LG 32LD450 shuts off after 15-25 seconds

[MisterD]

The longer it sits off it may stay on longer, I have had it stay on up to 5 minutes after sitting several days unplugged, once it gets to the point where it only stays on 15 seconds it will shut down and go to flashing red light but can always be turned right back on and will go about 15 seconds every time.

normal power when it is on has blue light, I did measure the power on (pson) voltage on ps board rail at 3.3v but once it shuts down the 3.3v is gone, could I take two AA batteries and wire in series to get 3 VDC, and hook to the 3.3 rail on ps board to see if it stays on and if it does would that indicate the main board is the problem? And ps board is good?
Sorry my camera is not here at this time to take pics of boards.
Thanks for all replies

[CapLeaker]

Re: LG 32LD450 shuts off after 15-25 seconds

you got to figure out why your PSon signal disappears. No wonder the PSU is shutting down if there is no PSon voltage sent from the main board.

[MisterD]

Re: LG 32LD450 shuts off after 15-25 seconds

The mainboard rail connector is Not labeled PSON but is actually labeled as power on, all voltages on the ps rail connector are normal As far as i know and all voltages drop to zero when the set shuts down, Except there are 4 pins with 3.4 volts and this voltage remains the same even when it shuts down.
Backlights either flicker or flash off then back on 3 to 4 times right before it shuts down.

Voltage across main caps flucuates from 781 down to 481 V AC while set is on and working, then drops to 222V AC when backlights are flickering, set shuts down then voltage across main caps is 368V AC, power light is flashing red like a car turn signal everytime it shuts down, but I can power right back on and it may last only 15 secs to several minutes depending on how long it was left off. Any help is appreciated

Also this TV stayed on this time with the back off for about 20-25 minutes before shutting down the first time then gradually had less time with each time I turned it back on, getting down to as low as only 15 seconds.

[MisterD]

Re: LG 32LD450 shuts off after 15-25 seconds

Here are some pictures of the boards and a drawing of the main rail connector on the power supply with voltages I read with volt meter, sorry but my camera will not take good close ups. The Brown colored wire is the power on 3.3VDC

Thanks in advance, your help is apprecitated

I hope budm will chime in, he is always very helpful.

[budm]

Re: LG 32LD450 shuts off after 15-25 seconds

"Voltage across main caps flucuates from 781 down to 481 V AC while set is on and working, then drops to 222V AC when backlights are flickering, set shuts down then voltage across main caps is 368V AC, power light is flashing red like a car turn signal everytime it shuts down, but I can power right back on and it may last only 15 secs to several minutes depending on how long it was left off. Any help is appreciated" You need to verify the DCV between the two legs of the main filter cap to see if the PFC Voltage circuit is working or not when TV is turned on. The Voltages you report are not right. Make sure to set the meter to DCV and measure right between the legs of the cap.

[MisterD]

Re: LG 32LD450 shuts off after 15-25 seconds

Sorry Bud my meter was set on AC, measured across main filter caps got 360 VDC while set was on and working, It was on about 15 mins this time and shut down with flashing red light, while shut down voltage across main caps is 168 VDC.
After unplugging and letting caps bleed down to 30 volts, plugged back in and got same 168 VDC while off and 360 VDC while on and working.

[MisterD]

Re: LG 32LD450 shuts off after 15-25 seconds

Deleted repeated reply.

[budm]

Re: LG 32LD450 shuts off after 15-25 seconds

OK, so we now know that the pfc is working but some how the Power ON (PS-ON) disappear and causes PFC and the rest of the switched power supply to turn off, but the standby 3.3V is always present, so something on the main board is going bad, it can be caps, IC, bad solder joints, etc.
You should also look to see if the INVR ON (BL-ON) also disappear or not too.

[MisterD]

Re: LG 32LD450 shuts off after 15-25 seconds

Yes the INVR ON is disappearing too whenever the tv shuts down, only the 3.4 standby remains, I have 2 AA 1.5 v batteries wired up in series to equal 3 volts, do you see any reason why I should or shouldn't cut power on (brown wire) from main board to ps and hook 3v battery to power on at ps connector and see if the tv comes on and stays on while battery is hooked up? Or jumping from 3.4 standby with 1k resistor to power on (PSON) ?

Thanks Bud.

[CapLeaker]

Re: LG 32LD450 shuts off after 15-25 seconds

Take the PSON pin out so you don't back feed power, then beige PSON with STBY voltage with a 500ohm resistor and the TV should be alive. You may also have to do that with the back lights BLon.

[MisterD]

Re: LG 32LD450 shuts off after 15-25 seconds

On the power supply rail connector I jumped through a 1k resistor from 3.5 to inv on/off, used 3v battery connected to power on, voltages remain on power supply rail but backlights are going off after about 15 seconds just like it did when tv was shutting down, now I wonder if the PS board is the problem, shouldn't the backlights stay on as long as I have the necessary voltage to inv on/off?
Also if I unhook power and let it cool down the backlights stay on longer the first time then the cycle gets shorter after it warms up.
I have the connector from the PS board to main board unplugged at the main board during this test. I don't see any bulging caps on the PS board. Need Help to diagnose the problem.
Thanks

[budm]

Re: LG 32LD450 shuts off after 15-25 seconds

So when you do the force on using the resistors on the POWER ON PIN and on the INVERTRER-ON pin the backlights still goes out after 15 seconds. So does the PF VDC on the main filter cap also drops down or it still stays at 360VDC or so?

[MisterD]

Re: LG 32LD450 shuts off after 15-25 seconds

The voltage across the main cap drops to 167 VDC, when the backlights go out, I just checked it with my meter hooked up it dropped suddenly to 167v, then backlights went out immediately after that, I noticed the voltage across the main caps climbing and dropping while ps is forced on and backlights go out, went from 167 to 377 then drops keeps fluctuating.

It didn't do the fluctuating immediately, I just left the ps forced on even after backlights went out, then after a few minutes it starts the voltage fluctuation. The connector to the main board is unplugged.

[budm]

Re: LG 32LD450 shuts off after 15-25 seconds

The backlights inverter circuit needs >360DVC to run so when that PFC Boosted Voltage drops down, then the inverter circuit will go into shutdown. The problem is that we do not know for sure if the PFC circuit drops out due to too much current demand by the inverter circuit or it drops down due to false in the PFC and cannot supply the needed current by the inverter circuits. Did you change all the caps on the power supply board? Inspect the main filter cap closely for the ESR and the capacitance.

[MisterD]

Re: LG 32LD450 shuts off after 15-25 seconds

Bud, I have not changed any capacitors on the power supply board none of them are bulged out, could they be bad anyway?
They are Su'scon brand caps, main filter caps (there are two) both are marked sk 105 degree C 1021M all others are marked SG 105 degree C, the pair of main caps are stuck down on the board with the value and voltage facing into the glue. What should I do now? Order and change caps anyway? Do you think the main board is ok?

Thanks

[MisterD]

Re: LG 32LD450 shuts off after 15-25 seconds

I had the 1k resistor leads shoved into the rail connector where the wire leads go in, and must not have been making a good connection because now that I have shoved them in more, the backlight has NOT gone off at all with the PS forced on and running about 45-50 minutes.
So now I'm thinking the PS is good and the main board is the issue after all.

[budm]

Re: LG 32LD450 shuts off after 15-25 seconds

Bad caps can look normal.

[budm]

Re: LG 32LD450 shuts off after 15-25 seconds

I had the 1k resistor leads shoved into the rail connector where the wire leads go in, and must not have been making a good connection because now that I have shoved them in more, the backlight has NOT gone off at all with the PS forced on and running about 45-50 minutes.
So now I'm thinking the PS is good and the main board is the issue after all.

Lost of command signal is not the function of failed the power supply if the STBY is good.

[MisterD]

Re: LG 32LD450 shuts off after 15-25 seconds

I had my meter hooked up monitoring the command signal and the backlights would flicker or flash on/off 3 or 4 times then go off first before the power on voltage dropped out but it did drop out immediately after backlights went off.

[CapLeaker]

Re: LG 32LD450 shuts off after 15-25 seconds

To get this straight. Since you have the PSU "hardwired" everything is OK?