testing the SMPS and Backlight Operation with No Main Board Connection
It‘s important to understand that disconnecting the Power Supply line to the Main Board and plugging in the AC Power Cord turns on the Power Supply circuits but does not light the Backlights in this model. Follow these steps to test the SMPS and Backlights:
1. With the AC Power Cord removed, disconnect the Power Supply Cable to the Main Board at the Main Board end.
2. Connect the AC Power Cord. This will turn on the SMPS Power Supply, but, because all four P-DIM Control Signals are at approximately 0V, there will be no Backlight LEDs lighting.
3. Check the four SMPS Backlight Power Supply Voltages at the four Electrolytic Capacitors: C9010, C9210, C9310 and 9410. They will have changed from 150VDC, the normal supply operation, to approx 95V VDC. The voltages may also begin fluctuating at each supply between 50-85VDC with the supply cycling on and off. This is also a normal possibility in this condition.
4. Check the four backlight strings one at a time. Using an external wire jumper, locate and connect A5V to the P-DIM.
1 control pins found on CNM803. This will light the left and right LEDs responsible for the top quarter of the screen.
The particular responsible supply, C9110 in this case, will jump from 95V to the normal 150VDC while the other three remain at approx 95V.
The SMPS supply may remain stable enough to simply light the responsible LEDs. However, the power supply error detection circuit may shut off the responsible supply just as it reaches near 150VDC, turn it back on, and then repeat the cycle causing the LED Back Lights to also cycle on and off and repeat indefinitely. This does not prevent the backlights and supply from being tested.
5. Connect A5V to the P-DIM 2 control pins on CNM803.The responsible Left and Right LEDs will now light the 2nd
quarter row of the screen. Its responsible supply will also jump from approx 95VDC to approx 150VDC normal operation.
6. Connect A5V to P-DIM3 and observe the 3rd quarter of the screen for proper LED and SMPS operation.
7. Finally, connect A5V to P-DIM4 and observe the bottom quarter of the screen for proper LED and SMPS operation.
Troubleshooting a Particular Failure
SYMPTOM:
The TV turns on with seemingly normal looking video of the Boot Logo for approximately 2 seconds, and then turns off. It remains off for approx 2 second and cycles back on with Boot Logo for about 2 seconds again with good video. This process continues cycling over and over again. The relay on the power supply can also be heard clicking on and off during this time. The Video may advance further to display the input video in addition to the Boot Logo during testing while it continues to cycle on and off indefinitely.
SOLUTION
1. Measuring the four Backlight Power Supplies at each capacitor (C9110, C9210, C9310, and C9410) reveals that 3 of them rise to approximately their normal supply voltage of 150V before dropping with the power cycle off, while the other supply, output at C9210 in this example, rises to over 290VDC before dropping with power cycle off.
OBSERVATION: This condition indicates that there is either a SMPS failure or an LED (Panel) open load failure condition. The supply is likely cycling on/off due to an Over Voltage detection condition. This can be verified by isolating the SMPS from the Main Board.
2. Removing the power supply connection to the Main Board and connecting A5V to P-DIM1 correctly lights L/R LEDs responsible for the top ¼ of the screen, even though in some cases it may power cycle on and off as in this example. Connecting A5V to P-DIM2, however, does not light the next quarter of the screen. Connecting A5V to P-DIM3 correctly lights the next quarter of the screen. Connecting A5V to P-DIM4 correctly lights the bottom ¼ of the screen.
OBSERVATION: Failure of the second quarter of the screen is obvious. The failure may point to either an open set of LEDs, in particular R3, R4, L4, and L3, on the Panel, or a possible Power Supply failure.
3. With the power supply connection to the Main Board still removed, each of the four Backlight supplies now measures approx 95VDC, which may fluctuate between 50VDC and 85VDC while powering on/off for circuit protection. Measuring the voltages at the suspected string set of LEDs reveals a normal 50VDC – 85VDC fluctuation at input R3+ but an improper 0V reading at each of the remaining R4, L4 and L3 pins located on CNL802A and CNL802B.
CONCLUSION: Confirmation of a properly operating SMPS Power Supply and open string set of LEDs is confirmed, since an open series set of LEDs would break the circuit and prevent normal voltage readings at R4, L4, and L3.
SOLUTION: The SMPS is OK. Replace the defective Panel.
POINT TO BE NOTED WHILE TROUBLESHOOTING
# D7000 Samsung LED TVs have four distinct Backlight supplies within the SMPS that control 4 separate sets of LEDs.
# Backlight supplies can be checked by comparing voltages at their related Electrolytic Capacitors on the SMPS Board.
# The Main Board controls the 4 separate sets of LEDs with P-DIM1 through P-DIM4.
# When the Power Supply connection to the Main Board is removed, all the SMPS Supplies will turn on but the Back light LEDs will not light.
# Each set of LEDs can be individually tested by supplying A5V to P-DIM1 through P-DIM4, lighting the combined Left/
# Right Panel LEDs one quarter at a time top to bottom and across the screen.
# Removing CNL802A or CNL802B, the Left and right side sets of LED, will create a no backlight condition since they are connected in series.
# Checking Voltages at the four individual supplies and CNL802A or CNL802B can determine a defective set of panel
# Back Light LEDs or a defective SMPS Power Supply.