52LG50 Has Mind of its own

[gfox]

Re: 52LG50 Has Mind of its own

Well the new 5W resistor has gotten my power back, but the television has started turning back on again after I shut it off. Also it is back to only doing this when the back cover is on. immediately after I take the back cover off it works fine.
Even when I have the back cover off, but then but a piece of cardboard big enough to cover the power supply cover up against the cover it starts acting up again. I am convinced it is a EMI issue.

fierygt
,
I am curious if you power cord has a ferrite bead on the wire? I am wondering if these televisions actually came with cords with ferrite beads and perhaps the cords have gotten switched out with cords without the beads?

I didn't buy mine new, I actually got this television from a guy who already had this issue with it turning back on after it was turned off.

[Agent24]

Re: 52LG50 Has Mind of its own

If it is an EMI issue, it could also be a failure of some EMI filtering component somewhere else which is making it more susceptible to a problem it should not be reacting to.

Also, I would have thought an EMI issue would show up with covers off.

[fierygt]

Re: 52LG50 Has Mind of its own

fierygt
,
I am curious if you power cord has a ferrite bead on the wire? I am wondering if these televisions actually came with cords with ferrite beads and perhaps the cords have gotten switched out with cords without the beads?

I didn't buy mine new, I actually got this television from a guy who already had this issue with it turning back on after it was turned off.

The cable I have been testing with doesn't, but I can see if I can find one with the ferrite core on it. I have bin full of those cables...

[gfox]

Re: 52LG50 Has Mind of its own

fierygt,
Just to confirm is your television turning back on after you turn it off, or is it turning right back off after you turn it on?
Mine has been turning back on after I turn it off, but has done both from time to time. Recently though it has just been turning back on.
I was wondering if you could check something else for me?
I have discovered that if I hit the Q Menu button, then select the Backlight and set the value down to 3, the problem has gone away. Mine was originally set to 6. I haven't plugged it in to the cable to see how dark this picture would be, yet, but it does still point me in the way of EMI, or possibly just back driven power somewhere, or in other words the extra power it takes to have the backlight brighter is causing some feedback somewhere on the board when it is having to dissipate that much power.
If yours is turning back on as mine is could you check to see what your back light is currently set to and see if your problem also goes away when you lower this value?

[truep]

Re: 52LG50 Has Mind of its own

does the set have a timer function?? Mu LG suddenly started coming on with full volume at around 2:30 AM. Somehow (power glitch?} the timer became set. problem was resolved by deleting the time on.

[fierygt]

Re: 52LG50 Has Mind of its own

Mine will cycle back on, after I turn it off. I will have to reinstall the power board, and solder back on the resistor, so it might take a few days to get back to you on the tests. (It's not my primary project at the moment). It makes some sense, when you turn down the brightness it "fixes it", as the problem seems to be in the power delivery to the backlights. I mentioned if I unplug the cables going to the inverters, mine would act normal. Other than the backlights off, obviously. But it would stop power cycling at that point.

[Alastair E]

Re: 52LG50 Has Mind of its own

You got any flourecent lamps,--including those 'energy-saving' CCFL types around in your house?
Try unplugging 'em.....

Maybe suspect LED type lamps these days too--Unplug the buggers......

Used to have all sorts of issues in the 80's with IR Remotes and those CCFL and flourecent lamps--Worth a try.....

[gfox]

Re: 52LG50 Has Mind of its own

I can send you one of those resistors if you want. I ordered 5. If I stuck it in the USPS mail, I would be out a whole .70. No big deal.

[fierygt]

Re: 52LG50 Has Mind of its own

I can send you one of those resistors if you want. I ordered 5. If I stuck it in the USPS mail, I would be out a whole .70. No big deal.

Sure, sorry been super busy. I can PM you my address if you like. If it didn't resolve your problem, it might not resolve mine, but I still am curious if it will.

[gfox]

Re: 52LG50 Has Mind of its own

fierygt,
PM me your address and I will get that resistor mailed to you.
Sorry I haven't been on here much, since I got mine working acceptable.
By lowering the Backlight setting from 6 down to a 3 I have not had a bit of a problem. The thing that really surprised me was that there was no obvious difference in the looks of the picture when I went from a 6 to a 3. I expected it to be much darker, but you couldn't tell a difference.
I no longer have the television, but now I am curious if it would have helped to put the 10W resistor in instead of the 5W. I bought some of them also, but obviously it would have stood off the board further, so I was afraid it would hit the cover unless I added longer leads. It may have helped dissipate the power on shutdown better though.

[fierygt]

Re: 52LG50 Has Mind of its own

gfox, pm sent. I mentioned I wouldn't mind trying a 10w resistor first, if you wouldn't mind sending one of those, too.

[fierygt]

Re: 52LG50 Has Mind of its own

Yes, it's either power supply or a main board.
1.Disconnect both sustain boards, and see if it clicks,it yes - one of the sustain board is bad,connect one by one and see which on caused tv make clicking sound - that one is bad.
If you disconnected both sustain boards and it still clicks - see if Va and Vs is there - if at least on of them is missing - Then The Power Supply need to be replaced.
2.IF BOTH Vs and Va come on, and TV is still trying to come on the shuts off, then the Main board is bad, and need to be replaced.
Good luck,check our website: www.tulsatvrepair.biz for more related articles on our Newsletter section.

The 52LG50 doesn't really have "sustain boards". It's LCD, not plasma. But earlier in the thread, I did mention removing the connections to the inverter boards, and that will keep it on. And gfox mentioned lowering the brightness halfway allowed his to function. In this thread, we are not really looking for "board replacement". We are trying to pinpoint the component(s) where this problem exists, because people have replaced their mainboards, which fixes it for a while, only to have the problem reoccur.

[jordan.sloan159]

Re: 52LG50 Has Mind of its own

I recently found myself with with the same off and on problem. If I find out anything I will post here! However through some quick research replacing the entire power supply may be necessary. Rather than replacing the caps by themselves

[mveinot]

Re: 52LG50 Has Mind of its own

Recently received a 52LG50 free from a friend of a friend who got fed up with its behaviour as described in this thread.

It had been sitting in her basement unused for months before I took ownership and when I first got home and plugged it in, it worked perfectly for a few days, then it started exhibiting the on/off/on/off/etc... behaviour discussed here.

I've taken a look at the power board and all the caps look OK, but I ordered a replacement kit anyway as failing caps don't necessarily always look problematic.

I've also ordered a batch of the 5W ceramic resistors.

Everything's on route but I don't expect them for another couple weeks.

Has anything more definitive been determined by anyone? Currently letting it cycle in the hopes it will stay on so I can try setting the panel brightness lower.

[dick_barton]

Re: 52LG50 Has Mind of its own

Reading through the thread the substitution of the main board does not appear to have cured the problem. Turning down the backlights appears to work. I would be considering there's a possible fault on the inverter or even with the CCF tubes. Just a thought.

[mveinot]

Re: 52LG50 Has Mind of its own

Finally got into the Q.Menu and backlight was disappointingly already set at 1. 0 didn't help either. Back to waiting for parts I guess.

[mveinot]

Re: 52LG50 Has Mind of its own

Took the power board off the panel and metered the resistors. the 6.8ohm one reads 6.9 - well within spec. The 0.01 ohm reads 00.0 on my DMM... not sure if that's an issue, or just due to the accuracy of my little meter (or lack thereof).

[densmith123]

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

[SLK001]

Re: 52LG50 Has Mind of its own

I suspect a bad optoisolator.

[budm]

Re: 52LG50 Has Mind of its own

i suspect a bad optoisolator.

+10, poor CTR due to aging.
The ACDET is part of the protection circuit to make sure the incoming AC is within the operating range, it will not allow the TV to come on if the AC is too low (brown out which will cause excessive current draw).
Resistors network in the primary side for the circuit that drives the LED inside the OPTO could also drift up in value too and can cause the same symptom.
This symptom also happens in Sharp TV with ACDET circuit.