Dell Monitor Repair (Likom L705xxx Model) With No Power SymptomMay 5, 2020
This DELL 17″ Monitor (Made in Malaysia under LIKOM model L705XXX, product no: L7055LP) came in with no power symptom. Normally after opening up the cover I would look at the power supply (primary side) for any burnt and cracked components first. If there are darkened area and burnt components, I would tell myself that this Monitor needs a little more time to repair it. If it is not (component’s intact) I would usually begin to discharge the filter capacitor and start checking the fuse.
Okay, it is confirmed that all the primary side components looks good in this Monitor.
First, discharge the big fat filter capacitor with a resistor (I know that the voltage already drained off but it is a normal procedure for me because what if the start up resistor has an open circuit?) then check the fuse.
If the fuse is okay, experience will tell you that the bridge diodes are also working fine. Place your black probe of your analog meter set to x1 to the center pin (drain) of the power FET and red probe to the other pin one at a time (gate and source). If the pointer of the meter doesn’t kick then 80 % of the Power FET is okay. If you want to 100% confirm if it is good then you have to solder it out and test. If the pointer shows a reading then most probably the power FET has developed a short circuit. The result that I got from this Monitor was the pointer don’t flick.
Now is the time for me to use the dick smith flyback tester to check the smps primary winding. The result that I got was, the led bar don’t lit. Ah ha! That was a clue and this clue tells you that it is either the smps primary winding is shorted or one of the secondary diodes has given way. Of course you can check all the secondary diodes with your normal multimeter first (remember, every electronic repairers have their own procedure methods of troubleshooting) but I prefer this way as I want to confirm also if the smps primary winding is working or not.
Usually smps transformer is very robust thus I didn’t suspect it to pull down the flyback tester reading. I straight away check all the secondary diodes with my analog meter and found one diode shorted (part number U202G). Since I do not have this part number, I use a UF5404 ultra fast recovery diode as a replacement. In fact I use this UF5404 to all 14 to 17″ Monitor as a replacement for all part number of secondary diodes. The spec of this diode was 400volt, 3 Amp with 50 nanosecond.
After the replacement of the diode, and placing the flyback tester probes again to the primary winding, the result shows full bar on the LED. Don’t be happy with the result yet because your job still hasn’t done.
Now I moved on to check HOT with the same procedure as checking the power FET. The HOT checked to be okay too. Next, I check the internal capacitance of the flyback transformer to see it is shorted or not or is the capacitance value run. For your information, internal capacitor in flyback transformer gave lots of problems in computer Monitor. I also scanned through the B+ FET and found it to be good too.
Now is the time to switch on the Monitor, however this round I don’t connect any bulb because it was only a shorted diode in the secondary side. I would only connect a light bulb after repair when there is major short circuit in the primary side such as a blown bridge rectifier, shorted fet and smps primary winding and also burnt components in the board.
The DELL 17″ Monitor worked beautifully and after long hours of burn-in test, I declared it okay and ready to send back to my loyal customer. Isn’t it fun to be able to repair a Monitor and at the end you get paid for something you like doing. I just love electronic repair!