Samsung LS24F354FHNXZA - partially working no standby

[vgamesx1]

I got this off ebay very cheap as no power, well turns out it does have power and even shows a display, but it's very random and will only work for at most about 30 seconds before it becomes fully dead/no power again, just to be certain I've unplugged the LED backlight and display cables but that has had no effect on functionality, it runs on roughly 14v, at idle draws 0.02W and will go up to 6W or more when functioning correctly.

Thus far as a matter of course I've replaced the main power rail capacitor and inductor (slightly higher value) and I double-checked the power mosfet for any shorts as well as gently pressed/shaken the board trying to find mechanical failures (cracked solider joints), I didn't notice any changes in behavior, so my best guess at this point is that a small cap or something has gone out of spec and is causing an issue with an enable signal or clock somewhere on the main board.

Can anyone please give me advice on what I should test next? I'm not very experienced in troubleshooting in general and know even less about TVs/monitors, note there is nothing (that I can see) on the underside of the board, everything appears to be on top and thank goodness for that since it appears to be held on by soldered clips.

[vgamesx1]

Re: Samsung LS24F354FHNXZA - partially working no standby

Whoops, surprise I'm just being dumb, and I didn't think the lower power rail was very important and also it's easier to see in a blown up photo where the 3.3v and 5v test points are... Well after checking it I'm getting 2.8v and 3.4v there, so both lines but mainly the 5v is pretty low, but it isn't shorted as far as I can tell, and I'm still not sure what's wrong yet.

[derka]

Re: Samsung LS24F354FHNXZA - partially working no standby

What about C807 on the top left? It does not look super healthy

[vgamesx1]

Re: Samsung LS24F354FHNXZA - partially working no standby

That's just some flux where I didn't do a great job of cleaning the board, sorry for the confusion, I double-checked it's certainly fine.

Although one thing I did find is on the 5v line, C610 was very low resistance at roughly 20ohm and removing it did bring it up to 3.8v and my 3.3v up to 3.19v, but it didn't fix my issue and one time it booted up I did confirm the line went up to 5v, so is the regulator going bad prehaps?

[momaka]

Re: Samsung LS24F354FHNXZA - partially working no standby

It appears someone replaced SMD electrolytic cap C801/C812 with a regular through-hole cap... and it looks much smaller than the footprint on the board. (Top-left corner of the board in your picture.) I suggest checking that cap and/or maybe even replacing it with something of bigger capacity.

C610 was very low resistance at roughly 20ohm and removing it did bring it up to 3.8v

Was this measurement in circuit or out of circuit?
If in circuit, did you check that cap again out of circuit?
Sometimes, caps may appear shorted in circuit, but could be actually fine due to something else causing the low resistance. I see C610 is connected to R615. I can't quite read the numbers of IC600, but I'm pretty certain it's a switching/buck regulator of some type and not an LDO. So that cap could be part of the compensation network or something else important. Do NOT run power to the board with any components missing around switching regulators like that - you can damage them.
If you can, please let us know what part number is written on IC600 so we can try to locate a datasheet.

[vgamesx1]

Re: Samsung LS24F354FHNXZA - partially working no standby

It appears someone replaced SMD electrolytic cap C801/C812 with a regular through-hole cap... and it looks much smaller than the footprint on the board. (Top-left corner of the board in your picture.) I suggest checking that cap and/or maybe even replacing it with something of bigger capacity.

Yes that's the cap I mentioned in the main post, it looked like it might've had a short, so just I went ahead and replaced it regardless and I'm fairly certain it's the correct value, but in hindsight you can see it goes straight to the backlight which works perfectly fine (when it wakes up) and that was likely a complete waste.

Was this measurement in circuit or out of circuit?
If in circuit, did you check that cap again out of circuit?

In circuit, I just tested it by itself and it's hard to measure, but I seem to be getting OL and nearly 100nF so good call, maybe it is ok, also R615 appears to be a 0ohm link.

I can't quite read the numbers of IC600, but I'm pretty certain it's a switching/buck regulator

Indeed it is, as best I can tell the IC is RT5038B (RT5038BGSP) from Richtek which I can't find a datasheet for it, even Richtek apparently doesn't know it exists so I guess Samsung custom ordered it? Although the few sellers who offer it do call it a voltage regulator. Oh and to save you some time the IC under HDMI is an EEPROM.

Thanks for your time, one question I know that C610 is the 5V line so would it hurt anything to inject 5v there and look for any component that gets hot?

[momaka]

Re: Samsung LS24F354FHNXZA - partially working no standby

In circuit, I just tested it by itself and it's hard to measure, but I seem to be getting OL and nearly 100nF so good call, maybe it is ok, also R615 appears to be a 0ohm link.

Yeah, whenever I see a cap and resistor in series on a switching regulator like that, I don't usually expect the cap to be shorted there. And moreover, that cap is likely part of a feedback/compensation/bootstrap circuit, so it's best not to run the regulator without it.

Indeed it is, as best I can tell the IC is RT5038B (RT5038BGSP) from Richtek which I can't find a datasheet for it, even Richtek apparently doesn't know it exists so I guess Samsung custom ordered it?

Looks like it. I couldn't find anything either.
Doubt Richtek developed this IC specifically just for Samsung though. Likely they have the same regulator with a different part number somewhere. Or maybe they even changed a few pins around.

Closest I can find is something like this, though the pins don't seem to match what's on your board.
https://cdn.badcaps-static.com/pdfs/...81de43d017.pdf

I suppose the question I'm still trying to figure here what this regulator generates - is it the 5V_MAIN rail only, or is it the 3.3V and the 5V_MAIN rails? Reason I ask is because I see a test point labeled 3.3V, and from it, you can see SMD inductor L601 goes to one of the pins of the IC. At the same time, 5V_MAIN also seems to be connected to the IC, as well as power on the board from the external power adapter.

On that note, what resistances do you get between 5V_MAIN & ground, and 3.3V & ground?

Thanks for your time, one question I know that C610 is the 5V line so would it hurt anything to inject 5v there and look for any component that gets hot?

That's what I was thinking myself.
Key question is where to insert 5V. I'm more keen on suggesting to go with inserting it at the test point where it says "5V_MAIN". Also, before doing that, check resistances to ground on 3.3V and 5V_MAIN, as suggested above. If you see any low resistances, it might be a good idea to make sure your external power supply has some kind of current limiting. Otherwise, you may end up with burned traces and components.

Also, if this IC generates both 3.3V and 5V, then you may need to insert 3.3V as well - at least if you want to test if the monitor works. Hard to say more. Can you try tracing the pinout of IC600?
From what I can make out so far:
Pin 5: Ground
Pin 4: external power adapter (12-19V?)

All others I can't make any sense why they are connected the way they are - well, pins 1, 7, and 8, in particular.

[vgamesx1]

Re: Samsung LS24F354FHNXZA - partially working no standby

Yeah, whenever I see a cap and resistor in series on a switching regulator like that, I don't usually expect the cap to be shorted there. And moreover, that cap is likely part of a feedback/compensation/bootstrap circuit, so it's best not to run the regulator without it.

Ohm my transistor! Yeah, C610 was 99% certainly A problem...
I couldn't fit it back on because I accidentally covered it in solder thus I bodged a thorough hold cap into its place so I could test IC600 and not only did it boot on first power on it stayed on at least until it went to sleep, and I unplugged it.

That's so weird though since I couldn't measure anything on that cap with my DMM out of curcit, further testing is of course needed, actually never mind, although it does seem to stay on it still has the issue of randomly booting, after unplugging it and setting it up for use it wouldn't turn on, so although I might could manage with that, I've decided to give up and just buy a replacement board as there's currently one available for just $24, thanks anyway momaka.

All others I can't make any sense why they are connected the way they are - well, pins 1, 7, and 8, in particular.

If anyone wants to know the pins are for IC600 they're:
1=5v
2=I was seeing ~9.8v but I'm guessing it's 10v for feedback
3=4.5v
4=VIN (14v)
5=GND
6=8v
7=3.4v
8=5v

Oh and one thing worth noting for anyone else trying looking to repair similar models is that all the S2xF350 boards appear to be the same, I can't verify anything but if you look at boards for sale, next to the power connector where the sticker is, there is a resistor which may set the resolution/scaling, so swapping that might be worth trying if you're having trouble finding the exact replacement.

[momaka]

Re: Samsung LS24F354FHNXZA - partially working no standby

I've decided to give up and just buy a replacement board as there's currently one available for just $24, thanks anyway momaka.

No problems!

Let us know how it works with the replacement board. I don't see any reason why it shouldn't, but still might be worth mentioning for thread completion.

[vgamesx1]

Re: Samsung LS24F354FHNXZA - partially working no standby

No problems!

Let us know how it works with the replacement board. I don't see any reason why it shouldn't, but still might be worth mentioning for thread completion.

Sure, it works great now and I haven't had any issues with it after the board swap, well aside from a minor nitpick where it locks you out of certain settings such as brightness if you enable other settings and the stand isn't great but ignoring that it's a very nice display and even with the added cost of a new board I still paid less than half the $100 retail price, so I'm quite happy with the result.