samsung c27f396fhn no power. Power Led is off

[waakye]

Hi the greatest repair community!
I have this samsung c27f396fhn with no power. The monitor has no standby led. When plug in there is no sign of life. I already check the power cable and it's outputting 14.5VDC. I need pointers on how to troubleshoot the circuit. When I opened up the display there is only one board. Photos are attach. Need help on troubleshooting this circuit or maybe a schematic.
Thanks in advance.

[CapLeaker]

Re: samsung c27f396fhn no power. Power Led is off

Semi assemble the monitor, plug in the power to the monitor, then measure and see if the 14.5v are still there on the main board plugged in. Not that there is a short on the board and the power supply turns off, or the capacitors aren’t up to snuff in the PSU…

[waakye]

Re: samsung c27f396fhn no power. Power Led is off

Hi CapLeaker, thanks for your quick reply. Really appreciate it. I took the board out of the chassis and apply the 14.5vdc to the board. I was feeling the board with my finger and noticed that L600 was very hotand the voltages at the capacitor where the power connector plugs in was reading 4.5vdc n keeps dropping. The inductor in question is L600. See attached picture the inductor is circle in red. Do you think that needs to be change to resolve my issue.
Thanks 👍

[R_J]

Re: samsung c27f396fhn no power. Power Led is off

It's not likely the coil that is at fault, that is a buck converter circuit, what is the number on ic600? I suspect there is a short on the converters output.

[waakye]

Re: samsung c27f396fhn no power. Power Led is off

Thanks R_J for your help. The number on the ic600 is D93291.

[lotas]

Re: samsung c27f396fhn no power. Power Led is off

D93291 - bd93291efj

[waakye]

Re: samsung c27f396fhn no power. Power Led is off

lotas,Thanks for the datasheet.

[waakye]

Re: samsung c27f396fhn no power. Power Led is off

So I look at the circuit under microscope and noticed that Capacitor C616 and C617 does not seem right. Like they are somehow burnt. The only problem is I do not know the value of these capacitors. See attach picture

[R_J]

Re: samsung c27f396fhn no power. Power Led is off

I doubt those caps are at fault, if they were bad I suspect the circuit would not be working and the coil would not be hot. I suspect there is a short on VOUT1 causing the coil to get hot. If there is not a direct short to ground from VOUT1 to ground, try tracing where that point goes on the board.

[waakye]

Re: samsung c27f396fhn no power. Power Led is off

Thanks R_J for your support. I measure the diodes D601 and D603 and both are reading 0volts. Do you think that might has cause the issue.
Thanks again in advance.

[R_J]

Re: samsung c27f396fhn no power. Power Led is off

Thanks R_J for your support. I measure the diodes D601 and D603 and both are reading 0volts. Do you think that might has cause the issue.
Thanks again in advance.

I don't understand this, are you measuring the resistance across the diodes? Are the diodes on the INPUT (pin3/4) or OUTPUT (pin8) or output (pin7)of the ic?

You said C616 and C617 did not look right, do they check shorted? I missed the fact this is a dual output ic so check if there is a short on VOUT2 to ground as well as VOUT1

[waakye]

Re: samsung c27f396fhn no power. Power Led is off

Yes C617 and C616 measure 0 ohm on both sides with the ground lead on the chassis. I also measured pin 7 on the ice D93291 and it reads 0 ohm. The L601 which is attach to pin 7 on the D93291 also reads 0 ohm.
Note that L601, C617 and C616 are in series on pin 7 of the D93291 ic.
What are you thoughts.

Fyi the diodes are all good.

Thanks again

[waakye]

Re: samsung c27f396fhn no power. Power Led is off

Good news. I changed the capacitor C617 and the monitor has standby light and it is now working. C617 was shorted.
Special thanks to R_J, lotas and CapLeaker.
R_J you deserve a beer bottle. Thanks for pointing me to the right direction.

[BraveAlex]

Re: samsung c27f396fhn no power. Power Led is off

I changed the capacitor C617 and the monitor has standby light and it is now working. C617 was shorted.

Hi there!

I'm trying to repair Samsung LC27F591FDMXCG / LC27F591FDM. It has a different motherboard (BN41-0251B) but with precisely the same problems - power LED is off, D93291 hot, L601, C617, and C616 0 ohm to ground.
I de-soldered both capacitors and found that C617 is shorted and C616 is okay. I want to change C617 and check if monitor will be fixed.

So, I have some questions:
1. I tried to trace C617 and found out that it is working in parallel to C616 on both sides. Is it correct, or do I have another shortage somewhere?
1.1. If point no. 1 is correct, is there any wise reason to put two capacitors in parallel instead of just bigger one?
2. I read the datasheet and suspect that these capacitors are C_CO2, and according to the Evaluation Board schematic on page no. 16, they should be 22 uF. Am I correct in that?
3. @waakye, I understand that it is a two-year-old story, but could you be so kind as to remember what capacitor you used instead of the original C617?

[прямо]

If your diagnosis is correct that only C617 is shorted and nothing else is broken, you can power it on without replacing the cap.
Whether you want to replace it later or double the capacitance value of C616, that's entirely up to you.

[waakye]

@BraveAlex, if the C617 is shorted as you say so, like прямо mentioned you can remove the shorted cap and see if it works. I think it will work without the C617 but I am pretty sure it is the same value as C616 but i really do not recall. If I find the value I will post it here.

[BraveAlex]

Sorry for the delayed reply.

@waakye, @прямо thank you for your comments. I had soldered back C616, and the monitor came to life. It's been working for two weeks now. (But I remembered how uncomfortable it is to work on low-DPI monitors. Meanwhile, I remembered how having some additional monitor space is worthwhile. So I will buy a new one soon. And I'll present this Samsung to someone who needs it )

But I slightly disagreed with you about the value of the broken C617. Because of two thoughts:

• C616 and C617 look a little different in my photos, as well as @waakye's photos
• the same C617 was broken in both cases, so I think it differed from C616.

If the monitor fails once again soon, I'll try to change the C616 to the 22 uF one and update this thread.