Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

[leggot]

Hi all,

I have Viewsonic VA2323wm and the unit seems to have intermittent power, as the lcd panel turns on and off, yet the backlight doesn't turn on. The power led on the front panel also flashes. Upon opening it up, I determined that the transformer (PT801 - highlighted in red) buzzes, seemly to when the front panel led light turns on and off. There are indications of the transformer getting hot. The 5v lines from the powerboard to main board are strong, but the on/off line fluctuates from 2-3volts, which seems odd as the board gets its full 5v power.

Now, here's the interesting part, I have another fully working monitor and have swapped the powerboards with the broken one and the broken monitor works, so it must be a faulty powerboard. Upon testing the faulty board, all the capacitors seem to have the same resistance going through them, about the same as the working board; however the transformer drops from 10M (good board) ohm to 6M (bad board) ohm resistance when tested across it and both have 510 ohms across them (see pic). This leads me to believe that it is a faulty transformer, but its hard to tell.

[boyebaran]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

seems el cap near transformer is bad.replace all electrolyte cap in output dc line.

[leggot]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

Hi,

Thanks, but this is actually a picture of the working board.

seems el cap near transformer is bad.replace all electrolyte cap in output dc line.

[cpt.charlie]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

Such high difference in resistance with the good transformer seems quite suspect, likely it is internally shorted, try to swap the transformers and test again.

[budm]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

Hi,

Thanks, but this is actually a picture of the working board.

So does that cap has domed top? if it does it may be working for now and the board in that inverter section will run hot as you can see the board is already turning dark brown so sooner or later the MOSFETs will be damaged, etc.
That reading of 10M Ohms, is that the primary winding/ I cannot really see the pins due top the red arrows in the pictures.
And those Ohms readings are from the working board or not working board?

[leggot]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

Hi,

Thanks for replying. I am planning to replace the bulging cap when I put all the monitors back together.

I am not particularly familiar with the internals of transformers, but there are 8 pins, two large ones with 510 ohms between them. There there are six pins that all seem to have very low resistance between them (about 1 ohm). Now the difference between any of the small pins and a larger one is 10M ohms for the good board and 6M ohms for the bad board. The picture is reposted without arrows for reference.

Are transformers something that are generic enough to buy on ebay, or should I pull it apart and examine the windings?

So does that cap has domed top? if it does it may be working for now and the board in that inverter section will run hot as you can see the board is already turning dark brown so sooner or later the MOSFETs will be damaged, etc.
That reading of 10M Ohms, is that the primary winding/ I cannot really see the pins due top the red arrows in the pictures.
And those Ohms readings are from the working board or not working board?

[boyebaran]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

if the transformer getting hot So you must check CCFL before changing . one reason for hot transformer is because of bad CCFL.
you can check temperature of transformer by using this https://www.badcaps.net/forum/showth...9987disconnect own ccfl and using device in this showthread

[leggot]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

Hi,

I'm not quite sure what you are saying, but I did try the good powerboard with the bad monitor and everything functioned correctly. The bad powerboard also failed when tried with the good monitor, thus I have tested the CCFLs.

if the transformer getting hot So you must check CCFL before changing . one reason for hot transformer is because of bad CCFL.
you can check temperature of transformer by using this https://www.badcaps.net/forum/showth...9987disconnect own ccfl and using device in this showthread

[boyebaran]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

usually primery (small pin)have near zero Ohm and secondary (large pin) about 1K ohm.
for sure you can replace high voltage transformer in good power with the bad power board transformer and testing it.if didnt work may 2 mosfet (near small pin)damage.

[boyebaran]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

As picture i show secondary and primery.

[leggot]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

Ok, its odd that you mention the mosfet near the transformer. I tested the resistance between the pins, but they seem to behave similarly. There is some heat damage near it, indicating that it possibly may be malfunctioning.

I suppose the only way to know for sure, is to swap the transformer and the mosfet to see which one solves the problem. In the event that the transformer seems to be the issue, what would be my best course of action? Transformers seem to be non standard - unlike capacitors.

Good:
2-3 : 0 ohm
1-3: 0.8 M ohm then drops to 5 k ohm
1-2: 3.2 k ohm


Bad:
2-3 : 0 ohm
1-3: 0.8 M ohm then drops to 5 k ohm
1-2: 3.2 k ohm

[boyebaran]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

first replace transformer.

[Lobo]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

I just saved the exact same monitor from e-waste bin at work. Same problem, no power, intermittent power. The exact same capacitor is bulging on mine as well. I'm going to replace that first, since it's the cheapest, easiest "fix" for now. If it works, I'll be replacing all of them Elite caps.

You may be on to something though. There is definitely some overheating issues. Right between that cap and the transformer the board is discolored, turning brown-ish. It looks like yours is too.

I'll let ya know how it goes

[leggot]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

I swapped the transformer, a good powerboard with the bad transformer and the unit fails to turn on, a swap the opposite way seems to have blown a resistor on the t-con board. This is just seems really sketchy and the transformers are a pain to unsolder. Perhaps Ill mess with these guys another time, but if you can figure it out, that would be great!

[TechGeek]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

Sounds like a partially shorted transformer getting hot and not providing sufficient current to operate the backlight, caused by a failed cap.

[budm]

Re: Viewsonic VA2323wm - No Backlight/Intermittent Power/buzzing transformer

What are the P/N of those two SMD Power MOSFETs?
And have any good clear and focus pictures around those areas of the MOSFETs?