AOC LM725 Dead

[borinaga]

Can anybody help me with my aoc LM725. No sign of power. Fuse is good as i check. I measured a 327vdc at the filter cap 100uf/400v. PWM ic SG6841 has a vdd measured 14vdc but the sense pin#3 measured 17vdc. is it an indication of a bad PWM IC? datasheet says that pin #3 should be between -.3 to 7vdc. any help is aprreciated. no output at the secondary as ive check..

[PlainBill]

Re: AOC LM725 Dead

Can anybody help me with my aoc LM725. No sign of power. Fuse is good as i check. I measured a 327vdc at the filter cap 100uf/400v. PWM ic SG6841 has a vdd measured 14vdc but the sense pin#3 measured 17vdc. is it an indication of a bad PWM IC? datasheet says that pin #3 should be between -.3 to 7vdc. any help is aprreciated. no output at the secondary as ive check..

Would you like nice help or sarcastic help?

OK, I'll be nice - you've done a pretty good job so far. Look at the reference circuit on page 11 of the typical application circuit on the Silicon General datasheet. Look at the value of R8, and tell me how much current would have to be flowing through a 0.3 ohm resistor to give 17 volts at pin 3.

PlainBill

[borinaga]

Re: AOC LM725 Dead

Ok thanks for the hint. it seems that the MOSFET P9NK60ZPF is bad since it draws much current and being sense by the PWM IC SG6841. By ohms laws there is a 56.6667 amperes flowing at R8. .3ohms am i right. So what should i do next do i have to test the MOSFET? how to test this kind of transistor? Can you please give me some advice to do the test. thanks in advance plainbill

[PlainBill]

Re: AOC LM725 Dead

Ok thanks for the hint. it seems that the MOSFET P9NK60ZPF is bad since it draws much current and being sense by the PWM IC SG6841. By ohms laws there is a 56.6667 amperes flowing at R8. .3ohms am i right. So what should i do next do i have to test the MOSFET? how to test this kind of transistor? Can you please give me some advice to do the test. thanks in advance plainbill

First ask yourself a few more questions. What is the capacity of the circuit this monitor is plugged into? What size fuse is on the input to the monitor? How is it possible to get 57 amps from a 15 or 20 amp circuit? How is it possible to get 57 amps through a 5 amp fuse?

With the monitor unplugged, check the resistance from pin 3 to ground. Odds are it is much higher than .3 ohms. The FET MIGHT be bad. Or the SG6841 may have been destroyed by the voltage on pin 3. But it is a safe bet that the sense resistor is open.

PlainBill

[borinaga]

Re: AOC LM725 Dead

So nice at this point. R8 value of AOC LM725 as i check on the service manual is .39 ohm / 2watt and I think it is a metalized or flameproof resistor that is not available here in the philippines. During resistance testing it shows that i get a 1 ohm value of that resistor designated as R919 of AOC it is a 5 color band resistor, the first band is orange followed by white, silver, gold and green. One question is it a characteristic of a metalized or flamefroof resistor than even it is open it still give a reading? These 2 watts 5 band resistor with respect to the color value it must be .39 ohm +-5% so i must have a reading more or less .4095 ohms. By the way resistance at pin#3 is 1 ohm as i've check when R919 of AOC is connected R8 of the application circuit mentioned earlier. But when i disconnect R919 pin#6 of SG6841 reading at pin#3 is .45Mega ohms. Any idea so far? Thanks for the fast reply to this problem.

[PlainBill]

Re: AOC LM725 Dead

So nice at this point. R8 value of AOC LM725 as i check on the service manual is .39 ohm / 2watt and I think it is a metalized or flameproof resistor that is not available here in the philippines. During resistance testing it shows that i get a 1 ohm value of that resistor designated as R919 of AOC it is a 5 color band resistor, the first band is orange followed by white, silver, gold and green. One question is it a characteristic of a metalized or flamefroof resistor than even it is open it still give a reading? These 2 watts 5 band resistor with respect to the color value it must be .39 ohm +-5% so i must have a reading more or less .4095 ohms. By the way resistance at pin#3 is 1 ohm as i've check when R919 of AOC is connected R8 of the application circuit mentioned earlier. But when i disconnect R919 pin#6 of SG6841 reading at pin#3 is .45Mega ohms. Any idea so far? Thanks for the fast reply to this problem.

You will have to check all components in the primary side of the SMPS for damage. I would pay particular attention to the power FET, and any components associated with the current sense resistor. The datasheet should help you with this.

PlainBill

[borinaga]

Re: AOC LM725 Dead

Ok thanks, I've check earlier the FET and only the drain to source has a reading of .4v at diode test but fail to drain as i touch the gate with my finger. Any recommended ciruit that might help in testing mosfet? THanks for the great help...

[borinaga]

Re: AOC LM725 Dead

Can I substitute the P9NK60ZPF with P5NK60ZPF? Any idea?

[PlainBill]

Re: AOC LM725 Dead

Can I substitute the P9NK60ZPF with P5NK60ZPF? Any idea?

Probably. The P9NK60ZPF seems to be rated for higher current.

PlainBill

[borinaga]

Re: AOC LM725 Dead

Yah P9NK60zpf is 7amps rating while P5NK60ZPF is a 5amp MOSFET. Now another question can i safely run the FET when i replace a 2watts resistor with a 5watts resistor of same value .39ohms?

[PlainBill]

Re: AOC LM725 Dead

Yah P9NK60zpf is 7amps rating while P5NK60ZPF is a 5amp MOSFET. Now another question can i safely run the FET when i replace a 2watts resistor with a 5watts resistor of same value .39ohms?

Definitely yes. The power rating of a resistor is the maximum power it can dissipate. If it has to dissipate less, there is no problem.

An equivalent question is "The floor in my home can support 300 pounds per square foot. I only weigh 190 lbs. Will I be safe?"

PlainBill

[borinaga]

Re: AOC LM725 Dead

Thank you so much sir plainbill, i will post my experience once i finish repairing this monitor. From now i'm awaiting for the PWM IC SG6841 since it is not available here in Philippines. Again thnak you so much for the time you share with me.


I salute you...

[severach]

Re: AOC LM725 Dead

An equivalent question is "The floor in my home can support 300 pounds per square foot. I only weigh 190 lbs. Will I be safe?"

My two shoes are 12" x 4" x 2 = 1/2 square foot. 190 pounds on those lying perfectly flat will produce 380 pounds per square foot which goes way up when rolling up on the ball and heel.

Everybody downstairs to clean up PlainBill.

[jeromy]

Re: AOC LM725 Dead

hi everyone , i had the same case as above is it be possible C913 is the cause of problem ? 0.0047 uf 250V capacitor

[Agent24]

Re: AOC LM725 Dead

I don't know. What makes you think it is C913?

What problems do you get with the monitor?

Please upload some photos too if you can.

[jeromy]

Re: AOC LM725 Dead

Hi Agent 24

good day , the board look exactly like this below , the c913 is the blue capacitor between the yellow square transformer and the connector

yeah, i maybe wrong, just a rough guess , the main problem is there is no voltage output in the secondary winding

I checked the capacitor this morning, it appears ok, so right now, dont know where is the culprit

[jeromy]

Re: AOC LM725 Dead

when you plug in the monitor, , its dead, no LED,

there is no power output from the power supply board (5 , 12v 2.5 etc)

when plugged in, the main filter capacitor read ard 340 Vdc

[Agent24]

Re: AOC LM725 Dead

Does the supply make any noises? Like a ticking noise? Or completely dead?

[jeromy]

Re: AOC LM725 Dead

no sound, working but no voltage output whatsoever in the secondary transformer winding , big filter capacitor is working ard 325 Vdc

[budm]

Re: AOC LM725 Dead

Do you get any DCV output if the you run the power supply board by itself without other board connected to it?
We need straight shot pictures of your board, top and bottom side of the board.