LG 47LB5DF blown power supply board

[cmlewis89]

Hi folks,

I'm new to this forum. So my trusty 17-year-old LG 47" LCD recently died. As I was booting it up, I heard a hum followed by a loud bang, sounds of pieces flying around, and loss of power to the unit. Inspecting the PSU, I saw two obvious problems: a blown thermistor (bang + flung pieces) and a visibly bulging main cap. Inspecting as many components as I could with my Fluke MM, I diagnosed that the following components on the "hot side" of the board were also bad: one of the 2 main MOSFETs, main 8A fuse, small cap (25V 47uF) connected to main caps, and a blown resistor (2.2 Ohm) connected to main caps (all marked with red "X" in pic). The bridge rectifier, diodes, Zener diodes, etc. test fine.

I replaced the MOSFET with an identical replacement, main caps (Samwha 450V 150uF "LP" 105C (M) 35x35mm) with (Chemi Con 450V 150uF "KMM" 105C (M) 30x35mm), and fuse/small cap/resistor with same value equivalents. Previous attempts at powering up the board resulted in blown main 8A fuses. Now that I've replaced the MOSFETs, the fuses remain good, and I'm getting a solid 120V into the board. The main caps seem to be charging fine at 160V DC (believe that's the correct standby voltage). However, I'm still not getting any output at all from the "cold side" of the board. It also looks like the replacement resistor (R508 - between 2 main caps) continues to blow upon powering up. It's rated for 1/4A 250V, so there must be too much current being pulled through.

I'm assuming there have to be more damaged components from the initial main cap failure. As a note, I've never tried to repair something as complicated as a TV before, so I'm still learning about more advanced electronics. Unfortunately, I couldn't find a schematic to upload, so I just took some photos of the board from various angles/lighting (hopefully they're clear enough, can always upload more). Also, yes, the bad resistor has a disconnected joint in the pic (desoldered for testing). The large blue and black diode was jerry-rigged to the underside of the board (the result of frustration trying to desolder prior to purchasing a desoldering station) - the connections all test good. The original red-striped 5A fuse also remains good, so I'm hopeful no power surges damaged anything downstream.

I believe that's everything I can think of at the moment. I'd highly appreciate any insights anyone has that can help me save "Ol' Reliable." Thanks!

[R_J]

If r508 is blowing the standby ic is bad R508 is a 2.2Ω fusible resistor

If the mosfet was shorted drain ~ gate, check all the components in the gate drive circuit, and it is still possible the drive ic could be damaged.
IC502 is a ICE3B1565J, there are also weak caps in this circuit, C506, C507, C508, C510 & C511
Check the resistance between pins 3 & 4 on ic502

[nomoresonys]

Also looks like some of your solder joints need attention.

[cmlewis89]

Thanks R_J and nomoresonys for the replies.

Yeah, I figured that IC must be bad since it's directly connected to a resistor that shorted (just didn't know how to go about testing). I measured the resistance between pins 3 & 4 on IC502 and got 1.3 Ohms.

The one MOSFET was shorted both drain ~ gate, and drain ~ source (the other MOSFET was fine). I'm still testing all the nearby components - don't see any obvious problems yet. Probably a very basic question, but would the drive IC be the one on the underside of the board (aka U600)?

As far as the small caps go, C510 was the one that shorted open and I replaced. These were the values I got (spec values in parentheses):
C506: 9.73uF, 2.9Ohm ESR (50V, 10uF) - RD V8C PET
C507: 44.8uF, 0.63Ohm ESR (50v, 47uF) - RD V3C PET
C508: 99.3uF, 0.29Ohm ESR (50v, 100uF) - RD V8C PET
C510: 47.9uF, 0.86Ohm ESR (25V, 47uF) - RD V8C PET (original)
C511: 1.006uF, 3.8Ohm ESR (50V, 1uF) - RD V7C PET

Agreed, my solder joints are a bit...amateurish. I tested to make sure the connections were solid, but I certainly intend on cleaning them up for the final build.

[cmlewis89]

As an update, I found that the standby IC (IC502) was indeed shorted. I replaced it, the connecting resistor, and the MOSFET drive IC (U600). Solder joints were also tidied up. The good news is that when I plug in the isolated PSU, I get no blown fuses/components/etc. I'm also getting a very steady 5.01V DC from the standby output pins (no voltage from anything else). The main 2 caps are humming very, very slightly and being charged/discharged at a steady 167V DC.

So, everything at first looks to be working. Unfortunately, the PSU fails once connected to the other boards. Testing the cables one at a time, the PSU fails once I connect to the main board. One MOSFET immediately blows along with the main 8A fuse (I don't see much other obvious damage). I'm not sure why the PSU board works perfectly fine when isolated but immediately fails when connected to a load. There's definitely a pattern where only the right (Q506) MOSFET blows when the PSU is connected to the main board. It either blows open gate ~ drain or damages the gate somewhat. The drain ~ source diode tests fine along with the entire other (Q507) MOSFET. I always replace both at the same time just to be safe.

I'd appreciate any helpful hints on some possible faults. Thank you.