LG 39LN5758 random reboot / off-on while watching

[TRANSLTR]

Hey guys,

a while ago my LG 39LN5758 started performing random reboots while I was watching something.
It started during phases when the volume was set to higher level / the movie had a very loud sequence. The TV would suddenly go off and then reboot.
After this event, it returned to nominal functionaltiy.
But now - a few months later - sometimes the TV sometimes is in a loop, performing dozens of reboots before eventually reaching a somewhat stable operation.

The error pattern let me thinking about a bad cap or some other issue with the Power Supply Board (PSB) - Like an semi-stable voltage rail collapsing when seeing a high load.

Inspection of the device gave me the following information:

- PSB: eax64905301 Rev1.0

- MB: 183X225 (The other numbers are covered by connectors - that I did not remove so far)

- Caps on those boards look good from visual inspection.


H/W Measurements performed:

- I checked the connection between Mainboard (MB) and the PSB. There is a 24p connector with the following pinout:

PINNUMBER: (PINNAME) V_MEAS_STDBY -> V_MEAS_ON

17: (GND) 0V
15: (12V) 0V ->12V
13: (12V) 0V ->12V
11: (GND) 0V
09: (24V) 0V ->24V
07: (GND) 0V
05: (3.5V) 3.5V -> 3.5V
03: (3.5V) 3.5V -> 3.5V
01: (PWR-ON) 0 -> 3.5V

18: (GND) ?
16: n.c ?
14: (12V) ?
12: (GND) ?
10: (24V) ?
08: (GND) ?
06: n.c ?
04: (PDIM #1) ?
02: (DRV-ON) ?

- I performed oscilloscope / multimeter measurements with the TV in standby and after switching it on. The results are written behind the pinout.

- The second row of pins could not been reached because of the connector attached (maybe I will repeat the measurement with another tip - but in the end, there seems to be no voltage on the second row, that is of interest)

- The PWR-ON Pin was oberved toggeling during the reboot loops desribed above. Everytime, the TV restarted, the PWR-ON Pin switched to low, coming back to high a few moments afterwards.

- Thus, also the 12V and 24V where switched on and off.

S/W Inspection:

- Entering the Service Menu gave me the following Power Off Status:
POWER_OFF_BY_ACDET
- This is a bit confusing, because it is not mentioned in the service manual - but google says for other LG devices:

"Power Off by AC OFF (Loss of AC) when the TV is in Compensation Mode. Micro Logs this entry Twice. SMPS is operational in Compensation, with AC loss, the Micro shuts off the SMPS then the TV."



Evaluating my knowledge so far, I am not sure if
- the PSB fails at some point, causing a breakdown of one or more supply voltages, causing a powerloss of the MB,
- the MB encounters an internal error and initiates a reboot or
- the PSB, supplying the MB with an unstable voltage and thus, causing the MB to perform a reboot.


At the moment, the TV is running stable again - so it is difficult to perform any error measurements. But experiance from the last month tells me, that this state it will not last for long :/


Please let me know what you would do next and what you think the root cause might be.
If you need any further information, please ask : )

[TRANSLTR]

Re: LG 39LN5758 random reboot / off-on while watching

Hi again,

Following the trace of the POWER_OFF_BY_ACDET entry in the Service Menu I found this post from densmith who performed an excellent analysis on a different model:

https://www.badcaps.net/forum/showpo...5&postcount=58

My case might be similar. However, the voltage detection seems to work a bit different on the LG 39LN5859.
I got my hands on the service manual for my TV. It features the schematics of the MB - I attached the part, where some kind of voltage monitoring is done.
In contrast to the 52LG50 analyzed by densmith, the LG 39LN5859 seems to not monitor the AC voltage directly (via some Opto-coupling-device).

Instead it monitors the 12V and 24V rail and uses a NCP803 (a reset monitor, which issues an open-drain reset signal when the input voltage goes below 2.93V).
The 12V, 24V and 3.5V_ST have their origin on the Power Supply Board. The POWER_DET is connected to the IC3000 - which probably is responsible for managing the power status of the TV (It also has the RL_ON as an ouput, which drives Pin 1 on the PSU connector and switches on the 12V / 24V rails on the PSU)

What I do not understand looking at the circuit: Why are the Resistors R2413 and R2400 (value both 0R0) hard-pulling the signal to 3.5V_ST. That whould render the 12V/24V voltage dividers (R2412 / R2411 and (R2410 / R2409) useless.
Am I missing something?

Thanks for your interest : )