Samsung 2493hm flashing backlight

[radicalmjp]

Re: Samsung 2493hm flashing backlight

Sorry for any confusion.
I mean the 24v_DET pin on the logic board as it appears in the top of the image in post #11
I have not done any measurements on the power supply output towards the inverter.

[newbie1]

Re: Samsung 2493hm flashing backlight

Budm, yeah. i think that is what he is saying..

Radical, looks to me like the problem lies on the power board. maybe the more experts guru (especially Toasty for this model) can confirm that. Can you get hold of better caps than the green Lelon?. if you can, it is better to replace them although it may or not be the rootcause of your problem. i would also scrape off ALL the yellow glue on the power board. What is the resistance reading of the 2 resistors at the blackened glue area?. i think that should read 100k ohms right.

Fyi. i failed to repair two of these due to my transformer getting blown. heez

edit : i believe you should be measuring the output to the inverter. How about the 18v rail in the picture on post #11?

[radicalmjp]

Re: Samsung 2493hm flashing backlight

The two resistors measure (with the meter setting at 200k) 98.1 (left) and 100 (right)
The colours (not clear from this angle in the picture) are Brown-Black-Yellow-Gold which I found out that it amounts to 9.5 and 10.5 Ohms, so I guess that is correct.

Yes I must figure a way to remove the glue without causing more damage.

As I said earlier I am not very good at soldering/unsoldering and neither is my equipment, so I would really like to avoid replacing the caps, if you guys here think that the caps I used as replacements can cause the exact same problem as the original ones then I will get any brand you suggest, but I would like to make sure before changing them again that the brand I used is the source of the problem. If I manage to get the screen working with these caps I will definitely invest into getting better caps for the final repair.

I have realised this power board that samsung used also in other 24" displays is problematic and I have read many other posts on this forum proving it, they seem to be able to have a great variety of problems. And it is discouraging that even gurus like Toasty and PlainBill among others have a hard time diagnosing this power board (ok their hands are tied if they have to go through questions instead of actually checking things themselves).


I remeasured the values on the pins with the screen on.

With the inverter disconnected

The Logic Board gives on the pins from post#11
Pin: 24V 4.7V
Pin: 15V 5.5V
Pin: 5V_MICOM 5.3V
Pin: 18V 0.1V

The power board gives from the cable to the inverter
Pin: 24V 21.8-22V

-------------------------------------------------------------------
With the inverter connected

The Logic Board gives the following
Pin: 24V 4.7V
Pin: 15V 5.7V
Pin: 5V_MICOM 5.2V
Pin: 18V 0.1V

The power board gives from the cable to the inverter
Pin: 24V 21.5-21.7V

I have two questions:
1) Is it normal that the pins state values like 24V but measure only 4.7V?
2) How can I measure the diode in the image infront of the two resistors? My meter has a diode symbol and with that it shows that it works in only one direction (as supposed)

[budm]

Re: Samsung 2493hm flashing backlight

The 24v_det reading of 4.7V is correct, it is the logic level signal to tell the main board that the 24V power supply section in putting out the voltage.
You can use two 12v car lamps in series to chek the out put of the 24v power supply, if it is fluctuating then you can easily see with the lamps connecred.
For example:
http://s807.photobucket.com/albums/y...ply%20testing/
ALL the glues should also be removed as you can see in this thread:
https://www.badcaps.net/forum/showth...965#post274965

[radicalmjp]

Re: Samsung 2493hm flashing backlight

Ok, I think I might find some old working 12V car lamps somewhere.....will try tomorrow hopefully.
This is to check the 24V output to the inverter, right?

[budm]

Re: Samsung 2493hm flashing backlight

Just do not use the head lights, too low cold resistance which will cause the power supply to shutdown. Not sure what you have in Greece, I use 1157 lamps low elements to start with in the US.
http://www.run-n-lites.com/bulbspec.asp

[radicalmjp]

Re: Samsung 2493hm flashing backlight

I have not abandoned this task....I am just swamped for now.... will get back to it with car lights to test... (anything else I could use to test except cart lights?)

[radicalmjp]

Re: Samsung 2493hm flashing backlight

Ok, I dont have at hand car lights, so I guess I will go buy some cheap ones for my bikes turn signals.
I did try two 12v 10W ikea lights and they light up nice and bright with no flicker at all when connected to the 24v output to the inverter (inverter disconnected)

[radicalmjp]

Re: Samsung 2493hm flashing backlight

Ok , removed most of the glue and where I didn't yet I at least cut and separated it from most components(will remove it all), this damn thing is like a chewing gum stuck under a cheap cafeteria table for 10 years... the purpose of this yuck eludes me as it seems randomly dropped on the board....why does a simple resistor need to be secured with glue, or a single capacitor but not the rest of the resistors?

Did the test with the two 12v lightbulbs from my motorcycle turn signals
With the inverter disconnected the lights turn on nice an steady and bright.
With the inverter connected the lights turn on nice an steady and bright.

I assume these results mean the power supply is ok and that the inverter should be the cause of the problems, right?


Sorry for the delayed reply/test but this was a week of chaos for me.

[budm]

Re: Samsung 2493hm flashing backlight

That sounds like it.

[radicalmjp]

Re: Samsung 2493hm flashing backlight

Update!
I ordered a used inverter (yes I cheated) from ebay and installed it today. The screen works, turns on but if no input is connected it scans for inputs (digital/analog/hdmi) and after few seconds it starts flashing, until it is completely black. If there is a video signal from a PC then it turns on for less than two seconds and goes black, so I guess unless anyone suggests something I will study the "two seconds to black" thread.
Note: if the screen has been resting for some time it takes longer to go blank. If it's still `warm` it goes immediately into flashing.... as if something is overheating.

[Mike Jenk]

Re: Samsung 2493hm flashing backlight

If you have some type of video source hooked up to it when the screen goes blank, it is easier to see a picture in the background when shine a flashlight around the screen, just ensure your in a dark room.

[selldoor]

Re: Samsung 2493hm flashing backlight

As above - as you have replaced a board you should really do all the
voltage checks from the power supply again.
Also - and I cant just spot of you did this on the old inverter board, can you check the inverter transformers.
Power off - you can even try this on your old board as well. Meter on OHMS2000 or autorange check resistance on pins S1>S2 then S3>S4 as shown in pic then post the results for each transformer

[radicalmjp]

Re: Samsung 2493hm flashing backlight

Ok guys I will do the measurements....just give me some time....going through some difficulties in life.

[radicalmjp]

Re: Samsung 2493hm flashing backlight

Transformer measurements
-------------------------
Old inverter board (fully disconnected from power and ccfl's)
transformer #1, S1+S2: 1712 ohm
transformer #1, S3+S4: 1700 ohm

transformer #2, S1+S2: 1754 ohm
transformer #2, S3+S4: 1719 ohm

transformer #3, S1+S2: 1747 ohm
transformer #3, S3+S4: 1694 ohm

New inverter board (fully disconnected from power and ccfl's)
transformer #1, S1+S2: 1662 ohm
transformer #1, S3+S4: 1654 ohm

transformer #2, S1+S2: 1640 ohm
transformer #2, S3+S4: 1634 ohm

transformer #3, S1+S2: 1664 ohm
transformer #3, S3+S4: 1654 ohm


Power board output to inverter measurement
------------------------------------------
Measurements of new (used) inverter installed and plugged
when ccfl are on 27.1V
when ccfl turn off with video input 27.9V

could it be some over voltage cut out thing?

( I feel I have to thank anyone taking the time to suggest things to try, so...thanks everyone!)

[radicalmjp]

Re: Samsung 2493hm flashing backlight

Forgot to mention!!
Yes, when the screen goes blank I can see the desktop with a flashlight.

[McEugene]

Re: Samsung 2493hm flashing backlight

solved case that worked for me after all procedures above failed in my case:
my psu is a BN44-00195A model 245B2 (slightly different part numberingfrom schematics above)
symptom was:
a black screen with blue power light steady on.
5,2V standby power was ok, primary PFC voltage was ok (380V for my european 220V input monitor). but the backlight 24V was not there ...(5.3V was not their either since it is a byproduct of the main 24V output...).

what I did:
A) replaced most polarized cap though all my caps looked ok (no bulging top)---> still black screen
B) checked for the yellow glue becoming conductive, clean up a couple of components ---> still black screen (no 24V out)
C) checked the operation of the MC33067P ICM801
pin 14 was always high and pin 12 alway low...check the oscillator (pin 2 ) and their was no oscillations there, so I started looking at reasons for not startup of the IC in the datasheet...
pin15 12V was ok (power supply)
pin 8 non inverting output of error was 5.1V =(ref voltage of the IC) --> force max oscillator frequency in principle...

pin 10 fault input was low when I mesured it...so in principle all was ok , no fault detected ...
By reading the datasheet of the IC , I noticed that if there was a pulse greater than 1v on the fault pin the IC would lock itself to prevent serious damage and remain in that state until power is removed...
It was a long shot but I decided to connect my DSO (! use double 12V-220V AC 3A transformers back to back on the 220V ac input to allow the whole supply to float and prevent to blast my scope inputs!!!! Ov of the scope is connected to earth in principle...)
So, i measured (captured) the Pin 10 level as I flipped on the power switch of the PSU and I noticed a tiny 10ms 1.1V pulse on the fault line after 200ms then the line was perfectly below the 1V threshold of the fault input pin (pin10)...I was nearing the solution.
There are two separate circuits tha can trigger a fault condition on pin 10:
1)- the overvoltage monitoring circuit via opto PC802S on the schematics(PC804S on my PSU model)
2) the "feedback/monitoring circuit" from the primary of the TM801S transformer (DM805-CM809-RM815 on the scematics)

on the overvoltage monitoring circuit, I disconnected the 0 ohm resistor (it not on the schematics, but on the PCB is is easy to find by following the tracks)
between the PC802S opto led cathode and the LMBT2222 transistor to desactivate that circuit and measured again the power up sequence on pin 10 ---> the 10ms pulse high was still there....
Then,
I checked carefully feedback components from the the primary of TM801S transformer and noticed nothing abnormal at first, then I notice some of that yellow hardened glue on the 47pF 1KV capacitor (CM809 on the schematics) in the feedback circuit. That glue was covering both legs of the capa!!! It was a long shot but I decided to desolder the capa and clean it up as nice as possible without breaking it...soldered it back...and it WORKED! The MC3306P was oscillating nicely and the 24V was back, I reconnected the 0 ohm resistor of the other monitoring circuit and all was OK.
My 2493HM monitor is back to life and so far it as been working perfectly again...

So my advice is CHECK the Yellow glue on all critical components and remove it if you can!!!

Now I know that PSU quite well since I have been working on it for a complete day or so....Since I have two of those monitors I am ready for the second one when it fails (hope not but...)
McEugene is online now Report Post

[budm]

Re: Samsung 2493hm flashing backlight

Bad glue

[radicalmjp]

Re: Samsung 2493hm flashing backlight

Hello again,
Thank for the posts to McEugene and budm, sorry I didn't notice them, I didn't get a mail notification and I had lost hope about help on the forum. Good news is that I managed to repair another LG screen.... but that was easy, bulging cap, 0.30euro to repair.....felt really nice.

As I mentioned my screen now turns on and after few seconds it starts flickering. Since I am revisiting the problem and I refuse to throw the screen I will scan for and remove any tiny amounts of this horrendous brown glue and remove it.

Thanks for the replies