Need help diagnose Samsung Syncmaster 153V

[mikey5791]

Hi all ,

A friend sent me this old Samsung Syncmaster 153V LCD monitor to repair. The problem is no power,completely dead no light whatsoever. He had sent to a shop for repair before and the monitor came back to life for a short while, later he kept it aside as spare unit but later found out the unit is non working.

I had some basic skills in repairing dead monitor with a soldering iron,solder pump,a digital multimeter (DMM), and a digital capacitor meter. After opening up the plastic case, I see no bulging caps or burnt marks and try search online for a service manual/schematic but none found.

Testing at component level on the power supply board at position D660=Schottky Diode STPR10 is open, failed diode test.

Also at position D440=LT 350 SBL1040CT Schottky Rectifier Diode is open and failed diode test. (refer image 1450) The new parts will arrived in about 2-3 weeks from now. I have the following questions:

1) What are other components that are likely failed and needs checking?
2) How do I removed the two faulty diodes(refer image 1450) which are placed in a tight spot, my philips screwdriver can 't reach the screw as they are blocked by the yellow square transformer.
3) How do I use light bulb series to limit the current going to the power board
after putting in new parts?

Attached image 1411 as board top picture,image 1440 as power &VGA board
and image 1456 as VGA board.


Appreciate for any help or suggestions.Thanking in advance.

[stj]

Re: Need help diagnose Samsung Syncmaster 153V

ouch.

firstly remove the transformer to get your screwdriver in,
you need to resolder all the transformers anyway.

second, the 4 caps in the enter with a pink tint are done, they are for the invertor function.
given the age and heat i would just do a full recap.

the lamp thing, you make up a power cable with a 60w lamp inline with the live so it acts a bit like a fuse.

[mikey5791]

Re: Need help diagnose Samsung Syncmaster 153V

Hi stj,

Appreciate your reply.

The 4 caps you mentioned in the center with pink tint, is it the image 1465 which
i circled in red on the right side of yellow transformer. Those are 47uf caps which
I will recap.

I ghetto work a Light bulb tester as per image 1463, diagram is per attached. Is this
the right way to assemble ? I tested on a faulty crt monitor, the light bulb gives an
initial bright, then shuts off after about 2 second. Is this meaning the test equipment as in the crt monitor is operating normal?

Eager to know and hope for any suggestions and answer. Thank you.

[stj]

Re: Need help diagnose Samsung Syncmaster 153V

yes, that's all correct.
some stuff wont work properly with the lamp, but your only using it to make sure the psu wont explode when you power it up.

if the lamp goes full-bright and stays that way - you just saved yourself a fuse & other stuff!


those caps went pink from the heat btw - they never made them that colour!

[mikey5791]

Re: Need help diagnose Samsung Syncmaster 153V

Hi stj,

I resolder the transformer (image 1609), and some 3 pieces 470uF25V caps at position
C665,C662 &C661, also remove a 330uf25V cap at C17 and the 4 small caps (each is 47uF25V). Tested using my only capacitor meter & Replaced those 470uF that reads less
than 10% off scale (like 425uF)..... surprisingly the 4 small caps (47uF25V) in the center
still reads within range, so i didn't replaced them.

Tested continuity of transformer with an analogue multimeter, it shows continuity; so i put it back. After replacing some problem caps, I desolder the round pink fuse near the main cap, and connect to a 60w light bulb (as a series current limiter). Switched on 220AC main without connecting to LCD panel or vga board, the light bulb light on a second then lights off, followed by flickering(oscillating siren like) light bulb. This indicate somewhere there is still a fault. Also I noticed some buzzing/hissing noise near the yellow transformer area.

Any idea where to check now. I really appreciate any help or suggestion.
Thanks.

[charis]

Re: Need help diagnose Samsung Syncmaster 153V

The switching mosfet on the heatsink is gone.
Why you put out the transformer...
Change it and if you have a noise check the x capasitor and the two suppression caps.

[Joe Black]

Re: Need help diagnose Samsung Syncmaster 153V

What Charis sad is likely to be true.
That is a 3-legged component bolted on the heat-sink. The one that got yellow-ish colour on pics looks like it failed, but can't tell for sure.

Google on how to test it. If those are OK, check for short on graetz, and so on

[budm]

Re: Need help diagnose Samsung Syncmaster 153V

Run the power board by itself without any cable between the power supply and the main board connected, if the LAMP did not stay on bright then check the DC output of the power supply to see if they are present. You are testing it this way so the inverter circuit will not be turned on to see if the power supply is even working or not, you may have problem in the inverter section which will draw lots of current when the main board sends the signal to turn it on.
The power supply is an always on power supply.
BTW, to have BOTH Power rectifiers to be bad (open per OP) is very strange.

[mikey5791]

Re: Need help diagnose Samsung Syncmaster 153V

Hi,

I desolder the switching mosfet STPR10, off circuit tested ok, but anyway I replaced
with STPR1020CT. Also i replaced new 470UF 25V & 1000UF 25V caps at C661&C441 just in front of the new mosfet.

Charis mentioned about noise check on x cap, is it the yellow square one near the main cap? where is the two suppression caps, sorry i got no clue. I use cap meter to check the yellow square x cap, reading is 0.33UF as printed on the cap.

How do I check output DC voltage on the white connector ? Do i put positive probe to third wire from right side marked +5v? then negative probe to screw hole ? sorry can't upload photo yet.
Thank you for any help.

[budm]

Re: Need help diagnose Samsung Syncmaster 153V

Hi,

I desolder the switching mosfet STPR10, off circuit tested ok, but anyway I replaced
with STPR1020CT. Also i replaced new 470UF 25V & 1000UF 25V caps at C661&C441 just in front of the new mosfet.

Charis mentioned about noise check on x cap, is it the yellow square one near the main cap? where is the two suppression caps, sorry i got no clue. I use cap meter to check the yellow square x cap, reading is 0.33UF as printed on the cap.

How do I check output DC voltage on the white connector ? Do i put positive probe to third wire from right side marked +5v? then negative probe to screw hole ? sorry can't upload photo yet.
Thank you for any help.

STPR1020 is not MOSFET it is diode, so I have no idea at this point as to what you have replaced. MOSFET will usually shorted out when they go bad, you need to look up the spec sheet on the parts you are replacing,

[mikey5791]

Re: Need help diagnose Samsung Syncmaster 153V

Hi budm,

Sorry my mistake, the original STPR10 is a 10A 100V Silicon rectifier diode,
but my supplier do not have this stock. So I no choice have to use STPR1020CT as nearest substitute. Off circuit, I had tested the diode with diode test on DMM, is tested ok. Earlier I was referring to charis on post No.6 " The switching mosfet on the heatsink is gone."

But sadly after replace this diode, the light bulb in series still lit brightly flickering.
Something else is shorted. The next diode is SBL1040 which I use diode test on DMM, also tested good. So I run out of idea, maybe someone can guide me on voltage test on output.

Thanks.

[mikey5791]

Re: Need help diagnose Samsung Syncmaster 153V

Hi,

Uploaded some more photos after replacing the original diode STPR10 with STPR1020CT, and changed a new 470uF 25V & a new 1000uF25V cap.

Refer image 1683,how do i test for DC output voltage at the square white connector? Do I just put red DMM probe to third right wire labelled +5V,and black probe to cold ground?

Feeling stumped and not sure where to look for fault in this power supply since after I switched on AC main, the light bulb series lighted a second and off, then 2 seconds later flicker brightly. This indicate there is still a short circuit. Or do I need to connect to main vga board and connect to the 4 ccfl lamps as load for testing?

Appreciate for any help on what to do next and I am willing to learn patiently.
Thanks in advance.

[mikey5791]

Re: Need help diagnose Samsung Syncmaster 153V

Hi,

Last night I retried switched on AC main, the light bulb series glow a second then shuts off; this time no flickering. Is this indicate there is no more short circuit on component? But there is still hissing noise somewhere near the yellow transformer- unsure what is this mean.

[mikey5791]

Re: Need help diagnose Samsung Syncmaster 153V

Hi,

Did some volt test on primary bridge rectifier=232vAC, main cap=317vDC
There is power from primary output.

On secondary side,diode near yellow trans(transformer)=0vDC, on diode D320=-101vDC (why negative? strange) On diode D440 on heatsink(SBL1040CT)
middle pins=0 vDC(zero volt)

Can hear audible hissing suspect from trans but not sure what the noise means.
Anyone pls help diagnose/solve this no pwer from secondary output.
Thanks.

[budm]

Re: Need help diagnose Samsung Syncmaster 153V

OK, I am a little confused, so just having the power supply-inverter board running by itself without having the Main/Logic board connected and you still have the lamp on brightly?
As shown in this picture?
https://www.badcaps.net/forum/attach...9&d=1460555768

[mikey5791]

Re: Need help diagnose Samsung Syncmaster 153V

Hi budm,

The series lamp lit a second, then off(no light),no flicker(blinking like ambulance light). Then i did the voltage test and found no power to
secondary side.
Kindly comment and advise.Thanks.

[mikey5791]

Re: Need help diagnose Samsung Syncmaster 153V

Hi,

This is what I found on secondary power output:
On secondary side, red DMM probe to diode cathode & black probe to chassis near yellow trans(transformer)=DMM reads 0vDC, same probe testing on diode D320=-101vDC (why negative? strange)

Next, red DMM probe to middle pin & black probe to chassis ; On diode D440 on heatsink(SBL1040CT) middle pins reading=0 vDC(zero volt)
Is the result clearly mean secondary side no power output ? somewhere along the line
there is a fault. Can anyone list out voltage test point to do further check why no power? Also, noted the power switch led no light whatsoever during testing or after switched on main. I thought there should be a blue led light on switch if there is power.

Thank you for help.