AOC LM725 Dead

[jeromy]

Re: AOC LM725 Dead

Sorry Guys,

I am just a Novice here, at times cannot catch up hehe ..

yes, i was afraid the damage might affect the main board, I have it main board disconnected since i got into said trouble

the portion in Yellow circle, the reading is 320V DC , I observe the Pin 3

of the driver IC is just around 3v dc ( yellow line)

the entire secondary winding ( red circle) is dead ( see front and back of board pic)

i try testing the components in the primary side, all appear to be ok
resistor, diode and the 2 transistor

Previously i had experience blown fuse, wherein the entire rectifier/filter capacitro ended up short, i replace the Mosfet , it stop blowing , could it be that controller IC got damaged as well

Could it be the IC is faulty , any suggestion i try replace it ?

thanks

[jeromy]

Re: AOC LM725 Dead

The IC 901 was indeed the culprit !! SG6841DZ I searched the whole electronic parts district , said IC was not yet available locally Except i dropped by a electronic shop that sold it , the shopkeeper told me, they import it online, since there were some inquiries on it , sadly the price is 3x the price sold in Aliexpress and other online services . the consolation is , its readily available, ( took weeks even months if to buy online from China ) , and best of all, it finally work !!

thanks

[Agent24]

Re: AOC LM725 Dead

Yes, if the switching MOSFET shorts then you end up wtih that 320VDC going straight into the drive pin of the switching IC (IC901 here) and it will be destroyed.

Would have helped if you told us in the beginning that you had to replace the MOSFET\Bridge\capacitor... then we'd have said straight away to check the IC.

Good to hear you got it fixed though! Thanks for letting us know how it went.


PS: While you're in there, check the solder joints around the inverter transformers and drive transistors. These parts run hot and the solder joints can become bad after a while. If the solder joints go bad it's a risk that the drive transistors will blow.

[jeromy]

Re: AOC LM725 Dead

Yes, i was indeed a Novice ,

Yes, do encounter the shorted MOSFET, ( result is the blown fuses)
I replace the mosfet taken from another old board, ( also a AOC LCD monitor, but older model ), also swap the transformer, several mylar capacitor as well

I also encountered the case, where the monitor kept on turning on and off after it was powered up , took time to found out, there are cold solders ( which appears intact at first glance) located in the one of the step up transformer connecting to the lamps

yes, for reminder , I check over it

rgds

[jeromy]

Re: AOC LM725 Dead

Yes, if the switching MOSFET shorts then you end up wtih that 320VDC going straight into the drive pin of the switching IC (IC901 here) and it will be destroyed.

Would have helped if you told us in the beginning that you had to replace the MOSFET\Bridge\capacitor... then we'd have said straight away to check the IC.

Good to hear you got it fixed though! Thanks for letting us know how it went.


PS: While you're in there, check the solder joints around the inverter transformers and drive transistors. These parts run hot and the solder joints can become bad after a while. If the solder joints go bad it's a risk that the drive transistors will blow.

yes, thanks for the reminder, i think you are referring to the 2 pairs of 5706 transistors , I have them replaced some weeks earlier, when i encounter a pair is defective , the symptom then is , once power up, the screen will black out after 2 second , i learned to fix it from this Forum as well

[Agent24]

Re: AOC LM725 Dead

Yes, those 2SC5706 transistors which drive the inverter transformers.