AOC LM2028 - no voltage

[dondrusco]

Hi,

I repaired one AOC LM2028 - there were problem with display connector - backlight and control OK - but no display.

When I completed it, monitor stops working.
There is problem with power supply, I think. There is no voltage at output.

I checked voltage at big capacitor - there is 320V=.
At secondary circuit of transformer there is 0V~

Then I checked voltage at MOSFET(Q901) S-D - there is also 320V.

It looks like FET is dead.

Have I to replace PWM controller IC901, too?

[PlainBill]

Re: AOC LM2028 - no voltage

Hi,

I repaired one AOC LM2028 - there were problem with display connector - backlight and control OK - but no display.

When I completed it, monitor stops working.
There is problem with power supply, I think. There is no voltage at output.

I checked voltage at big capacitor - there is 320V=.
At secondary circuit of transformer there is 0V~

Then I checked voltage at MOSFET(Q901) S-D - there is also 320V.

It looks like FET is dead.

Have I to replace PWM controller IC901, too?

I think you are getting ahead of yourself. Note that the FET won't do anything until the PMW controller starts. What is the part number of the PMW controller? Download the datasheet and make sure it has the proper voltage. Depending on the PWM controller, a bad capacitor can cause this problem. Pictures of both the top and bottom of the power supply will help.

PlainBill

[dondrusco]

Re: AOC LM2028 - no voltage

Hi, PlainBill,

nice idiom - I try to remember

I replaced this K2996 FET - no change.

Even worser thing is, that I'm not able to read PWM type number.

Power board type is PWPC19625FJ1 - no info available.

I try to ask my friend at AOC authorized service here in Slovakia - to get more info.

[dondrusco]

Re: AOC LM2028 - no voltage

The PWM controller (SMD in the left) is SG6841

I try to exchange it, but I have to collect more items for the next order.
It is too much for P&P (6EUR) for one (2,8EUR) item.

[neutrino68]

Re: AOC LM2028 - no voltage

dondrusco,

Sorry for bumping this old thread, but were you able to figure out what caused this problem, and fix it? I have a dead LM2028 here, which seems to show exactly the same symptoms on the power board as you described here, e.g. 0 volts on secondary side of the transformer, 320VDC on the primary side but Q901 seems dead.

This monitor has been showing signs of problems for a long time, sometimes it would lose power for some seconds, switch back on and off for a random number of times, and then resume normal operation. Now it has gone completely dead though.

[momaka]

Re: AOC LM2028 - no voltage

hey neutrino68, welcome to the forums.
Does your monitor also have a SG6841 PWM controller?

If so, the SG6841 datasheet states that there should be a filter cap (usually 10-47uF) on pin 7 (Vdd). From dondrusco's pictures, this looks like the small cap above the main switching transistor between the heatsink and the transformer. Check the voltage on that cap. Should be no more than 30v DC**. Post here what values you get. Also check the diode that supplies current to this cap (i.e. test if it's shorted - with the power disconnected from your monitor of course). Again, from dondrusco's pictures, this looks like the diode right above the small cap I mentioned.

**Note: When checking for this voltage, you need to have the negative lead of your multimeter connected to the negative on the primary side, not to ground. Be very careful around the high voltages there.

[retiredcaps]

Re: AOC LM2028 - no voltage

I have a dead LM2028 here, which seems to show exactly the same symptoms on the power board as you described here, e.g. 0 volts on secondary side of the transformer, 320VDC on the primary side but Q901 seems dead.

1) Please start by posting pictures and the location of Q901. The pics in dondrusco's posts are a bit small and hard to make out the details.

2) Post clear focused pictures after reading

https://www.badcaps.net/forum/showthread.php?t=1868

3) Please do not post pictures inline as they slow down the loading of pages.

4) For best picture quality and clarity take your boards to a window on a sunny day, turn flash off, and use macro mode. Take a top down view of all your boards (front and back). Make sure the photo is legible so that we can read the PCB printing clearly. A shutter speed of 1/125 or faster will produce nice clear focused pictures. Try to get a photo that is 2000x2000 resolution or as close as possible.

Here is an example of the pictures we want.

https://www.badcaps.net/forum/showpo...94&postcount=1

5) momaka makes excellent suggestions on the next steps to try.

[neutrino68]

Re: AOC LM2028 - no voltage

@momaka: Thanks, this forum seems like a gold mine, I'm sure I'll spend hours and hours reading here. Regarding the cap you're talking about, I'm not quite sure which cap you are referring to, maybe the pics I've attached now can help you point it out to me using the board marking, so I can measure it.

@retiredcaps: Hi, and thanks for the hints re pics. I've taken some pics now, unfortunately I had to use the flash, light in here is not very optimal to say the least. Q901 is the main switching MOSFET, you can see it marked (text upside down) in the middle of solderside_pwmcircuit.jpg, its Gate, Drain, Source are marked there as well.

These are pretty sharp close-up pics (sorry for the flash glare though), hope they help.

edit: momaka, could it be C922 (cmpside_hivolt_area.jpg, right above the upper-right corner of the switch transformer) you are referring to?

[retiredcaps]

Re: AOC LM2028 - no voltage

I'm not quite sure which cap you are referring to

It is either C907 or C922.

I don't know if it is the lighting or not, but there seems to be a lot of ring cracks on the solder side.

[retiredcaps]

Re: AOC LM2028 - no voltage

From tracing pin 7 of the SMPS controller, it looks like C907 is the one.

See yellow line on pic showing pin 7 to C907.

[retiredcaps]

Re: AOC LM2028 - no voltage

Does your monitor also have a SG6841 PWM controller?

BTW, PlainBill does a "walk thru" of the typical application circuit at

https://www.badcaps.net/forum/showpo...6&postcount=30

The Daytek uses the same SG6841 PWM so you can read the entire thread and look for clues that may help you.

[neutrino68]

Re: AOC LM2028 - no voltage

From tracing pin 7 of the SMPS controller, it looks like C907 is the one.

See yellow line on pic showing pin 7 to C907.

I read 11.3 volts between the + side of C907 and negative on primary side. I'm checking out the Daytek F19AH thread, thanks for the pointer!

[retiredcaps]

Re: AOC LM2028 - no voltage

I read 11.3 volts between the + side of C907 and negative on primary side.

From the datasheet, pin 1 is GND, and pin 7 is VDD. So put your black probe on pin 1 and red on pin 7. What is the voltage measurement?

edit: While you are measuring these pins, you might as well measure and report them all. Watch out for pin 3 (VIN) it is mains * 1.4 so be CAREFUL.

[neutrino68]

Re: AOC LM2028 - no voltage

First I have to correct myself a little; the 11.3 volts reading was without the video logics board connected. So I figured I should connect everything and measure.

Now, doing the same measurement as before, I get fluctuating values, between 8 volts up to 12.5 volts. Sometimes it stabilizes around 10-11 volts. Measuring between pin 1 and 7 directly on the controller gives the same fluctuating voltage.

edit: I'll redo the measuring tomorrow to make sure it wasn't me who was unstable, it's 2:53 AM here and my eyes are getting sore too.

[retiredcaps]

Re: AOC LM2028 - no voltage

Now, doing the same measurement as before, I get fluctuating values, between 8 volts up to 12.5 volts.

I would start with changing cap C907. These little guys don't have to be bloated to be bad. They can be out of uF tolerance or have high ESR (ohm).

After you change the cap, retest.

BTW, what is the uF, voltage, brand, and make of the existing C907?

[neutrino68]

Re: AOC LM2028 - no voltage

I would start with changing cap C907. These little guys don't have to be bloated to be bad. They can be out of uF tolerance or have high ESR (ohm).

After you change the cap, retest.

BTW, what is the uF, voltage, brand, and make of the existing C907?

It's 4.7uF 50V, "Elite". I'll replace it and see what happens.

[momaka]

Re: AOC LM2028 - no voltage

edit: momaka, could it be C922 (cmpside_hivolt_area.jpg, right above the upper-right corner of the switch transformer) you are referring to?

Hi, sorry for the delay, my internet went down shortly after I posted my message yesterday and didn't come back until a few hours ago.

Yes the cap I was talking about is C922 indeed. Check the voltage on it.
If you get a voltage, what is it? If you get none, test diode D902 with the diode check on your multimeter.
Also, thanks for posting high resolution pictures. Makes troubleshooting much easier.

BTW, PlainBill does a "walk thru" of the typical application circuit at
https://www.badcaps.net/forum/showpo...6&postcount=30
The Daytek uses the same SG6841 PWM so you can read the entire thread and look for clues that may help you.

Thanks for the link. It could come in pretty useful in here. As usual PlainBill did a great job of explaining everything.

[neutrino68]

Re: AOC LM2028 - no voltage

Didn't have time to get a new cap today because I worked too late, but I did some more probing now, and I think the fluctuations I saw yesterday may have been the fault of my digital multimeter. I turned the voltage selector back and forth a couple of times and then measured again:

Between pin 1 and 7 on the SG6841 controller, I read stable 13.4 volts.

Between + side of C922 and primary -, I read 14.1 volts, also seems stable.

I checked diodes D902 and D904 (right where C922 is, D904 is on solder side), they are both okay according to my diode check function.

[retiredcaps]

Re: AOC LM2028 - no voltage

I think the fluctuations I saw yesterday may have been the fault of my digital multimeter. I turned the voltage selector back and forth a couple of times and then measured again:

Between pin 1 and 7 on the SG6841 controller, I read stable 13.4 volts.

Between + side of C922 and primary -, I read 14.1 volts, also seems stable.

Okay, before you buy any capacitors, can you re-measure all the pins on SG6841 and report the measurements. Also specify if they are stable or not.

edit: I'll have to read through the datasheet again regarding the above voltages.

[retiredcaps]

Re: AOC LM2028 - no voltage

Here is what I think is the relevant portion of the datasheet.

== snip ==

Under-voltage lockout (UVLO)

The turn-on and turn-off threshold of SG6841 are fixed internally at 16V/10V. During start-up, the hold-up capacitor must be charged to 16V through the start-up resistor so that SG6841 will be enabled. The hold-up capacitor will continue to supply VDD before the energy can be delivered from auxiliary winding of the main transformer. VDD must not drop below 10V during this start-up process. This UVLO hysteresis window insures that hold-up capacitor is adequate to supply VDD during start-up.

=end snip ==