Wednesday, January 11, 2017

Samsung plasma TV – Logic board troubleshooting tips

Logic Board Troubleshooting
A defective logic board will often display the video signal but with the wrong color or a smeared picture. If you can recognize the video pattern but there are errors in color , smearing or jittering, suspect that the Logic board is defective.
The logic board creates the timing pulses to create the X, Y and Address waveforms. Additionally the logic board is involved in controlling the power on operation. Logic board defects will usually cause the screen to show a full image but with some type of error. The error can be inverted video, jitter or improper color or luminance. If the logic board is defective all of the pixels will be illuminated but the video will be incorrect. The error may be sectionalized or across the entire panel. Another common error is related to a loss connection on the LVDS cable. It is possible to isolate the Logic board from the Main board by accessing the test pattern on the logic board.
Isolating the Logic Board
1. Access the service menu by pressing Mute + 1 + 8 + 2 + Power with the power
off.
2. Access the PDP option sub menu item.
3. Access the Pattern select submenu item.
4. Scroll through the different patterns. These test patterns are generated on the
Logic Board. If these patterns are displayed properly, the problem is before the logic board. This can be problem with the Logic-Main Board LVDS interconnect cable or a problem with the Main board. If the patterns are not displayed properly, the problem is probably caused by a defective Logic Board. Additionally a dead set may also be caused by the Logic board not sending out the relay on command. This can also be a set that cycles on or off quickly at startup.
To troubleshoot video problems or to make sure every board after the main board is working properly, the internal test patterns can be accessed in the logic board then move to the main board test patterns. Even the customer menu's can be used as a test pattern.
Logic Board Forced Operation
Step 1- With the Main PCB removed, short pins 1 & 5 on the 24P ribbon cable to the SMPS to turn the panel on. This will turn on the secondary voltages of the SMPS. Shorting the POWER ON connection to ground is the same as the power on from the power switch.
Step 2 - Carefully short pins 5 & 6 of the 6 terminal test point on the Logic PCB together. When the panel is powered on, the PDP will automatically “play” the internal test patterns stored on the Logic board that would normally be accessed only through the service menu. This will force the logic board into a loop where the internal test patterns will be displayed in sequence.
Step 3 - After the proper pins are shorted, apply power to the AC input to the panel. The TV will power up by itself and if all is well with the logic, X, Y buffers and address boards, the panel will “play” the test pattern loop. If everything looks normal the possible problem is the main board which is the only thing left not in the circuit.
The patterns that will be displayed from the logic PCB are located after the LVDS cable. If the patterns look normal, the SMPS, logic, X, Y, buffers and panel are all working properly. Use this method in cases where the video is so distorted that the menus can not be seen to access the test patterns using the service mode.
Logic Board Failure Examples
Logic PCB failures The test patterns located on the Logic board are being displayed along with the incoming video.
If the relay cycles repeatedly or will not close, verify the Standby +5VDC supply is not being pulled down by a defective Logic Board