Samsung 245BW power led comes on but no backlight

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

heres a little bit better pic

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

i noticed something strange. when i first started working on this monitor, the blue led came on as soon as i applied power (didnt have to push the button)
but now when i apply the power, the button stays dark until i press it. then it lights up blue. and then after a minute the blue light flashes on and off. pressing it again turns it off. and then i can turn it back on right away...and another minute before it starts flashing again... what's going on here? maybe changing the resistor made this happen? thats the only thing ive changed besides removing glue....


edit: i figured out it was because the computer i had it hooked up to went into power save mode so it turned off the monitor. i guess the blinking light is normal in that case.

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

oh and i get 3.3v on the PS_ON pin and 5.3v at the SB pin....

[budm]

Re: Samsung 245BW power led comes on but no backlight

Are you getting about 292VDC right at the two legs of the main filter cap CP803?
Also the another common failure for not having the 24VDC is the bad capacitor CM802 (18nF 630V) you will need the cap to verify the value. Just remember one thing when making DC voltage measurement in the primary side, the negative leg of CP803 is your ground ref for your meter.

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

how do i tell which side of the psu is the primary side and which is the secondary side? if i have the black probe on the negative lead of the big cap (cp803) and i try to take a measurement of something on the secondary side, what will happen?

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

ok on cp803 i get 390v ... i saw this in another thread too...it says its supposed to be 290 somewhere but its actually 390...

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

and what did you mean when you said "CM802 (18nF 630V) you will need the cap to verify the value"

what do you mean i need the cap? i have the cap. its right there on the board. what do i have to do with it to test it?

[budm]

Re: Samsung 245BW power led comes on but no backlight

I mean meter that can read the capacitance of the cap. You have to remove it to test it off the board. Did you verify your meter battery?

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

yeah my volt meter is fine i tested it by testing some batteries with known voltages

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

i dont have a capacitance feature on my DMM ... got this from ebay but didnt put it together yet...they didnt send any instructions!
http://www.ebay.com/itm/281262386306...%3AMEBIDX%3AIT

anyone build one of these yet?

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

i desoldered the cm802 cap... a friend of mine supposedly has a DMM with a capacity feature so im waiting for him to come over with it

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

so my friend came over with his DMM that can supposedly test capacitance but i cant seem to get a good reading out of it. i tried testing caps i salvaged, and even some brand new caps i ordered to fix my xbox360 but havent put in yet. i guess its autoranging..
with the probes probing the air, and the mode set on what i assume to be capacitance (says nF in the upper right corner of the display)
it reads .39
sometimes it reads .44 or something close around that
anyway i put the probes on the capacitor leads
black to the one with a stripe (shorter lead)
red to the other one
the display on the multimeter usually does not change.
once in a while it does change but to some number like 1.8 or 3.7 or something meaningless like that. it will stay at that number for about 10 seconds then it switches to saying OL (open line?)
if at any time during this "charging process" i remove the probes, i get a random reading of anything from 5.something to 27.something ...i cant see a pattern really. i think the longer i hold the probes there before it says OL the higher value i get but i cant be sure.

when it switches over to saying OL the little icon in the upper right of the display switches from nf to uf...

do i just not know how to use this thing, or is it broken?

*frustrated*

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

oh yeah i should mention i tried the same thing with the probes reversed and it makes no difference

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

i guess youre supposed to zero it out first...but i did that and it still pretty much does the same thing...

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

ugh i think i figured out why its not working. on page 7 of the manual for this DMM it says it can only read up to 100 uf ... is that right?
pretty useless...why would it only go up to 100uf???

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

oh wait but if the thing im trying to test is only 18uf then that should be fine.
so i tested a bunch of smaller salvaged caps and most of them come up close within a couple microfarads of what they are supposed to be... what is the tolerance on these things? anyway i think i verified that my DMM is reading capacitance mostly accurately up to 100uf anyway. so trying the test on the 18uf capacitor in question for this tv, i find it goes to OL after some time and thus... it is most likely bad. but without being able to test it on the board before removing it, what confidence can i have that i didnt ruin it myself with too much heat or pressure while removing it?

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

oh man i just realized i was misreading.... its supposed to be 18nf not uf ... either way they are both in range of this DMM which i have verified as giving accurate readings... so i think its safe to assume the capacitor is bad... but as i still would like to know how to tell that i didnt destroy it myself while removing it...seeing as i cant test it prior to removal...?

[triplefour]

Re: Samsung 245BW power led comes on but no backlight

oh and what should i do for a replacement? can i find this from scavenging or should i order it?

[bammbammfran]

Re: Samsung 245BW power led comes on but no backlight

Since you have 5.3 at the connector and 3.3 at the on-off
the power supply is partly on.
Now see if the on/off pin changes voltage while you push the power button
also check to see if there is a dim pin next to the on/off and if that has any voltage
If dim has voltage and on/off changes, unplug the inverter board and recheck for 24volts at the inverter plug of the power supply when the power button is on.
If the 24 is there, then the inverter may be shorted.
if it is not there, then we have to dig further into the power supply.

[C152Heavy]

Re: Samsung 245BW power led comes on but no backlight

why would i put the original resistor back in when i can clearly test it with the DMM and see that it has infinite resistance! (open) i see your point on changing the caps cause theyre cheap but hey if i can test them first to be sure it would save a lot of soldering and a few bux. my goal is to identify the problem and only change what needs to be changed, not make guesses!

You said:

this is baffling: with the resistor i was just measuring at 420 this is when i have my multimeter in 2000 ohms scale. on 200 ohms scale it reads infinite (makes sense cause its over 200)
but if i put the scale up to 20k ohms i should expect to see less than 1...somethink like .4kohms, right? but it says 5.85 kohms
and it i put the scale up to 200k ohms i get 32k ohms
and if i put it up to 2000k ohms i get 221!

so which of these readings is correct? why am i getting wildly different numbers just by changing scale?

I suggested putting the original back in because it's quite unlikely it failed and you're having trouble measuring or reading its value so it's reasonable to put it back to get back to where you started and look for a more likely failure mode.

It only reads infinite because you're using the wrong scale (200 Ohm) and as another poster pointed out you need to keep your fingers clear of the test leads so you don't corrupt the reading.

The most common failure mode for monitor power supply boards and a lot of computer gear is bad caps, that and the symptoms (power light etc.) are the basis for an educated guess to start troubleshooting. That's different from a wild guess. I put a resistor which has mysteriously gone open without physical signs of failure (discoloration, charring, fracture) as a long shot guess. Resistors just don't fail like that.

FYI, that type of resistor has an MTBF of about 3.343573e+7 Hours, the cap would be about 4.972205e+6 hours so the MTBF of the resistor is about 6.72 times as long as the cap but we see that Samsung has used poor quality caps in their monitors so their MTBF will be worse.

To verify the condition of the caps you need to remove them (cannot in circuit test) so you might as well replace them right off because that's the most common failure mode. If measuring them after they are removed demonstrates that they have not failed then go from there knowing that's not the problem.

By the rest of your posts I'd suggest to take some time to get more familiar with how to use your test gear by making measurements on some known good components. Once you can reliably make repeatable valid measurements then you can be confident to start digging into the monitor to do diagnosis.

Good luck & keep at it, it's a great learning experience!

C152Heavy