LG 47LS4500 PSU Problem

[msft_engr]

Hi all,

I have an LG 47LS4500 that I am pretty sure has a problem with the PSU board. I've isolated it (unplugged from other boards) and have a good steady 3.5v rail, but not output on the 12v or 24v rails, nor do I have voltage on the primary side of T101.

I do have 168.3VDC at F501 and F101, so the output is good from the full wave bridges out.

I'm suspecting something wrong with the circuit that powers T101 and wanted to see if anyone with experience with this unit had suggestions on where to look before I dig deeper.

(And yes I did search thoroughly here and other places).

Thanks so much for any and all assistance.

--Henry

[budm]

Re: LG 47LS4500 PSU Problem

You have the standby power supply but the rest of the power supplies will not come on UNTIL the main board send the PS-ON signal to turn on the rest of the power supply.
Typical PS-ON DCV is <1V = OFF, > 2V = ON. So you need to supply good clear pictures of the whole power supply board and the closed up around the connector of the power supply board that goes to the main board so we can see the pin labels. You should read the basic LCD TV troubleshooting guide.

[msft_engr]

Re: LG 47LS4500 PSU Problem

Thanks for the reply, much appreciated. I'd looked at the excellent guides posted in your signature (thanks for those!), I should have figured that the PSU would only keep the bare minimum rails on and that the others would be energized based on logic. I've attached pics of the PSU and controller here. I'd like to be able to test both of these boards to understand which one needs to be replaced (or, more preferably, which components are suspect - I am adept at soldering, have a great temp controlled wand and an SMD hot air rework station).

I really do appreciate the help, even just getting in the right direction (man how good schematics with test point values would help :-) ).

Thanks again.

[dick_barton]

Re: LG 47LS4500 PSU Problem

Maybe worth checking your diodes first. D607 tends to be a problem on this power supply.

https://www.youtube.com/watch?v=j_G3Flclj0o

[msft_engr]

Re: LG 47LS4500 PSU Problem

Thanks for the suggestion Dick. I've already done an in circuit ohming of all of the diodes and fuses, hoping it would be an quick fix, no joy.

One thing I just thought of - when everything is plugged up and I plug in the power, I can hear a quick high pitched whine from the controller board around the area of the heatsink in the center (using a piece of flexible tubing as a stethoscope). At that time the 12VDC and 24VDC rails power up, but only for a half second or so before going flat. Maybe that'll help with the diagnostics...

And I do truly appreciate any and all ideas! :-)

--Henry

[dick_barton]

Re: LG 47LS4500 PSU Problem

When you unplug the mainboard and then switch on do you then have 3.3V, 12V & 24V ?

[msft_engr]

Re: LG 47LS4500 PSU Problem

Nope, when I unplug the mainboard (and others, isolating the board) I only have the 3.85v bus - nothing on the others, and 168VDC at the input to the stages that power those busses (at the outputs of F101 and F105).

[dick_barton]

Re: LG 47LS4500 PSU Problem

What does C603 look like which appears to be another problem with this supply.

Photo's courtesy Shopjimmy

Edit
What DC voltage do you have across the two pins of C610

[msft_engr]

Re: LG 47LS4500 PSU Problem

Haven't pulled it to do an ESR or leakdown test but being that most of these typically torch themselves when they puke this one "looks" good, and no hot/bad traces or cold joints on the backside around C603...

Really appreciate everything you're helping me with!

--Henry

[dick_barton]

Re: LG 47LS4500 PSU Problem

Ok then, start troubleshooting. What DC voltage do you have with the mainboard disconnected from the power supply across the 2 pins of C610

a) without the PS_ON connected to 3.5V
b) with PS_ON connected to 3.5V via a 1K resistor.

[msft_engr]

Re: LG 47LS4500 PSU Problem

Thanks for the idea. After forcing the PSU on with the 1K resistor between the 3.85V bus and PWR_ON I am getting steady voltages on both the 12V and 24V busses. Appreciate that help, I'm confident the PSU is good, the problem I think has to be in the control board.

Anyone have schematics for that board handy (EAX64437505 (1.0) )?

Thanks...

[dick_barton]

Re: LG 47LS4500 PSU Problem

So it looks as though your power supply is OK so you must be missing the PWR_ON from the mainboard which is fed back to the power board.
You could try reconnecting the power board to the main board but lift out the PWR_ON wire from the connector and use the 1K as you did above and then see if the set powers up.

[msft_engr]

Re: LG 47LS4500 PSU Problem

Excellent idea, I'd planned on doing exactly that as soon as I get a chance today, will keep you posted as to what happens. Hopefully no loss of Magic Smoke in the process.

[dick_barton]

Re: LG 47LS4500 PSU Problem

Excellent idea, I'd planned on doing exactly that as soon as I get a chance today, will keep you posted as to what happens. Hopefully no loss of Magic Smoke in the process.

If it does work (fingers crossed) then Q401 & Q402 on main board need to be checked.

[msft_engr]

Re: LG 47LS4500 PSU Problem

Ack. I'll take some with the DSLR and post after I test, fingers crossed!

[msft_engr]

Re: LG 47LS4500 PSU Problem

Delayed by life, back to this. After testing with the control board plugged in, all of the supply rails on the LV side of the PS fluctuate - they will come up to their proper levels and then drop to 0V on about a 5 second cycle. It sounds like the control board is suspect, I hear what sounds like an HF sound from the area of the chip under the heatsink. Without the control board plugged in and the PWR_ON pin shorted I get all of the LV rails at their proper voltages.

Here are high res pics of the control board front and back. Hope you can help, and thanks again!!!

--Henry

[dick_barton]

Re: LG 47LS4500 PSU Problem

Repeat as in post #21 to bring on the secondary voltages and in addition also apply 3.5V to INV_ON using another 1K resistor. Main board disconnected and check if the backlights come on.

[msft_engr]

Re: LG 47LS4500 PSU Problem

Excellent, thanks, proceeding, will update.

[msft_engr]

Re: LG 47LS4500 PSU Problem

Just tested as recommended, adding the 3.85V bus to the DRV_ON without the main board plugged in. Backlights function properly, rock solid.

Thoughts?

Thanks again!

[kramer123]

Re: LG 47LS4500 PSU Problem

Haven't pulled it to do an ESR or leakdown test but being that most of these typically torch themselves when they puke this one "looks" good, and no hot/bad traces or cold joints on the backside around C603...

Really appreciate everything you're helping me with!

--Henry

Hi, anyone know the value of c603 I got one that is shorted burnt beyond reading?
nevermind found a thread says 220pf @ 2kv
https://www.badcaps.net/forum/showth...highlight=c603