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Need help diagnose Samsung Syncmaster 153V monitor

Discussion in 'Troubleshooting and Repair' started by mikey5791, Apr 23, 2016.

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  1. mikey5791

    mikey5791

    173
    16
    Jun 7, 2013
    A friend sent me this old Samsung Syncmaster 153V LCD monitor to repair. The problem is power board no power, completely dead no light whatsoever. the power switch led no light during testing or after switched on main.

    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 case, I see no bulging caps or burnt marks on the power supply board and try search online for a service manual/schematic but none found.

    I ghetto work a 60W Light bulb tester(series current limiter) as per image 1644,I resolder the
    transformer (image 1629), 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 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.
    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.I desolder the rectifier diode 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 diode.

    Did some volt test on primary bridge rectifier=232vAC, main cap=317vDC
    There is power from primary output.
    On secondary side,voltage test by red DMM probe to diode cathode & black probe to chassis on diode near yellow trans(transformer)=0vDC, same probe testing on diode D320=-101vDC (why negative?strange) On diode D440 on heatsink(SBL1040CT)middle pins=0 vDC(zero volt)
    Is the result clearly mean secondary side no power output ? somewhere along the line there is a
    fault.
    How do I check output DC voltage on the white connector(image 1685) ? Do i put positive probe to third wire from right side marked +5v? then negative probe to chassis ?
    The light bulb in series still lit brightly flickering. Something else is shorted.

    Anyone pls help diagnose/solve this no power from secondary output or list down voltage test point.
     

    Attached Files:

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