LG 37LG3000 weird mainboard dtv turn on/off loop fault!

[jesterace]

Hello,
I've got this TV with two faults originally. One was the master inverter blown a ceramic cap. I've fixed that and the TV works fine again. The second issue is really confusing me. I'm not sure where to start.

Basically the TV gave this fault where you would turn it off expecting it to stay in standby, but instead it would turn back on again immediately. I checked the PSU board all over, re soldered a few cold joints but no change. Disconnected the power button, no change. Checked PSU caps, all good ESR, no bloating.

Then i proceeded to disconnect the POWER_ON wire going from the mainboard to the PSU to see if it was the PSU restarting itself or if the mainboard was indeed signalling for the TV to turn on again. After disconnecting POWER_ON, the TV stayed off properly with the standby led static. So this lead me to believe the fault is with the mainboard.

Now it gets weirder. Sometimes i noticed this fault never occurred, it's intermittent... BUT after lots of head scratching ive now discovered that this fault only happens if:

*The antenna is connected and the TV is on digital tv source*

If i disconnect the antenna, all is well. The tv can be turned off and it stays off.

I can also leave the antenna connected, provided that the tv source in NOT on dtv. So if it's on AV1 for example, i can leave the antenna connected and the TV will stay off when turned off.

Iv'e also cleaned all the connectors on the mainboard, HDMI's, Composite's etc. Turned off the "auto on" feature that can make the TV come on when a device is connected. Updated firmware to latest. No change.

Any ideas where i might be able to start troubleshooting the main board?

I'll get a picture taken asap, but any guidance would be appreciated.

Thanks,
James.

[jesterace]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Also should mention that the 12v feed from PSU to mainboard shows 0.9v in standby, shouldn't this be 0v??

[vinceroger69]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Hi whats the mainboad number? can you upload a good clear picture of it. Im also wondering if theres voltage regulators on the board that might be slighly out of spec to the dtv tuner maybe? its a odd fault.

[jesterace]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Will get a picture up in the next few mins.

I just disconnected the 12v wires from PSU and metered the 12v lines as they enter the mainboard in standby. They are 1.4v?!? Even when PSU disconnected! Very odd.

[jesterace]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

pictures as promised...

[vinceroger69]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Im not sure too be honest but im sure a more experienced member may have ideas what to check. main board faults seem hard to trace/find hence why a lot of us end up board swoping we would all like too fix the old one ofcourse but some times we just cant i guess.

[jesterace]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Hello,
yes its a weird fault but it's making more sense now. As it turns out the main board isn't the cause because I've just put a replacement one in. Same fault.

I'm quite happy to throw money at it to learn, it's a fault I've never encountered before. I sooo wanna find out what's causing this!

Anyway, the whole idea of this fault occurring when using DTV when the aerial is plugged in seems to be because when there is a picture, the back light starts getting controlled to the set user back light brightness and the brightness is obviously much lower when there is "no signal". It's when the back light brightness goes up that the fault is triggered! No wonder the TV doesn't cause any problems when i disconnect the inverters, as I've just found.

So for starters i've experimented with lowering the backlight brightness in the TV's menu. If it's set between 0-30% the TV works absolutely fine. If it's over 30%, then i get the fault of the TV turning itself back on when i put it into standby.

Bear in mind i now have a new mainboard and new inverters, so these can be ruled out.

Does this sound like a possible bad CCFL tube or something?

I have noticed that sometimes when turning the TV on it may take 2 attempts to startup. The back light will flicker, return to standby and then everything starts up again. Usually when this happens it will power up fully on it's second attempt. Bear in mind all the PSU caps are not bloated and ESR looked fine when i tested them...

Hoping someone can help. I'd really like to fathom it out if possible rather binning it

Cheers,
James.

[jesterace]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Wow i guess this one has stumped everyone and not just me?

[CapLeaker]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Did you try to disconnect the IR and keypad? Maybe something is stuck there?

[jesterace]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Yep tried that no difference. Like i said, if the back light brightness is kept below about 30% all is well. Anything over that and the TV won't stay in standby, it just wants to keep turning back on again.

I've got all day today to have a tinker with it. Haven't got a clue where to begin though because the symptoms just don't make sense to me.

At the end of the day it's the mainboard's job to turn on the PSU right? and i've changed that. I know the back light brightness control runs from the mainboard, through the PSU and to the backlight inverters... but what's that got to do with causing the mainboard to fire up the PSU again when turning the set off?? and since the mainboard and inverters have now been changed, that basically leaves me with the scenario of "the PSU is turning itself back on of it's own accord when backlight is over 30%" what the hell??

I've even pulled out the TCON cable to see if that helps. No difference.

So confusing...

[vinceroger69]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

when it goes off do any of the power supply voltages change? im wondering if one could fail under the load when the backlights on full brightness

[jesterace]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Not sure what you mean by going off, the TV doesn't turn itself off off at all. Much the opposite. It turns itself on a few seconds after pressing the standby button. TV goes to standby, but then TV starts again immediately (but stays on as normal).

Haven't noticed much in the way of voltage changes, 5v standby solid, 24v to inverters solid. The 12v to main board seems to hang around 0.9v after entering standby, don't know if that's normal. Saying that though, i measured the 0.9v when the brightness was set low and TV stayed in standby fine fine so it must be normal?

The PWM_DIM works fine, changes between about 1v and 3.3v depending on menu backlight brightness. I pulled out the PWM_DIM wires from the PSU (inverter plug and mainboard plug) which seemed to just force the inverters to go full brightness (the PWM_DIM on the inverter boards read 3.3v even when wire was disconnected from PSU).

So still not much further on, but it's definitely a case of, the backlight brightness/power draw is screwing things up...

Only other observation is that when backlight is low brightness, the inverters seem to be quite noisy. Again i'm not sure if that is normal, i suppose if the transformers are being driven slower they will whine more?

Another bright idea of mine at the moment is to try and run the 24v to inverters from a bench supply. I've got a 30v 10a variable supply, hopefully the result of that might explain more...

[jesterace]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Hi i plugged up my variable supply to the master inverter for a start, just feeding it with 24V/GND... and left the slave inverter running on the TV's 24V supply.

All works great!! set the brightness to 75% or more and no problems at all. The set turns off nicely and stays in standby.

So it's smelling like the TV PSU isn't liking the high power draw from two inverters at high brightness. Although it's all still very weird to me why the fault only shows when turning off and not while it's running.

Any ideas what i might be able to do from here?

[CapLeaker]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Check the caps on that circuit on the PSU

[ReeceyBurger123]

Re: LG 37LG3000 weird mainboard dtv turn on/off loop fault!

Check caps on mainboard as well they are prone to failing on this model as well all all the Ldo regulators.