LG 47LM4600 Blown TCON fuse and Main Board Transistor

[jason123]

Hi friends..
The TV in my son's room just lost the picture... I thought it was backlight since audio is on...

Backlight is on however no picture. First check is the TCON fuse. Sure enough the 2A 125V Fuse was open. I noticed a transistor (?) Q409 on the main board (EBR74404304) by the LVDS connector is also blown. See photos.

Questions:
1. Anyone with a schematic of the main board so I can find a replacement for that transistor?
2. Replacing the fuse on the TCON, I'm assuming is going to result of that transistor blown again. What is the next step?

Thank you

[budm]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

Q409 is the switched MOSFET for supplying 12Vdc to run the T-CON board, some thing is badly shorted out on the T-CON, or the parts on the tab bonding boards whcih are connected to the T-CON has shorted parts. You have check the T-CON board for shorted parts, you can started by checking the resistance of various caps to see which one will show very low resistance first.

[jason123]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

Thanks budm,
Any idea what markings are on that FET so I can replace it? it's pretty charred on my board
Thanks again

[jason123]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

As it turns out the schematic is in the back of the service manual. Here it is for anyone looking (hopefully link is allowed on this forum, if not, please remove and I'll upload the file)

[budm]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

T-CON power.

[jason123]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

Thank you budm!!

[budm]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

AO3407A P-CH MOSFET by Alpha-Omega.

https://www.digikey.com/product-deta...6-1-ND/1855948

[jason123]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

Thank you budm. Found replacement and it's on order.

I tested all caps on the panel boards - All seem to check out. I took the TCON board out and externally powered it with 12 V, and it seem to have a dead-short on it. Testing diodes (prior to current-limited test) and one seems "Short" and the other not identified as a Diode by my meter. caps got damage during my test as well and I would like to know their specs

By any chance, would you have a schematic for that board?
it's 6870C-0148A

I'm buying one online, but curious to see if i can fix this one. Diodes are marked with 3125 but I can't seem to find them online.

[jason123]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

Issue was a bad TCON card apparently. Replaced it and all is good - replaced the FET on the main card, Q409 first, of course. Thanks budm

[budm]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

So what RESISTANCE do you get on the 3125 Schottky diode (2~3A 100V)? If it tested bad then you need to remove it and test it off the board.

[jason123]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

116 Ohm off the board. Diode tester is not detecting it as a diode

[budm]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

If you test the resistance of the empty pads where the diode was soldered to, what do you get?
I suspect that the original shorts may be due the MLCC (Multi Layer Ceramic Cap), those brown rectangular caps.

[jason123]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

2.06k Ohm across pads. ESR on all the caps is similar in the range of few 100s mOhms. DC resistance across caps in the area is in the kOhm range. I can replace all of these, I just don't know the rating and value of them.

[budm]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

We looking for something that is really shorted out that cause the MOSFET on the main board to burn out like that.

[jason123]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

A shorted Diode doesn't doesn't do that??

[budm]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

If you apply 12V to the two legs of that BAD diode that shows 116 Ohm, the current will be = 12V/116 Ohms = 0.103A.
But at this point, you should replace the know bad part.
We are looking for low resistance around 1 ~ 2 Ohms.
1 Ohm will draw 12A from 12V power supply.
2 Ohms will draw 6A from 12V power supply.
Typical fuse on T-CON is 4A or so.
BTW, what happen to the MLCC cap that you blown up during the test?

[jason123]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

Cap just had dark spot on the board, but ESR and Capacitance test shows it acceptable (not short at least)..

I'm trying to replace that Diode.. Only marking on it is line 1 is 31, and line 2 is 25.. can't seem to find this part number

[budm]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

It is just Schottky diode for the switching power supply in that section, 2~3A 100V rating or higher will be fine, just have to match the dimension.
BTW, that the diode that shows 116 Ohms in both direction when using the Ohm meter to read the resistance, you can try hooking it up to variable power supply in the reverse bias and turn up the power supply output to see if the current starts going a lot when Voltage is turned up toward 12V.

[jason123]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

I will try that in the morning.. Thanks budm. Vfd is important at all??

[budm]

Re: LG 47LM4600 Blown TCON fuse and Main Board Transistor

The Vf of Schottky diode is lower than the Silicon, the important is the switching speed, regular diode will heat up if used in high speed switching power supply.
Vf rating is also based on the If, so you have to look at the graph.
If you have spare switching power supply you can use the through hole Schottky diode from it for testing.
Read the example of the TPS IC spec sheet for the type of the diode to be used.