Samsung Monitor repair

[R791945]

I thought it would be some fun giving me a sense of success to try to repair the LED monitor I have for my computer.

One morning this week it started and then within seconds went black. This happened subsequently on each start up of the monitor.

I found an article which suggested that it was probably capacity problems and that they should be changed:
http://510x.se/notes/posts/Repair_LCD_monitor_with_bad_capacitors/
I have isolated the power supply in my model and the capacitors look in very good condition: no liquid, no bulging cap.

What should I do next to find the fault? I have a volt meter that measures volts, ohms, mA, uA. I also have a digital probe, otherwise normal tools.

How can I test if the power supply is truly working ok?
One route occurs to me is that they may be a dry joint on one of the capacitors. Should I re-solder them before i know if the board is not the fault?

 

[R791945]

I tried to add a picture to the first post above but as you can see it was crossed out. What are the rules about picture posting?

 

[bertus]

Hello,

You can upload a file to the forum with the upload a file button.
Keep the size below 300k, so resize the picture if needed.

Bertus

 

[R791945]

 

[pharaon]

first you should have a multimeter, and the after connecting the power supply to power source make sure if there's an output voltage or not

 

[Bluejets]

One morning this week it started and then within seconds went black. This happened subsequently on each start up of the monitor.

It could be backlight problem.
With the monitor turned on, if you look at an angle to the screen with daylight reflection, can you see any really dim image on the screen..??

Failing that, you will need a multimeter and with it set on DC voltage, run through the measurements on the terminal strip (1 to 12) in the photo bottom left. There is a list of what should be there.
This assumes everything is connected and turned on.
Most will be with reference to Gnd, terminal 3 or 4.

Warning that there are some high voltages on the power board upwards of 300v dc not to mention mains supply voltage,(anything in the zone above the white demarcation line) so if you are not familiar with it, then leave it alone.

 

[R791945]

It could be backlight problem.
With the monitor turned on, if you look at an angle to the screen with daylight reflection, can you see any really dim image on the screen..??

I looked for that and didn't see anything

Failing that, you will need a multimeter and with it set on DC voltage, run through the measurements on the terminal strip (1 to 12) in the photo bottom left. There is a list of what should be there.
This assumes everything is connected and turned on.
Most will be with reference to Gnd, terminal 3 or 4.

1. Do I need to connect to the screen and the computer for this voltage test? I ask because some computer type PSU's will not work unless they sense a pull on their current? Will this type of PSU output current even if nothing other than the volt meter is connected?
2. If the voltage test is ok will that also confirm that the capacitors are working properly?

Warning that there are some high voltages on the power board upwards of 300v dc not to mention mains supply voltage,(anything in the zone above the white demarcation line) so if you are not familiar with it, then leave it alone.

Noted. I may have to ask for some guidance if that zone has to be entered.

 

[Bluejets]

1. Do I need to connect to the screen and the computer for this voltage test? I ask because some computer type PSU's will not work unless they sense a pull on their current? Will this type of PSU output current even if nothing other than the volt meter is connected?
2. If the voltage test is ok will that also confirm that the capacitors are working properly?

It may be that all the monitor would need to be connected to get the power supply running but I don't see why it would need to be connected to the PC.
Monitors usually have a menu screen which would be sufficient to tell if operational.

I may have to ask for some guidance if that zone has to be entered.

Advice already given there.....point is, you may unintentionally enter that zone, and chances are you'll never know about it as it could be too late.

Start with the low voltage strip, see what you get there.

 

[R791945]

Advice already given there.....point is, you may unintentionally enter that zone, and chances are you'll never know about it as it could be too late.

I had noticed the zones but thought the hot area related to heat not danger of electrocution. I am very grateful for your warning. Sincerely, thank you.

 

[R791945]

It may be that all the monitor would need to be connected to get the power supply running but I don't see why it would need to be connected to the PC.
Monitors usually have a menu screen which would be sufficient to tell if operational.


Advice already given there.....point is, you may unintentionally enter that zone, and chances are you'll never know about it as it could be too late.

Start with the low voltage strip, see what you get there.

As a first stage I have tested the the connections to the terminal strip, without connecting the screen. All the + outputs correspond to their rating. None were dead. The connection to the +16v was about 17.5v. Do I need to proceed to connect the screen and redo the test?

The capacitors on the board are presumable all full. Is it safe to discharge them by using the DC setting of the voltmeter across the terminals of the capacitor?

 

[Cirkit]

The capacitors on the board are presumable all full. Is it safe to discharge them by using the DC setting of the voltmeter across the terminals of the capacitor?

You can measure the state of charge of the capacitors by setting your multimeter to the appropriate voltage setting and measuring across both terminals.

If they still hold a charge, you can discharge with an automotive bulb, 5k to 10k power resistor or the blade of an old screwdriver (this should be your last resort).

 

[R791945]

Can I please clarify if your comment relates to capacitors still soldered to the board?

 

[Cirkit]

Yes, they can be discharged while connected to the board. C1 is of most concern.

Make sure that the mains power inlet is disconnected before going anywhere near the capacitor.

 

[Bluejets]

or the blade of an old screwdriver (this should be your last resort).

This should never be done.

Some of the voltages should not be present unless the power button is operated.
What indication do you get there?
Should be some LED indicator somewhere, possibly dual colour.

 

[73's de Edd]

Sir R791945 . . . . .

So, you have yourself a 27 inch display screen there, with that being quite blasé as a TV screen size, but starting to be quite impressive when being a computer monitor.
After my initial observations and resultant thoughts, I highly suspicion that you have had a cumulative failure of your units Hi Voltage inverter for powering up your units Compact Fluorescent Lamps that illuminate your forward light transmissive display screen.
Since all of your supply voltages provided are present at the test points , one then needs to look at the right half . . . "cold" side half of the circuit board and inspect the dual inverter transformer that is located at board center right and denote its dominant BLUE / BLACK color scheme.
Printed at its very front is being its specs of using 5VDC at 3A ( THAT'S BEING 15 WATTS ! BABYCAKES ! ) into its primary winding (of each separate transformer) along with a measly 10 ma drag being needed from your 16VDC supply.
That then creates an outputted hi voltage on the secondary winding of each transformer of 750 volts @ 7.5 ma . . . which is also being ~ ~ ~ ~ 15 watts ! . . . but that's being derived by implied Reversed Polish Logic computations.
Those supplies outputs are series transferred via the BLUE C75-76 and C77-78 ceramic capacitors to the mating CN72-73 onnectors which in turn feed into your bank of uniformly spaced apart, UBER skinny, CFL display tubes, that are lining the back of the display.
Applied 750V is being high enough to cause their gas fill to initially ionize and do a start up flash, then the inverters electronics cumulatively adjusts for a corrective constant current / voltage feed to the tubes .

Now time for the BIG SHOW . . . . . thereby, imparting your astute powers of observation . . . . and possibly with some analytical sniffle-wiffles being thrown in additionally.

Look at the longitudinal BLUE tape wrap of theT-former . . . . . the top half looks DARKLY discolored like its been runnin' hotter'n hell.
Additionally . . . do a fingernail pressure test of the center of the darkened area and relate it to the comparatve same spot of the
other half of the transformer. The dark spot was less flexive or even fracted under pressure . . . right?

IT'S SHOWTIME ! . . . . .

Now peel back the BLUE tape to reveal the two separate secondary windings and compare their relative sameness of appearances.
Also a good time to have the set in an unplugged into AC power state and then be able to take ohms readings of the two secondary windings . . . . expecting the darkened one to be APPRECIABLY lower, due to its magnet wires insulation breakdown and cumulative shorting between turns.
Your SNIFFER placeed right down onto the windings should tell you a lot also.

Now you initially failed to show it, but another good photo of the BOTTOM / FOIL side of the PCB should show a controller IC and some power FETS that drive these inverter transformers.
I hope that none of them crashed and burned from a volumetrically increasing current demand . . . before having the small 4A GREEN Pico fuse, located between C54-55 E-caps . . . going . . . PING! . . .open circuit.
The white silk screening rectangular markings also shows an alternate use of a brownish / maroon chemical fuse unit . . . . but they go / fail with a W h o o o o o o o s h.

So o o o o o o . . .go ye forward and enlighen yourself, with your newly to be found . . . findings.

As for me, give me feed back of the voltage ratings being marked on that inverter portions E-caps . . ...

Plus . . . I suspect that the parallel shunted D21-22-23 (as is being done for current capability multiplication) are creating the 5VDC supply from their silver banded cathodes to feed into C54-55 for filtering.
Likewise, the same, for D24-25-26 to have their cathodes be outputting into C21-22 for filtering and then into orange topped ferrite bead inductor and finally into C23 for the 3.3VDC logic oriented / based supply.
I'm uncertain on the 1 lesser diode and minor E-cap needed for the minor 16 VDC supply, as I see no other smaller diodes and E-caps . . .BUT . . . .one of the sets that I had just assigned to the former 2 supplies might be used instead.

Another shot of the involved chassis . . . . but STILL . . . .being WITHOUT the money shot . . . .the foil pattern side..
BUT this further confirms that your Inverter transformer part number is being QGAH02113 . . . . . by virtue of its its duplicity.

PRE HOSTED IMAGE SOURCE . . . . .

Thaaaaaaaaaaasssit . . . . . . ( Th . . .th . . .th . . .Thaaaaaaaaaaaats all Folks )

73's de Edd . . . . .


Life is being all about perspective, when you think about it, the sinking of the Titanic was a God sent miracle to all of the multitude of live lobsters in the ship's kitchen.


.

 

[R791945]

73's de Edd
Thank you for your impressive response.

1. C-caps marked: BD 3KV
2. Per attached the windings on the left are burnt out. I did not test the voltage. See pic attached
   
3. I have not tested the PICO fuse. Would a continuity setting on the v meter be safe?
4. C1 cap shows 2.10 volts. I did not see the voltage start to drop when I touched the terminals with the V-meter
   
5. My pics of the back are not coming out well. Maybe in daylight it will be better.

 

[Bluejets]

Last pic looks pretty conclusive to me, even without any further testing.

Board similar to this advertised on Ebay from the States for US$29.00.

https://www.ebay.com.au/itm/Samsung...306199?hash=item3d7940f8d7:g:xJUAAOSwzcNb4hdW

 

[R791945]

Last pic looks pretty conclusive to me, even without any further testing.

Board similar to this advertised on Ebay from the States for US$29.00.

https://www.ebay.com.au/itm/Samsung...306199?hash=item3d7940f8d7:g:xJUAAOSwzcNb4hdW

I would buy one but the postage to the uk plus import duties work out to an extra $54!
As you found one similar what did you search on? I will try it on Ebay.Uk.

Correction to post 16 item 4
I rechecked if the cap at C1 does discharge during voltage checking via a V-meter. It did do so though sufficiently slowly for me to not have realised it was happening.

 

[R791945]

Bearing in mind that I know the output to the terminal strip is OK, is there a way to simply check if there are other faults on the reverse transformer side? I have found some suppliers offering just the reverse transformer. I will of course have to check if the rating is the same as on the board.