52LG50 Has Mind of its own

[densmith123]

Re: 52LG50 Has Mind of its own

Thanks budm & slk001.

Something I did not mention regarding the opto-isolators...

In my troubleshooting effort I stumbled into Tom66's post about failed opto-isolators in similar TVs. I checked all 4 pins of opto-isolator PC100, PC200, and PC500 on the oscilloscope and they each had normal readings. However, I did NOT read them out during an attempted power up or shutdown sequence to see if they remained stable, and now I wish I had that data too.

I did swap PC100 with PC200 to see if the problem would go away, and nothing changed. This is the configuration I left the board in before moving on with the capacitor.

It is quite possible that the 22uF capacitor I installed is masking another intermittent problem occurring elsewhere on the board (such as those mentioned budm), by "buying some time" and storing up enough voltage to stabilize the ACDET signal.

I told TV's owner that if the problem returns, we will need to replace the power supply board.

-Denny

[dzooem]

Re: 52LG50 Has Mind of its own

Badcaps,

I seem to have solved the 52LG50 power cycling problem on a TV that I repaired for a friend. I first came to this thread looking for answers, and now I think I've found the culprit.

Here are the details:

Model: 52LG50-UA

Problem: TV intermittently powers off during startup or powers back on after shutdown.

Note:
To access TV Service menu, simultaneously press Menu on remote and on the TV front panel, and enter password "0000". Check the Power Off history (there seems to be 2 different service modes, so if this option doesn't appear exit out and try again)

Diagnosis:
1) An initial check of the power off history in the TV Service Menu showed multiple instances of “POWER_OFF_BY_ACDET.”

2) All power supply voltages coming from power supply board to main board checked good and were stable

3) I used an oscilloscope to monitor the ACDET signal during startup and shutdown. This was on connector P203 of the PSU, 4th pin down labeled "ACD". I set the scope to trigger with a single sweep on any drop in voltage, about halfway between 0 and 5V. Here is what I found:

a) signal was steady at 5V whenever power was not being cycled

b) on instances where the TV powered on and off normally, there was a stable 5V ACDET signal with no dips

c) on instances where the TV acted up and started to cycle (shut down on startup, or comes back on after shutdown), I observed a momentary drop to 0V in the ACDET signal lasting for 10ms

d) I've attached photos/sketches of 3 waveforms I observed: 1) how the ACDET signal looks when TV malfunctions - I accidentally deleted this off my scope so this one is drawn by hand to give you the idea; 2) a degraded ACDET signal dip that did not affect the TV power sequence; 3) how the ACDET signal should look

4) Knowing there was a problem in the ACDET signal, I spent 3 hours drawing the ACDET signal circuit by hand (see attached schematic for your reference - the drawing is crude but effective)

5) I concluded that the ACDET signal line intermittently had insufficient capacitance to maintain the 5V ACDET signal during power cycles

6) After replacing all electrolytic capacitors on the ACDET circuit one at a time, the fault went away when I soldered a 22uF electrolytic capacitor in parallel with ceramic capacitor C219, on the bottom of the power supply board. C219 was measuring 91nF at the time, and was sometimes not holding the 5V ACDET signal. I am not sure if this is a design flaw or if the ceramic capacitor had degraded over time.

7) After installing the capacitor, I observed a stable 5V ACDET signal with no voltage drops throughout multiple power cycles, and problem did not return

8) I experimented by installing a 4.7uF surface mount capacitor, but the problem returned; I reinstalled the 22uF electrolytic capacitor and left it. I am not sure what value was originally there, all I know is that it measured 91nF and did not work sometimes. Your methods may vary, but I'd bet a 10uF would do the trick

Theory about cause of this problem:

You will notice that whenever you unplug this TV and plug it back in, it reverts back to whatever state it was in before unplugged. If it was on before you unplug it, it comes back on when you plug it back in; vice versa is true if the TV was off. The standby microcontroller, IC407 on the main board, monitors the state of ACDET on pin 29 to determine if it has AC power, and what to do when it regains AC power if lost.

For the case where the TV momentarily loses the ACDET signal during a power cycle, here is the sequence I believe is happening:

a) TV is plugged in and turned off; there is a red standby light; ACDET measures a steady 5V at this time

b) User turns TV on - microcontroller recognizes current state as "TV on" and activates Pin 30, "RL_ON", energizing the remaining PSU voltage rails

c) As power supply activates and various transistors switch on, ACDET momentarily drops to 0V due to insufficient capacitance; the drop I observed on the oscilloscope was only 10ms long

d) With ACDET at 0V, the standby microcontroller aborts the startup thinking it has lost AC Power, and shuts down (I think the TV is designed to do a controlled shutdown on capacitance if the TV actually loses AC Power, as detected by ACDET on pin 29)

e) During the shutdown, ACDET pops back up to 5V after 10ms, and the standby microcontroller reverts back to its last known state, "TV is on"

f) This cycle repeats - On, off, on, off, on, off... the ACDET signal dropping to 0V for a few milliseconds is what causes the whole thing; the cure is to provide enough capacitance on the ACDET circuit so it can maintain 5V during a power cycle

g) You can follow this same logic through when the TV starts out on, and the user commands it off: the result is off, on, off, on, off, on...

I hope this helps someone else out there, because it took me 2 weeks to figure out. See attached photos.

-Denny

Not resolve the original problems but Add 22uf cap. It work to me . Thanks densmith123
Here link to down load LG schematic 52LG50 .
https://www.electronica-pt.com/esque...is-la84a-7968/

[Truncatedhose]

Re: 52LG50 Has Mind of its own

Badcaps,

I seem to have solved the 52LG50 power cycling problem on a TV that I repaired for a friend. I first came to this thread looking for answers, and now I think I've found the culprit.

Here are the details:

Model: 52LG50-UA

Problem: TV intermittently powers off during startup or powers back on after shutdown.

Note:
To access TV Service menu, simultaneously press Menu on remote and on the TV front panel, and enter password "0000". Check the Power Off history (there seems to be 2 different service modes, so if this option doesn't appear exit out and try again)

Diagnosis:
1) An initial check of the power off history in the TV Service Menu showed multiple instances of “POWER_OFF_BY_ACDET.”

2) All power supply voltages coming from power supply board to main board checked good and were stable

3) I used an oscilloscope to monitor the ACDET signal during startup and shutdown. This was on connector P203 of the PSU, 4th pin down labeled "ACD". I set the scope to trigger with a single sweep on any drop in voltage, about halfway between 0 and 5V. Here is what I found:

a) signal was steady at 5V whenever power was not being cycled

b) on instances where the TV powered on and off normally, there was a stable 5V ACDET signal with no dips

c) on instances where the TV acted up and started to cycle (shut down on startup, or comes back on after shutdown), I observed a momentary drop to 0V in the ACDET signal lasting for 10ms

d) I've attached photos/sketches of 3 waveforms I observed: 1) how the ACDET signal looks when TV malfunctions - I accidentally deleted this off my scope so this one is drawn by hand to give you the idea; 2) a degraded ACDET signal dip that did not affect the TV power sequence; 3) how the ACDET signal should look

4) Knowing there was a problem in the ACDET signal, I spent 3 hours drawing the ACDET signal circuit by hand (see attached schematic for your reference - the drawing is crude but effective)

5) I concluded that the ACDET signal line intermittently had insufficient capacitance to maintain the 5V ACDET signal during power cycles

6) After replacing all electrolytic capacitors on the ACDET circuit one at a time, the fault went away when I soldered a 22uF electrolytic capacitor in parallel with ceramic capacitor C219, on the bottom of the power supply board. C219 was measuring 91nF at the time, and was sometimes not holding the 5V ACDET signal. I am not sure if this is a design flaw or if the ceramic capacitor had degraded over time.

7) After installing the capacitor, I observed a stable 5V ACDET signal with no voltage drops throughout multiple power cycles, and problem did not return

8) I experimented by installing a 4.7uF surface mount capacitor, but the problem returned; I reinstalled the 22uF electrolytic capacitor and left it. I am not sure what value was originally there, all I know is that it measured 91nF and did not work sometimes. Your methods may vary, but I'd bet a 10uF would do the trick

Theory about cause of this problem:

You will notice that whenever you unplug this TV and plug it back in, it reverts back to whatever state it was in before unplugged. If it was on before you unplug it, it comes back on when you plug it back in; vice versa is true if the TV was off. The standby microcontroller, IC407 on the main board, monitors the state of ACDET on pin 29 to determine if it has AC power, and what to do when it regains AC power if lost.

For the case where the TV momentarily loses the ACDET signal during a power cycle, here is the sequence I believe is happening:

a) TV is plugged in and turned off; there is a red standby light; ACDET measures a steady 5V at this time

b) User turns TV on - microcontroller recognizes current state as "TV on" and activates Pin 30, "RL_ON", energizing the remaining PSU voltage rails

c) As power supply activates and various transistors switch on, ACDET momentarily drops to 0V due to insufficient capacitance; the drop I observed on the oscilloscope was only 10ms long

d) With ACDET at 0V, the standby microcontroller aborts the startup thinking it has lost AC Power, and shuts down (I think the TV is designed to do a controlled shutdown on capacitance if the TV actually loses AC Power, as detected by ACDET on pin 29)

e) During the shutdown, ACDET pops back up to 5V after 10ms, and the standby microcontroller reverts back to its last known state, "TV is on"

f) This cycle repeats - On, off, on, off, on, off... the ACDET signal dropping to 0V for a few milliseconds is what causes the whole thing; the cure is to provide enough capacitance on the ACDET circuit so it can maintain 5V during a power cycle

g) You can follow this same logic through when the TV starts out on, and the user commands it off: the result is off, on, off, on, off, on...

I hope this helps someone else out there, because it took me 2 weeks to figure out. See attached photos.

-Denny

Thank you so much for your hard work and dedication! This worked for me. Now I will have a solution for these models (and the 52LG70) whenever I see one, which is sometimes frequently. Thank you!

[budm]

Re: 52LG50 Has Mind of its own

Read post #61 and keep an eye on it.

[boypony]

Re: 52LG50 Has Mind of its own

Badcaps,

I seem to have solved the 52LG50 power cycling problem on a TV that I repaired for a friend. I first came to this thread looking for answers, and now I think I've found the culprit.

Here are the details:

Model: 52LG50-UA

Problem: TV intermittently powers off during startup or powers back on after shutdown.

Note:
To access TV Service menu, simultaneously press Menu on remote and on the TV front panel, and enter password "0000". Check the Power Off history (there seems to be 2 different service modes, so if this option doesn't appear exit out and try again)

Diagnosis:
1) An initial check of the power off history in the TV Service Menu showed multiple instances of “POWER_OFF_BY_ACDET.”

2) All power supply voltages coming from power supply board to main board checked good and were stable

3) I used an oscilloscope to monitor the ACDET signal during startup and shutdown. This was on connector P203 of the PSU, 4th pin down labeled "ACD". I set the scope to trigger with a single sweep on any drop in voltage, about halfway between 0 and 5V. Here is what I found:

a) signal was steady at 5V whenever power was not being cycled

b) on instances where the TV powered on and off normally, there was a stable 5V ACDET signal with no dips

c) on instances where the TV acted up and started to cycle (shut down on startup, or comes back on after shutdown), I observed a momentary drop to 0V in the ACDET signal lasting for 10ms

d) I've attached photos/sketches of 3 waveforms I observed: 1) how the ACDET signal looks when TV malfunctions - I accidentally deleted this off my scope so this one is drawn by hand to give you the idea; 2) a degraded ACDET signal dip that did not affect the TV power sequence; 3) how the ACDET signal should look

4) Knowing there was a problem in the ACDET signal, I spent 3 hours drawing the ACDET signal circuit by hand (see attached schematic for your reference - the drawing is crude but effective)

5) I concluded that the ACDET signal line intermittently had insufficient capacitance to maintain the 5V ACDET signal during power cycles

6) After replacing all electrolytic capacitors on the ACDET circuit one at a time, the fault went away when I soldered a 22uF electrolytic capacitor in parallel with ceramic capacitor C219, on the bottom of the power supply board. C219 was measuring 91nF at the time, and was sometimes not holding the 5V ACDET signal. I am not sure if this is a design flaw or if the ceramic capacitor had degraded over time.

7) After installing the capacitor, I observed a stable 5V ACDET signal with no voltage drops throughout multiple power cycles, and problem did not return

8) I experimented by installing a 4.7uF surface mount capacitor, but the problem returned; I reinstalled the 22uF electrolytic capacitor and left it. I am not sure what value was originally there, all I know is that it measured 91nF and did not work sometimes. Your methods may vary, but I'd bet a 10uF would do the trick

Theory about cause of this problem:

You will notice that whenever you unplug this TV and plug it back in, it reverts back to whatever state it was in before unplugged. If it was on before you unplug it, it comes back on when you plug it back in; vice versa is true if the TV was off. The standby microcontroller, IC407 on the main board, monitors the state of ACDET on pin 29 to determine if it has AC power, and what to do when it regains AC power if lost.

For the case where the TV momentarily loses the ACDET signal during a power cycle, here is the sequence I believe is happening:

a) TV is plugged in and turned off; there is a red standby light; ACDET measures a steady 5V at this time

b) User turns TV on - microcontroller recognizes current state as "TV on" and activates Pin 30, "RL_ON", energizing the remaining PSU voltage rails

c) As power supply activates and various transistors switch on, ACDET momentarily drops to 0V due to insufficient capacitance; the drop I observed on the oscilloscope was only 10ms long

d) With ACDET at 0V, the standby microcontroller aborts the startup thinking it has lost AC Power, and shuts down (I think the TV is designed to do a controlled shutdown on capacitance if the TV actually loses AC Power, as detected by ACDET on pin 29)

e) During the shutdown, ACDET pops back up to 5V after 10ms, and the standby microcontroller reverts back to its last known state, "TV is on"

f) This cycle repeats - On, off, on, off, on, off... the ACDET signal dropping to 0V for a few milliseconds is what causes the whole thing; the cure is to provide enough capacitance on the ACDET circuit so it can maintain 5V during a power cycle

g) You can follow this same logic through when the TV starts out on, and the user commands it off: the result is off, on, off, on, off, on...

I hope this helps someone else out there, because it took me 2 weeks to figure out. See attached photos.

-Denny

Denny-
YOU DA MAN.
Been fighting this problem for a year. New PS caps helped for about a week. New inverter was a waste. But your 10 cent cap FIXED the LG!!!!!!
This needs a Youtube Video.
THX!!

[Bruster55]

Re: 52LG50 Has Mind of its own

Nice detective work, this is kind of what i was thinking as well but your write up is probably correct for most of the situations. I have already replaced all of the PSU caps and was looking at the relay turn on circuits as they may be flaky or bad relays.
GOOD JOB