LG 42LA620S TV - No Backlight Issue, Individual LED Check, Power Supply Voltage (EAX64905301)

Hello,
I have a LG 42LA620S TV in which the backlight does not work. The image is (you can see it shining in the matrix with a flashlight), the sound also. At the beginning I checked the status of the LEDs - visually they all looked ok. However, I decided to check each LED separately with two AA batteries. Each LED is on individually.
The next step was to examine the voltage at the power supply with the LED connector (EAX64905301 power supply). An example photo of this power supply is below:

This is a 42-inch TV, so this connector should be 157.5V. Instead, when the TV is turned on, the output voltage to the LEDs is around 126V (EDIT: Exactly 118V).
If each diode works separately, can a diode fault be excluded? Unfortunately, I don't have any LED testers to check if they work all together.

Greetings,
Cuba

Disconnect the backlight, connect the meter to + LED and ground, turn on the TV. Enter the result.

The voltage after disconnecting the backlight is the same, exactly 118V - the same as when the backlight was attached (I waved about these 8V in the first post).

What matrix?
How many LED lines, how many LEDs in the line, how connected.?
What about the control signal on pin.1 MAP3202 from main ... through the transistor
This model of the power supply (your photo) is
- LGP3739-13PL1 EAX64905301 (2.3) with LED driver for MAP3202



In 42: LG with this PSU was matrix LC420DUE (SF) (R1)
-5x line of 10_LED> 50x LED (3-3.5V) which gives approx. 150-170V



time between field Q401 and MAP output is an additional PNP 2907A 60V / 0.6A 0.4W transistor link

The matrix in this TV is LC420DUE (SF) (U2).
There are 50-5 lines of 10 LEDs in total, of which two halves of 5 LEDs each (as you wrote).

There, the LEDs are connected in series!

Look at this bipolar what I wrote, field
- transitions between the "halves" of the Led lines

MAP3202 has 2.48V on 1 pin. 11.9V is on 2 pins (VCC).

Revelation! Keep it up.

I use the scheme available here http://www.go-gddq.com/upload/2015-09/15091816267835.pdf on page 19.
I checked the Q801 transistor and after switching on the TV on the drain-source connector I have 120V. According to the specification available here: https://4donline.ihs.com/images/VipMasterIC/IC/ADPO/ADPOS00529/ADPOS00529-1.pdf, the maximum voltage on this connector is 200V.
On the other hand, the Q803 transistor has no voltage on any of the connectors (collector-base, collector-emitter and emitter-base).

Replace Q803, improve soldering to MAP, see the transition between the halves of the lines (purge) and it should be ok.

ps
What do you have after the pin. OVP and FBN?
Read the datasheet of this MAP3202 link

Example voltages with damaged Q803

Quote:

1 - 2.7
2 - 11.3
3 - 1.2
6 - 2.94
8 - 4.94
9 - 9.5
10 - 1.1
11 - 2.85
12 - 2.35
13 - 0.73
14 - 0

Thank you for your answer. The voltages on individual MAP3202 pins are as follows:
1 - 2.48
2 - 11.89
thirty
6 - 2.97
8 - 4.94
9 - 0
10 - 1.45
11 - -0.34
12 - 0.72
13 - 0.64
14 - 0,
so on OVP I have 1.45V and on FBN 0V.
Solders on MAP3202 at first glance seem fine:

Even today or tomorrow I will drive after Q803, solder, clean the transitions between led halves and let me know if it worked.

In your place, I would check the diodes again, but not with the batteries, only for the power supply.

Q803 replaced, but no backlight yet. The voltages on the power supply are also the same as before the exchange. I looked at the diodes again. I attach a photo of my "suspect":

For comparison, the second LED without a "cap".
Can the LED on the left be a potential culprit for the lack of backlight? In its upper left corner a brown "dot" is visible, which is not next to the diode on the right.

kuba_n wrote:

Can the LED on the left be a potential culprit for the lack of backlight? In its upper left corner a brown "dot" is visible, which is not next to the diode on the right.

Please stop further work and have the equipment serviced.


3.1.17. Do not send very basic questions that can be found in the instruction manual or publicly available sources. Do not take the attitude that I deserve. Keep the level of questions and discussions. Thank you.

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