LG 60LB5200 will not turn on

[Unspun01]

UPDATE:

I ordered a replacement "U1" IC (TEA1755T) from China. Replacing U1 had no effect. 12V and 24V still low, power LED still lights up green and flickers faintly.

Originally Posted by tvfixer007

I got a good board to work off of but my multimeter crapped out.

tvfixer007:

I hope you still visit this forum. Can you perform some testing of your faulty and working power supply?

I wonder if there is a zener diode that is not working correctly?


Can you please make the following voltage measurements across the following ZENER diodes (BOTTOM OF BOARD) of both your faulty and working power supplies. TV plugged in, standby mode.

WORKING BOARD:
ZD2 =
ZD4 =
ZD6 =
ZD7 =
ZD8 =
ZD10 =
ZD11 =
ZD12 =

FAULTY BOARD:
ZD2 =
ZD4 =
ZD6 =
ZD7 =
ZD8 =
ZD10 =
ZD11 =
ZD12 =

See attached image highlighting the zener diode locations.

[nomoresonys]

Did you check all your diodes to see if any have 0 or very low ohms?

[Unspun01]

Originally Posted by nomoresonys

Did you check all your diodes to see if any have 0 or very low ohms?

Yes, I checked continuity of each zener diode. None measure as a short or suspiciously low ohms. But I am aware that a zener may still appear as a regular diode, but not work correctly as a zener at the specified zener voltage.

That is why I am requesting someone to post voltages of their working (and non-working) board. I do not know the required zener voltages of the diodes marked ZD, but if something doesn't make sense, then it may be a troubleshooting lead.

[Unspun01]

I wanted to post an update to this thread to post solution to my power supply problem:

To summarize:
My TV is 60" LG model 60LB5200. My power supply had low 12V and 24V. They measured 8V and 16V respectively.

I replaced the main IC TEA1755T, but there was no difference. Low 12V and low 24V is not due to the main IC.

The problem is bad/leaky surface mount NPN BJT transistor marking 2X at position Q13.

2X marking is a MMBT4401 NPN BJT transistor 40V 600mA.

This transistor was not shorted in my power supply and i thought it was ok and overlooked it. But nearby resistances did not measure correctly in-circuit.

Measure resistance of highlighted components in-circuit: R79 [102] = 1kohm, R84 [102] =1kohm, R87 [222] =2.2kohm. If in-circuit resistance of any of these do not appear to match their stated value, remove Q13 and re-measure the resistances.

In my TV, the in-circuit resistances were way off. With Q13 removed, these resistances matched their stated values exactly. When I replaced Q13, the power supply correctly output 12V and 24V once again.

None of the resistances needed to be replaced or removed. I only highlight them to assist anyone in determining if Q13 is bad.

This does not preclude the other nearby transistor (Q12) or 431A IC (U3) from being bad, but they were not the issue in my TV.

See attached photo describing the locations to test.

[Banner]

Thanks, Unspun01. Today I picked up an LG 60LB5200 in the dump with a message posted that doesn't work. It had all the symptoms mentioned by the previous posters. Power supplies cold side has only 3 transistors and one ic which is an adjustable Zener voltage reference ic TL431A. All transistors checked ok in circuit. As the previous poster mentioned, I have replaced the q13 with a W1A SMD (2n3904 generic NPN equivalent) transistor and it works only to find the LCD panel has a crack.

[nomoresonys]

hate when that happens, well at least you learned something. a backlight tester probably would have shown the cracked screen, around 20 bucks on ebay or amazon.