Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue light.

[robertp]

I read all of these previous threads:
https://www.badcaps.net/forum/showthread.php?t=15796
https://www.badcaps.net/forum/showthread.php?t=9425
https://www.badcaps.net/forum/showth...t=6930&page=24
https://www.badcaps.net/forum/showthread.php?t=15843

and a few more that were less relevant.

Admittedly, I barely know what I'm doing here but I tried my best to understand the threads.
It seems the results were bad MOSFETs, bad inverter, and maybe the long blue high voltage capacitor(?).

This board seems to be a bit of an "unsolved mystery" compared to the earlier board (BN44-00173A) with the common 3 bad resistors.

In this thread: https://www.badcaps.net/forum/showth...t=15796&page=3
@epicelite sent one of these boards to @PlainBill, (https://www.badcaps.net/forum/showpo...2&postcount=60)
who confirmed it was somewhat functioning,
then it was sent to @Toasty (https://www.badcaps.net/forum/showpo...6&postcount=63)
who said the inverter FETs were dead and apparently solved it with a new inverter (https://www.badcaps.net/forum/showpo...0&postcount=65)


The capacitors are all fine from what I can tell, though I only have a cheap $20 multimeter from Frys and just did the apparently-unreliable "set to High Ohms" test.

Resistors and Fuses all seem fine too.

MOSFETs might be shorted, they are less than 30 Ohms. I don't know if I tested them correctly though. The results were very similar to a mentioned thread (https://www.badcaps.net/forum/showth...t=15843&page=4), something like 0.05 Ohms across the ones on the heatsink.

The Video board is getting 3.75V(IIRC) and 5.25V where it is supposed to, but nothing on 5.0V rails.

The Inverter is getting a short spike of >12V on the 24V rails which immediately dwindles down to 0.01. Those capacitors are fine, and MOSFETs(?) seem fine too. (With my limited testing ability)

Is there a way to determine if the problem is local to the power board or inverter? I will probably try to just replace one of those unless the problem is fairly obvious. (or most likely just toss the monitor, as much as I hate to.)

Could I use the earlier power board unit with the common bad 3 resistors? (BN44-00173A) https://www.badcaps.net/forum/showthread.php?t=9343
Are these units interchangeable?

The larger cap on the right (first pic) had a black marker dot on it. I marked one of the smaller caps on the left with black marker myself, but it turned out to be "probably OK" upon testing, same results as the others.

I suppose it could be the glue?

Any rough ideas of what to do to test if the PSU itself is functioning correctly?

[selldoor]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Hi You keep adding bits as I am reading. The Pictures missing are the bottoms of the boards.
I suggest your read this:https://www.badcaps.net/forum/showthread.php?t=10419
by retiredcaps. It includes lots of tests and you can see if you tested the mosfets correctly.
Yes it could be the Glue - did you read the post where it had gone conductive - suggest
as a first step you carefully remove as much as you can- looking for black marks where it may have gone conductive. When removed re check the 24v.

Can you look at C812? with a magnifying glass the end looks a bit odd and it is leaning towards a heat sink.

[robertp]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Reading that thread now.

I should mention the "flashlight test" shows no image, and the blue light does not turn amber or go into sleep mode.

I'll take some pics of the bottom of the boards later today if I can.
In case it helps before then: the PSU bottom is identical to previously posted pictures, all traces, no components.
The inverter is the "strange one" with all the traces on the top side, the bottom is blank.

[robertp]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

CM812 (lower left) was the one I marked with a black marker, which is why it looks strange in the picture. It is not bulging and appears OK, (inaccurate) testing with a multimeter gives the same response as the others, with Ohms set to 20K it increases from 0 to 0.66 and slowly increases from there.

[selldoor]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Could I use the earlier power board unit with the common bad 3 resistors?

This site is selling them as fully compatible :http://www.iccfl.com/product_info.php?products_id=6573

[robertp]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

That's great, thanks for finding that!
If we can determine whether it is the PSU that needs replacing I will just buy that, since this particular PSU seems more difficult to troubleshoot.

[PlainBill]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

A quick test can be performed by disconnecting the cable to the signal card and on the power supply connect the PS_ON pin to 5V_SB with a 3.3K resistor (these may not be the exact labels on the pins). If you get 24 volts to the inverter the power supply is probably fine.

Another way to check is to CAUTIOUSLY measure the voltage across the large cap, and the voltage on PS_ON. If the voltage at the large cap is about 385 volts the proper signal is coming from the main board.

CM812 can best be tested with a DMM with a capacitance range.

PlainBill

[robertp]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Thanks Bill!

I connected the SB to PS_ON and tested 24V, got 0.1V.
The voltage at the large blue capacitor was 1V, I think.
Seems like it's the PSU. Without being able to test the caps properly (and since it seems the MOSFETs are probably bad too) I'll just go ahead and order a new board in the next couple of days, unless anyone advises against it.

[PlainBill]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Thanks Bill!

I connected the SB to PS_ON and tested 24V, got 0.1V.
The voltage at the large blue capacitor was 1V, I think.
Seems like it's the PSU. Without being able to test the caps properly (and since it seems the MOSFETs are probably bad too) I'll just go ahead and order a new board in the next couple of days, unless anyone advises against it.

Did you do this test with the inverter connected? Don't forget that's what tripped up someone else. If it does not produce 24V with the inverter connected, but does with the inverter disconnected, the problem is the problem.

There has been some confusion; the capacitor I wanted you to check is the large black cylindrical electrolytic cap, not the large blue film cap. If your DMM has a capacitor test function, remove the large blue cap and measure it. If I recall correctly, the value is printed on the side.

The odds are this is NOT a bad mosfet.

PlainBill

[robertp]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Without the inverter connected (with PS_ON and SB connected with 3.3K resistor) the 24V rail gave me >12V which quickly decreased to 0.1V (only after a cold start), and the large black capacitor gave me ~385V.

With the inverter connected (with PS_ON and SB connected with 3.3K resistor) I got the same results for both 24V rail and 385V capacitor.

Both measurements were with the video board disconnected.

Does this tell you anything?

I tested the long blue capacitor with 2000k Ohms on my DMM, it increases to infinity, as do all the other capacitors- except CM812 and CP804 (the one above CM812), which stop at 6, same as before. Perhaps this could mean they are the problem? Or could it just mean they didn't get charged when it was powered? (Or perhaps my readings for the others comes from the large 385V cap discharging?)
Unfortunately I don't have access to a better DMM.

[Toasty]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Back of the PSU board photo please. Using picture 1 as a reference, I need to see the lower right corner under the transformer to the 2 connectors.

[robertp]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Here are alternates just in case something isn't readable, hope I got the right section.

[Toasty]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Lift either end of jumper J830. Using the 3.3kΩ resistor method PlainBill described to turn on the supply, measure the right hand end of the jumper to ground. See if it reads 24v and that it comes up and and stays.

[PlainBill]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Without the inverter connected (with PS_ON and SB connected with 3.3K resistor) the 24V rail gave me >12V which quickly decreased to 0.1V (only after a cold start), and the large black capacitor gave me ~385V.

With the inverter connected (with PS_ON and SB connected with 3.3K resistor) I got the same results for both 24V rail and 385V capacitor.

Both measurements were with the video board disconnected.

Does this tell you anything?

I tested the long blue capacitor with 2000k Ohms on my DMM, it increases to infinity, as do all the other capacitors- except CM812 and CP804 (the one above CM812), which stop at 6, same as before. Perhaps this could mean they are the problem? Or could it just mean they didn't get charged when it was powered? (Or perhaps my readings for the others comes from the large 385V cap discharging?)
Unfortunately I don't have access to a better DMM.

The voltage readings indicate the PS_ON signal IS being processed, otherwise the voltage across the large cap would have been lower (165 to 330 volts, depending on where you are located). I'm having a senior moment. Several (many?) months ago I came across the schematic for another Samsung power supply that uses the same design as this power supply. I was sure I downloaded and saved it; of course now I can't find it.

If you did the capacitor test out of circuit the results would indicate it has a reasonable amount of capacitance.

PlainBill

[robertp]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

@Toasty: It is indeed constant 24V. Now to figure out how to reconnect the jumper!

@PlainBill: I did the capacitor tests in-circuit so far, I'll take them out if it sounds like it'll help narrow it down. I'm not too great with a soldering iron so I'm afraid to mess up the board too much.

[Toasty]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Ok. Resolder the jumper? How did you un-do it?

Your pictures have a strange prismatic color split to them that makes them look jittery.

Check RT852 for a burn mark in the center. It looks like it is, but the picture isn't sharp enough and that is a -really- tiny resistor. Get the magnifying glass out for that inspection. Or, just measure it. It should be ~100Ω or less.

BTW: Where are you located? Update your profile if you would please.

Toast

[robertp]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

The jumper is underneath the heatsinks the MOSFETs are screwed to, and there is another component blocking the screw... I pushed it up through the bottom but I'll probably connect it back across the bottom rather than unsolder a bunch of other stuff to get to the screw.

My multimeter only goes down to 200 Ohms and all those tiny resistors all flash ~50-150ish and then shows "1 .", RT852 has the same result as the others.

The prismatic split is from taking the pictures through a lighted magnifying glass, it's the best light source I had available. I'll take some direct pictures.

I'm in the SF Bay Area.

Added some pictures of RT852 through a loupe, I'm not sure if it's solder or not. Amazing you saw that!

Edit: Looked through a microscope, pretty sure it's a burned pit.

Now the question is, how do I replace that- and what do I replace it with?

[Toasty]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Yup. That resistor is toast.

Amazing? No, I've seen it before. I have one of these monitors that was a subject of a previous thread that I bought off a member here. PlainBill has seen this PSU too and had it on his bench for a bit.

It is a 100Ω resistor. Marking is 101.

My suspects here are the Zener diode, DZT851, located just up and and right from J830. I can't see it clearly, but it might be marked WS. The other is QT851, right next to the burned resistor. That's a 1P, NPN type.

Gotta find the part numbers and get back to you. Shame you're not closer. I'd have you mail me the board as I have all the parts.

Here.

Somewhere....

Toast

[robertp]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

DZT851 has a bubble of magic yellow goop hanging off the side, QT851 looks physically OK to me. Any way to test these?

[shovenose]

Re: Yet Another Samsung 245BW - Power Board BN44-00195A - No display, solid blue ligh

Glad you found the issue!
Though it would still be smart to replace the secondary caps on the power supply if they are a lousy brand...