The complaint for this Samsung 510BN LCD monitor was no power, after the cover was opened I found that the power filter capacitor still hold some charge and the voltage was quite high. A good monitor by right the filter capacitor stored voltage should be discharge in less than 10 seconds. This indicates that there was something not right in the circuit. If you happen to repair this type of power problem, make sure you discharge the filter capacitor first before checking on others components. After the filter capacitor voltage already discharged , I used dick smith flyback tester to measure across the primary winding of the switch mode power transformer to see if there are any short circuit occurred.
The result of the test was the flyback tester LED light all goes off which means there must be components shorted either in the primary or secondary section, sometimes the primary winding can go shorted also. A shorted power ic (this monitor is using the 4 pins 5L0365R power ic) in the primary section or a shorted diode in the secondary section would pull down the flyback tester LED bar to the minimum.
If the flyback tester LED have 8 full bars, this means that the components such as power ic (power fet if have), power transformer primary winding and all the secondary diodes are not shorted. To isolate if the shorts is either in the primary or secondary section, you must desolder the power transformer. Check the primary winding of the power transformer off board first before you test on all the secondary diodes using analog meter set to times 10 K ohm.
A shorted diode will show two reading registered at the analog meter. In this Samsung 510bn LCD monitor power supply, the caused of the no power and filter capacitor take longer time to discharge was due to a shorted secondary diode which is from the schottky type with the part number of SB5H100. Searching the internet, I found this diode technical datasheet; it is a 100Volt 5A schottky diode. This shorted diode is located at the 12 volt rail which means it converts the ac to 12 Volt dc.
After the replacement the LCD monitor work perfectly fine! Whether you are troubleshooting the Samsung 510BN Lcd monitor power supply or any type of switch mode power supply, basically the procedure of testing it is the same. Of course there are many more methods to locate the shorted secondary diode such as directly test the diodes, but this method can check for components shorted even without touching the soldering gun.
This means, if the flyback tester shown 8 full bars, then don’t waste your time to test on all the secondary diodes again. Get one dick smith flyback tester and you could see how valuable this meter is in performing variety test for you such as checking the computer monitor B+ coil, flyback transformer, crt horizontal yoke coil, ballast and many more.