LG32 no 24v line

[yogurt]

Hello all,

My problems is like this:

LG32 LG5000 turns on but picture is black. Backlight is not working. I found 4 A4614 transistors shorted and have taken them out. Still 4 left. I used an external power supply to check if the lamps are ok and yes, they all work. Because there is nowhere i can find A4614, a friend told me that IRF7389 will work the same. Oh boy was he wrong.... I soldered 4 new transistors and TV turned on for couple of seconds and BANG. A fuse blows. I test why the fuse blew, and I find 2 shorted mosfets. OK, so I replace the mosfets and the fuse, TV turns on but there is no 24v anymore only 1.2v. I disconnected the inverter board thinking maybe something was drawing power and triggering overload protection. Still no 24v. Then I start looking around to see if any resistors might have gone with the mosfets and the fuse but no luck. Everything tests out fine. What else could be damaged when the mosfets have blown to take out 24v line?

[yogurt]

Re: LG32 no 24v line

Pictures

[PlainBill]

Re: LG32 no 24v line

The pictures help a little.

The 'A4614' are probably AO4614; they are available from a number of sellers on eBay - 10 will cost about $11 delivered.

On the power supply, check the diodes (they look like power transistors) on the heat sink closest to the connector for the inverter.

Compare the original A4614 to the Alpha and Omega datasheet, paying particular attention to the logo to confirm that is the correct part.

If you have difficulties with the diodes, a picture of the bottom side of the power supply will help troubleshoot.

PlainBill

[yogurt]

Re: LG32 no 24v line

Thank you for the quick reply,

I checked the diodes out of circuit MBRF10100CT and they test OK.

Yes it is indeed the AO4614, sorry for the typo. We can't order from ebay in my country so I am forced to find something locally or find some distant relative that I can pay to mail me the parts.

I forgot to take the bottom picture, i will upload the photo tomorrow when my camera is available to me. Any other things i could test in the mean time?

[PlainBill]

Re: LG32 no 24v line

Thank you for the quick reply,

I checked the diodes out of circuit MBRF10100CT and they test OK.

Yes it is indeed the AO4614, sorry for the typo. We can't order from ebay in my country so I am forced to find something locally or find some distant relative that I can pay to mail me the parts.

I forgot to take the bottom picture, i will upload the photo tomorrow when my camera is available to me. Any other things i could test in the mean time?

The 1.2 volts is instructive. Was that with the inverter connected or disconnected? 1.2Volts suggests the SMPS controller is working, but is current limiting. Eitehr something (the inverter) is overloading it, there is a damaged current sense resistor, or there is a drive problem.

PlainBill

[yogurt]

Re: LG32 no 24v line

Before the mosfet accident, I had 24v with and without inverter board. Now it is 1.2v with and without inverter hooked up. I took out all fuses on the inverter to stop the mosfets from blowing up.

Good news is I found a way to order stuff from ebay so those ao4614 are coming

If now only we could get it to fire up 24v the tv would be fixed.

One thing i noticed and it might be helpful info to you> Mosfets are now giving out erratic voltages-from 0-150V DC. Maybe it is something before the large transformer underneath the inverter connector that is the culprit?

Pictures are coming within the hour too. Found a camera

[yogurt]

Re: LG32 no 24v line

More pictures

[PlainBill]

Re: LG32 no 24v line

Probably not the mosfet. If it were bad you wouldn't get anything. Most SMPS designs have a low value (< 10 ohm) resistor between the source pin of the MOSFET and Ground. The SMPS controller monitors the voltage across that resistor and will reduce the drive to the FET if the voltage gets too high. That resistor may be damaged, or the SMPS controller may be damaged.

PlainBill

[yogurt]

Re: LG32 no 24v line

I was not able to find a bad smd resistor between replaced mosfets source and ground. They all checkout. But, I have found two probably bad resistors marked A and B in the picture:

A: colors: Blue, Violet, Black, Gold = 5,5 Ohm Should be 68 Ohm
B: colors: Red, Red, Gold, Gold = 0.03 Ohm Should be 2,2 Ohm

I will replace them later today. Can they be the source of my problems, or have I discovered something even more wrong with this board?

[PlainBill]

Re: LG32 no 24v line

I was not able to find a bad smd resistor between replaced mosfets source and ground. They all checkout. But, I have found two probably bad resistors marked A and B in the picture:

A: colors: Blue, Violet, Black, Gold = 5,5 Ohm Should be 68 Ohm
B: colors: Red, Red, Gold, Gold = 0.03 Ohm Should be 2,2 Ohm

I will replace them later today. Can they be the source of my problems, or have I discovered something even more wrong with this board?

The possible bad resistor would not be an SMD resistor; the two you identified would be potential suspects, but it is unlikely they are the cause for several reasons.
1. I would expect them to be a lower resistance. I just checked the power supply schematic for a similar TV; they used .12 ohm resistors.

2. The resistor would fail high

3. I would expect the resistors to be in the area of Q110.

4. R123 (hidden under a dab of glue) is a possible suspect. THAT IS A GUESS; I have not had my eyes open wide enough to do a decent examination of the pictures.

PlainBill

[yogurt]

Re: LG32 no 24v line

No luck, replaced those resistors and the same problem exists. I checked and there are no other resistors near Q110 except that R123. It has 120K ohms on the meter and its colors say that aswell.

Do you have any other ideas what i could check? How can we test that SMPS controller?

I checked all other mosfets for shorts and they are all ok. All diodes checked and ok. What can I do to give you more info, so we can trace the problem?

[PlainBill]

Re: LG32 no 24v line

Since you ask, I need a decent picture of the bottom of the power supply. The pictures you provided of the top were quite good; those of the bottom were inadequate.

I need something well lit, and sharply focused. The recommendation is to put the camera in macro mode and turn off the flash. The limits on this site are 2000 x 2000 pixels; get as close to that as possible. Before you upload one, take a look at it. If you can't read component IDs, it doesn't really help me.

PlainBill