Replacing all the caps on the BenQ SW2700pt's power board didn't help...

[Noodneek]

My 3.5 years old monitor (BenQ SW2700pt) started to black out for 1-2 sec every minute or so, it was doing this even with PC disconnected from it. I searched online and read that it's probably aging capacitors and there's a workaround to lower the brightness.

I lowered it from 100 to 65% and it indeed helped, but after a couple of months the issue returned... I continued to lower the brightness, but it helped only for a couple of hours at most, then I needed to lower it further... Turning the monitor off and waiting for it to cool down also helped for a short time, after which it continued to black out every 10-30 sec... I decided to take the monitor apart and look at the power board, and indeed saw 3 bulged capacitors.

I bought new replacement caps and went to a professional (soldering is not among my skills), and he replaced all the caps on the power board except the biggest one for high voltage, since it seemed fine and rarely goes bad according to my research. But he mesured it's capacitance and saw it was 120 uF instead of 150, but I decided to see if replacing every other caps fixed the issue (I also didn't have the replacement for the big cap)...

Unfortunately it did not, the monitor continued to black out periodically, I continued to lower the brightness for several hours untill I hit the bottom and at the end the monitor started to fade frantically and after a short time blacked out for good...

I replaced the last capacitor on the power board (150 uF 450 V), but it didn't help, my monitor now is permanently black and doesn't turn on at all (the indicator light turnes white and then red after a minute).

Is there anything else I can do short from trying to find the whole replacement power board? Can it be something other than faulty power board? This is a great monitor I payed a lot of money for...

Thanks

[Noodneek]

It is apparently a backlight issue, since I can see the image on the screen with a flashlight.

Here are some pictures:

[CapLeaker]

Replace the backlight LED strips or find the bad LEDs and replace them. This is an example of a classic failure of the backlights. Has nothing to do with the caps.

[Noodneek]

Replace the backlight LED strips or find the bad LEDs and replace them. This is an example of a classic failure of the backlights. Has nothing to do with the caps.

Thank you for the reply. How do I disassemble the panel and find matching LED strips?

[Noodneek]

Replace the backlight LED strips or find the bad LEDs and replace them. This is an example of a classic failure of the backlights. Has nothing to do with the caps.

OK, I found the tutorial on how to disassemble the display to get to the led strips... But even then I might have a problem identifying the leds... Here, I found this thread by someone who took apart the same BenQ SW2700pt display and couldn't figure it out:
https://www.badcaps.net/forum/troubl...00pt-backlight
So, again, how do I find the right LEDs??

Another thing, before that I have to make sure the PSU board is OK, right? So as I understand, I need to power up the board and measure output voltages on CN801... But I don't really sure how to do this and I haven't found the schematics for the board... I'd really appreciate if someone could explane me how to check if the board works fine, looking at the pictures I provided.
Thanks!

[ISquishWorms]

Keep in mind this is all from memory, but two of those pins on that connector will have about 65 volts on, I think it is the two middle ones, the others are the returns from the end of each of the 4 strings of LEDS. From what I recall the 65 volts powers two strings of the LEDS which in total is about 20 LEDS and results in about 3.25 volts per LED. The current again from memory is about 130 milliamps which along with other constraining factors is why I decided to use an LED which could handle 150mA. I do not know your experience but just take care when carrying out measurements of voltages etc.

[CapLeaker]

OK, I found the tutorial on how to disassemble the display to get to the led strips... But even then I might have a problem identifying the leds... Here, I found this thread by someone who took apart the same BenQ SW2700pt display and couldn't figure it out:
https://www.badcaps.net/forum/troubl...00pt-backlight
So, again, how do I find the right LEDs??

Another thing, before that I have to make sure the PSU board is OK, right? So as I understand, I need to power up the board and measure output voltages on CN801... But I don't really sure how to do this and I haven't found the schematics for the board... I'd really appreciate if someone could explane me how to check if the board works fine, looking at the pictures I provided.
Thanks!

LEDs are current controlled not voltage controlled. Thats how the PSU is able to shut down, otherwise the TV could burn your house down. Fix the LED's. The PSU is fine. Before you look for LEDs you got to be able to get to the LED strips first place without breaking anything else like the screen in the process.
I don't think finding a replacement LED isn't that big of a problem. A whole strip may be a tougher issue. What I am more afraid of is, that there was so much done to the PSU, so who knows if it still works.

[Noodneek]

ISquishWorms and CapLeaker , thank you very much for your replies!
I doubt that what was done to the PSU damaged it, it was just caps replacement done by a professional and he checked the board afterwards (not the output voltages and currents, though, since we couldn't power up the board).

About finding the strips/leds, I found a company that claims to be able to reverse engineer and recreate any LED strip for any display:
https://www.plazmo.com/collections/led-strips
Though, I don't know how much it would cost, I'm considering maybe buying a new display panel (for the third of the price of the new monitor) would be a better solution...

[Noodneek]

Keep in mind this is all from memory, but two of those pins on that connector will have about 65 volts on, I think it is the two middle ones, the others are the returns from the end of each of the 4 strings of LEDS. From what I recall the 65 volts powers two strings of the LEDS which in total is about 20 LEDS and results in about 3.25 volts per LED.

ISquishWorms, I just measured the voltages on my monitor and I get 29.3 V on the two middle red wires (contacts 3 and 4 on my last picture). Are you sure your readings were 65?..

[ISquishWorms]

I can't be totally sure as it was a little while ago now, chances are if you have some voltage then the PSU is at least working to some degree and probably fine. It will probably be ok because as CapLeaker said LEDS are really current driven. If you are really worried and at a push I could take the monitor apart again and measure the voltages on the pins and let you know. It might be that the PSU is dropping the current which in turn is causing a drop in the measured voltage as it has detected that there is an issue with the LED strip. As CapLeaker also pointed out that:

LEDs are current controlled not voltage controlled. Thats how the PSU is able to shut down, otherwise the TV could burn your house down. Fix the LED's. The PSU is fine. Before you look for LEDs you got to be able to get to the LED strips first place without breaking anything else like the screen in the process.

[Noodneek]

ISquishWorms, I really appreciate it!
I wrote to BenQ tech support and asked them to tell me the nominal output voltages from the PSU to the backlight. Haven't heard from them yet...
I get the same voltages readings with the power cable plugged in, regardless of whether the backlight is connected to the PSU or not, and whether I turn the monitor on or off with the button (it just drops from 29.3 to 29.2 V when I turn it off). I don't know, maybe when the current is cut off, it also alters the voltage in some way as well...
If I can't find out the nominal voltages in any other way, I might take your up on your kind offer and ask you to measure the voltages on your monitor. You don't need to disassemble it completely, only to remove the back plastic cover and the small black cover above the connector.
Thanks again for all the help!

[Noodneek]

I wrote to BenQ tech support and asked them to tell me the nominal output voltages from the PSU to the backlight. Haven't heard from them yet...
I get the same voltages readings with the power cable plugged in, regardless of whether the backlight is connected to the PSU or not, and whether I turn the monitor on or off with the button (it just drops from 29.3 to 29.2 V when I turn it off). I don't know, maybe when the current is cut off, it also alters the voltage in some way

ISquishWorms, BenQ tech support replied back with the data!


So my measured voltage is half of the nominal...
Does it really tell that there is an issue with the PSU board or it's low as a result of the current being cut off?

[Noodneek]

ISquishWorms, can I ask another question? I tried to check the electrical circuit continuity between the wires on the LCD side of the connector from my last picture (disconnected from everything else). But I only got the signal when I connected two middle red wires (and the resistance I measured between them is 2 Ohms)... So either both of the LED strips pairs have bad LEDs on them or there's a LED driver inside the panel that cuts the circuit in the absence of power.
Do you remember, are those wires go directly ro the strips or first to a driver inside the panel? Thanks.

[ISquishWorms]

The wires from that connector go directly to the LED strip within the panel at the top of it. Sorry for the delayed reply, I have sent you a PM hope that you get it. I will take those readings you requested in a day or two and get back to you. If you need any other readings let me know and I will take those as well while I have the montor open.

[harp]

Check first small caps on logic board... the issue is connected to ambient temperature change and degenerating caps simutaneously
https://m.youtube.com/watch?v=aPAeZWeJ-nw

[Noodneek]

Check first small caps on logic board... the issue is connected to ambient temperature change and degenerating caps simutaneously
https://m.youtube.com/watch?v=aPAeZWeJ-nw

This is interesting, thank you!
Though, I don't understand Croatian well anough (I know Russian and Ukrainian) to get what he did to fix the issue permanently, and he didn't show what components he replaced on the T-con board.
And now my display is black permanently, I missed the chance to measure stuff when it was still in a partially working state...

[harp]

With dmm is monitored DIMM or BL-ON signal, while you check with changing temperature. This monitor is worrking ok when he heating up for few minutes, so to test instability is used cold ice cube in plastic bag to bring it back to fault.
If your monitor is working on warm period of year, and now not work, just do oposite, sligghtly heat (caps do not have to be hot, just warm over ambient temperature for test, hopefully it is not degenerate too much) a individual small caps with iron set to 100 deg C, or just replace all caps on logic board and see if helps.
In attach is marked caps who is only replaced in mentioned video to permanently fix issue.

[Noodneek]

I connected everything and measured the voltages on all the contacts on the mother board connector going to the CN701 connector on the power board, both in ON and OFF states. Here are the values from top to bottom as seen on my 1st picture below:

TP3: ON - 3.27, OFF - 0

TP2: ON - 3.28, OFF - 0

TP7: ON - 0, OFF - 3.28

-------: ON - 4.94, OFF - 4.98

-------: ON - 4.94, OFF - 4.98

TP4: ON - 14.07, OFF - 14.55

-------: ON - 14.08, OFF - 14.55

TP1: ON - 0, OFF - 0

-------: ON - 0, OFF - 0

-------: ON - 0, OFF - 0

-------: ON - 0, OFF - 0

So it seems like I supposedly get correct voltages, but the backlight is out... I don't know what to make of it and how I'm supposed to check the logic board with cold or heat as you suggested...

[CapLeaker]

Problem is we don't know what TP1 actually is. I'd be very tempted to take a 1k ohm resistor and put it on TP1 to the 4,98V. Turn it on and see what happens. Usually you need PSon, BLon and a DIM pin.
How much voltage do you get on the 450V capacitor on the PSU?