LG 37LH3000 dead

[Khron]

A friend brought me this 37" LG 37LH3000 from his folks the other day, said it was dead. Says it's made in march '09, on the label on the back.

A plugging-in to the mains confirmed it - totally dead, no stand-by light, no reaction to buttons or remote, nothing.

Took the back cover off, to probe the output voltages on the power supply connector. Nothing, zip, zilch, nada.

"Preventively", i changed the three small caps i saw on the primary (one near the PFC controller, and two near the main PWM), for peace of mind, but to no avail.

The main PWM's a Sanken STR-W6252.

Now, upon some further probing and a few lightbulb-tester plug-ins later, i discovered the main PWM showed a reading of about 2.36ohms between its drain and source (red probe on drain, black on source). Now, that's definitely not right, is it?


Slightly funny part is, neither of the two fuses (AC-to-rectifier, and B+ to PWM) had / has popped yet

Good thing is, i found spares on eBay, for ~20$ a pop. I'd rather get some sort of "second opinion" before i pull the trigger on one of those

[tom66]

Re: LG 37LH3000 dead

If you remove the main PWM IC, the set should still power up into standby mode if that's the only fault.

[Khron]

Re: LG 37LH3000 dead

Umm... The main PWM is one of those PWM+FET integrated jobbies, so... No PWM = no power, not even stand-by.

From what i could figure out from tracing some bits of the circuit, the STR-W6252 is "on" all the time; all the "power-on" signal from the mainboard does, is turn on the PFC controller.


EDIT: Dang, i took a couple pics to attach to the first post, but i forgot

[tom66]

Re: LG 37LH3000 dead

Ah, this set does not use a standby PSU. You can use a lamp in place of the fuse when you first replace the IC - it will save you from blowing the new IC.
http://www.kellerstudio.de/repairfaq...aq.htm#tvtslbt

(for a CRT TV but applies for almost any power supply.)

[Khron]

Re: LG 37LH3000 dead

Yeah, that's what i used for the "lightbulb-tester" bit i mentioned

Guess i'll go ahead and order one... Fingers crossed

[tom66]

Re: LG 37LH3000 dead

Ah, very good -- did not see that. Good luck. With those all in one ICs the damage is usually constrained to just the IC.

[Khron]

Re: LG 37LH3000 dead

I just ordered one off eBay.co.uk (gotta love it - ~1week delivery usually ).

There's no visible signs of damage whatsoever, no bulged caps or anything, and as i said, neither of the two fuses has/had popped

Ah well, i guess i'll be back with updates in a few days, as soon as i get the IC...

[tom66]

Re: LG 37LH3000 dead

There's possibly a burnt fusible resistor somewhere?

[Khron]

Re: LG 37LH3000 dead

Shyte... I think i might've found the ACTUAL cause

I just desoldered the PWM, and on its own, it shows over 6Mohm (mega-ohm) drain-to-source.

BUT...

It seems one or two of the four switching FETs from the inverter might be shot (they switch the B+, and their gates are driven with the two small transformers visible in the lower-left of the second picture.

Upon tracing the circuit, looks like those four FETs comprise a full-bridge circuit; the primaries of the two inverter trafos (bottom right in the 2nd picture) are in series.

I'll desolder those FETs too, and see what's what...

[tom66]

Re: LG 37LH3000 dead

Luckily, if they are driven by transformer, they should be the only casualty.
Really surprised fuse not blown -- are you sure??

[Khron]

Re: LG 37LH3000 dead

I'm positive neither fuse is blown - I used my multimeter to probe the B+ at various points, both before and after the DC fuse.

Commencing desoldering of the FETs...

[Khron]

Re: LG 37LH3000 dead

Confirmed - two of the four fets show ~1.16ohm drain-to-source.

Time to cancel that PWM order, and look for a quad of compatible FETs (FDPF5N50's)

[tom66]

Re: LG 37LH3000 dead

Sometimes there are diodes nearby them, check those too.

[Khron]

Re: LG 37LH3000 dead

The turn-off diodes all look (read: measure) ok, and the zener pairs on the gates also seem ok (forward-voltage-wise, anyway, and meter says OL the other way around).

[tom66]

Re: LG 37LH3000 dead

Then they are most likely OK.

[Khron]

Re: LG 37LH3000 dead

Ok, i found some pretty cheap STP5NK50ZFP's, and i ordered a set of 4pcs.

By a miracle, i'll get'em by friday, but i'm more realistically expecting them next week. Until then...

[Khron]

Re: LG 37LH3000 dead

Got the FETs today

Popped'em in, plugged in through the lighbulb tester - nothing. Voltage on the main filter caps = 305Vdc. Voltage between the Drain pin of the main PWM and ground = nothing.

Turns out the fuse between the B+ rail and the PWM/inverter sections was also blown. Didn't have any 3.15A at hand, so i popped in a 4A one.

Plugged it in through the bulb again - flickering, so the PFC was working. Mounted it back into the tv, plugged it in, and all's fine

Owner happily picked it up a few hours ago

[tom66]

Re: LG 37LH3000 dead

Glad you fixed it, thanks for posting back the fix.

[Khron]

Re: LG 37LH3000 dead

No problemo, just trying to give something back to this great community that's provided so much information and knowledge in this past half-decade