LG 26LE5300 Boot Loop after power surge hit

[Chaks]

I have a LG 26le5300 that gets stuck at boot logo after automatically rebooting for 4 or 5 times when switched on.
I suspect some blown components (see image in red and yellow markings on the pcb image and also on the pcb schematic image) in this area, the P channel MOSFET A03407 (Q7501). The voltage reading Vgs should be 10.8 for a 24 volt input bu I get +24.03 volts between Gate and Source. And -24.0 Volts between Gate and Drain. Measured with multi meter's Red lead on Gate and black lead on source and drain respectively.
Please help:

[Chaks]

Re: LG 26LE5300 Boot Loop after power surge hit

Can anyone please help me with the main board PCB of lg TV 26le5300. I want to compare my PCB with an actual working PCB to see if any components are missing on the PCB. Is there any complete service manual for the TV . please help.
Image of both sides of working PCB is what I need at least.

[nomoresonys]

Re: LG 26LE5300 Boot Loop after power surge hit

Try heating sections of the mainboard with a hairdryer, see if it stays on if no results try heating some capacitors on powerboard.

[Chaks]

Re: LG 26LE5300 Boot Loop after power surge hit

Q7502 2sc3052 npn transistor has the following readings.
Base +lead to emitter -lead = 0.6v
Base +lead to collector -lead =0.6v
Emitter +lead to base -lead =over limit
Collector +lead to base -lead = over limit

Collector +lead to emitter -lead = 0.98 volts
Emitter +lead to collector -lead = 0.68 volts

POWER DET circuit:

IC 7509 ( ncp 803 sn293)

Pin 3 vcc to ground of PCB is 3.5 volts
Resistance between pin vcc and reset pin 2 is 100k ohms
The TV resets when pin 1 gnd or pin 2 reset is touched by multimeter leads pos or neg.

IC 7510 (ncp 803 sn293)
Pin vcc to ground of PCB is 0.9 volts
Pin 3 vcc to gnd pin 1 is 1.9 volts
Resistance between vcc and reset pin is 355k ohms
Pin 1 gnd and pin 2 reset pin (+lead) is 3.6 volts
Reset pin to ground of PCB is 1.2 volts

Is there anything faulty here? Power-det circuit is on page 35 of the service manual.

[budm]

Re: LG 26LE5300 Boot Loop after power surge hit

https://www.badcaps.net/forum/attach...6&d=1544886413
For Q7501 (P-ch) the GATE Voltage has to be about 3 ~4 V < Source Voltage, you need to use circuit GND to verify the Voltages.
What Voltage do you have on that POWER_ON pin?
Q7500 has to be turned on by POWER_ON control Voltage for Q7501 to be on.
We also need pictures that show how all the boards are connected together.

[Chaks]

Re: LG 26LE5300 Boot Loop after power surge hit

I get the Vgs 10.8 volts on the a03407A G-S but only for a couple of seconds. The +15 Volts control chart in the service manual page 35.
I checked all the voltages as per the service manual charts on page 35. There are two areas Power-Det and 24-volt->15 volt chart where the readings does not match.
in power detect IC7510 vcc pin is not reading +3.5_ST but the IC7509 vcc pin measures +3.5_ST.
The other issue is that in 24-volt->15 volt control chart. the R7540 reads 32k Ohms only. the other resistors are ok and as per smd code/markings.The Capacitor 7522 has turned darkbrown seems over heated or about to die but but measures ok.Or it may be already dead. these 2 components along with Q7502 in the first chart in page 35 are abnormal. I will post an image of the pcb along with test voltages below.

[Chaks]

Re: LG 26LE5300 Boot Loop after power surge hit

Additional PCB Images

[budm]

Re: LG 26LE5300 Boot Loop after power surge hit

So what is the answer for this one: What Voltage do you have on that POWER_ON pin?
https://www.badcaps.net/forum/attach...6&d=1544886413

[Chaks]

Re: LG 26LE5300 Boot Loop after power surge hit

Here are the readings of the Q7500, Q7501 , Q7502, orientation as they appear in on the PCB. Thanks a lot for your time sir

[budm]

Re: LG 26LE5300 Boot Loop after power surge hit

So you do have 24V with reference to GND Source and also at Drain pin of Q7501, so what is wrong with that? That is what it should have when it is turned on.
BTW, you do know how to read schematic, correct? And you do understand how Transistor and MOSFET work, correct?
I still do not understand why you have to make other measurement that are not reference to GND, no need to do that right now.

[Chaks]

Re: LG 26LE5300 Boot Loop after power surge hit

Should I not worry about this
https://www.badcaps.net/forum/attach...0&d=1545156295
Is this normal?

[Biruslapio]

Re: LG 26LE5300 Boot Loop after power surge hit

Resistors are almost always in parallel with a lot of things in the circuit,especially main boards,it's not a problem if they read less than their rated resistance,but more,for example this 120k reading 200k,that would mean it is defective.
These capacitors if not reading 0 OHM are probably good,sometimes they are in parallel with some processor silicon which measures kinda low resistance(5 OHMs or more generally).

[Chaks]

Re: LG 26LE5300 Boot Loop after power surge hit

When the TV is connected to the ac using adapter the standb red led is ON and the optical output on the back of the tv usually glows red but in my case the optical output on my TV is off. The optical lack J1103's vcc pin does not have power. That's why I suspect that some part of the PCB is not getting power which is supposed to be there even when the TV in on standby mode. There is power in the USB ports of the TV.. In most of the TV's I have seen the standby led and the optical jack are red.In the present case it is not. That's what I am trying to first get the optical jack power up.

[Chaks]

Re: LG 26LE5300 Boot Loop after power surge hit

Can I use test points (TP) / JP to trouble shoot further. Can someone help me.

[Chaks]

Re: LG 26LE5300 Boot Loop after power surge hit

LG LOGO is displayed before or after the microprocessor lg engine lge101d is active .? Anyone has information on this.

[Chaks]

Re: LG 26LE5300 Boot Loop after power surge hit

Referring to the connector P7501
... the Pin 14 is at 3.3 volts when the TV is switched on (blue std_by light) this pin is Error_Out (output) to IC LGE 101D Error_Out (input at gpio 41).

This Pin 14 is at 3.3 volts even when the P7501 connector is not connected to the LCD_PANEL.

Pin 14 is at 0.9 volts when the tv is in std_by red light mode.
Pin 13 is at 1.03 volts when the tv is in std_by red light mode.

When the tv is completely off the Pin 14 to pcb Gnd measures 5.0Mega Ohms.

Is this also normal. I have attached the pcb as well as schema for reference.