Samsung LED Monitor Repair - Video Included

[_Sinusoidal]

Hello,

Currently trying to repair my monitor without a schematic. I have replaced all the large aluminum electrolytic capacitors, as well as the power mosfet for the backlight. Unfortunately this has not resolved the issue.

Here is a clip of my monitor turning on (sped up and trimmed, it took over 6 minutes to turn on):



When the problem first started to occur it took about 30 seconds to turn on, but has become worse over time. Does anybody on this forum have some ideas as to the particular circuitry that is failing? After the monitor has been turned on and is functioning correctly, I can turn it on and off again quickly without this issue occurring. However, if I leave it off or let it hibernate for longer, the issue comes back. To me, this strongly indicates a capacitor issue, but I'm not sure which cap could cause this, as I've replaced all the large caps on the main board.

Any suggestions would be appreciated!

Monitor: Samsung SyncMaster S22B350

*Quick Edit*

I've also load tested the power supply, virtually no voltage drop at rated load.

[retiredcaps]

You likely have an electrolytic capacitor that has high ESR.  After the cap warms up, the monitor works.  If cold, the monitor won't start.

I would suspect the startup capacitor that is next to the SMPS IC.  It might be 22 to 47uF in the 25 to 50V range.

[_Sinusoidal]

Thanks for the reply, retiredcaps. I have already replaced all the electrolytic capacitors on the board, to no avail.

[retiredcaps]

Initially you wrote

I have replaced all the large aluminum electrolytic capacitors, as well as the power mosfet for the backlight.

the startup cap is a small one on the power board.

If you replaced all the electrolytic capacitors with low ESR ones like Panasonic FM/FR series on the power board, then check the soldering on the power board.  You may have some cold joints?

[lowimpedance]

You likely have an electrolytic capacitor that has high ESR.  After the cap warms up, the monitor works.  If cold, the monitor won't start.

I would suspect the startup capacitor that is next to the SMPS IC.  It might be 22 to 47uF in the 25 to 50V range.

Would heating the suspected cap be useful in this case? If it starts up much quicker then it could confirm this hypothesis.

Or conversely with a warmed up unit then using a can of spray freeze on selected areas and the quick on/off sequence to see if the turn on slows down should help isolate the probable site of the fault. Be it a dodgy cap or joint etc.
Spray freeze can be very handy as can a hair dryer 

[_Sinusoidal]

Initially you wrote

I have replaced all the large aluminum electrolytic capacitors, as well as the power mosfet for the backlight.

the startup cap is a small one on the power board.

If you replaced all the electrolytic capacitors with low ESR ones like Panasonic FM/FR series on the power board, then check the soldering on the power board.  You may have some cold joints?

Ah, I apologize for my ambiguity. I meant to convey that I have replaced all electrolytic capacitors on the board. I will pull it apart later tonight and take another look, as well as some photos. Could the issue be from the display unit itself? I'm not familiar with the design of LED monitors or their potential failure modes.

What indicates that this issue is arising from a power supply fault? The monitor seems to be powered on fully, but it simply is not displaying the image, but rather random colours at normal intensity. I think I'll do some reading on how LED displays work.

[_Sinusoidal]

Opened it up again to take another look. Could not see any obvious cold solder joints. All electrolytic caps were replaced recently.

Here are some pictures:

Entire Board


LCD driver circuitry


Snapshot during monitor boot up


In the third photo, it's fairly obvious that the images being presented on the screen are more than just random colours. The image also has persistence that fades away over time, which makes me wonder if the LCD segments are not being refreshed for some reason. However, I noticed that sometimes the image randomly refreshes but becomes shifted vertically. I'm leaning towards a digital component issue rather than analog. With my limited experience I'm not sure where to look first. Any ideas?

[Rasz]

image also has persistence that fades away over time
 image randomly refreshes but becomes shifted vertically

tcon, chip on film or glass. something about bias voltage

S22B350  is a shit TN panel, probably same as T22B350EW = M215HW01 = tcon is glued to the glass with a hot bar = cant replace
and since its a shit TN monitor just replace it with something decent with MVA/IPS  panel