LG 32LN520B TV power up problem help requested

[dicky96]

I have this LG 32LN520B TV here and I have no experience of flat panel TV repairs though I have a lot of experience repairing other electronics.

PCB is marked LD36M

When I plug in the 24V PSU, the little red LED by the power button lights up, dims, lights again, dims once more then goes out.

That's it. The TV will not power up and does not respond to the on/off power button.

Quick poke around the PCB (with my meter not my fingers fortunately!!) found 24V supply on the input cap, and 120V across a couple other electrolytics the other side of what like a big square filter or inductor and a power FET. The 120V slowly discharges when I pull the 24V power supply.

So there is power in there, it just ain't starting up.

Any advice from you guys on how to diagnose this further?

Cheers, Rich

[dicky96]

Re: LG 32LN520B TV power up problem help requested

Hi Guys, I had a further look at this today and found this component lying loose in the front plastic casing near where the little PCB with the on-off switch goes.

Aha I thought, this looks like the IR sensor and there was a suitable vacant spot on the little PCB with the power switch (IC1) where it appears to have come from... So I soldered it on there. Think my soldering is OK, certainly looks OK under a jewelers loop.

See attached pics.

Anyway I thought this was probably going to fix the TV, but it hasn't.

I still get the same symptoms, power on, a little red LED on the switch pcb lights up... dims.... lights up... dims then goes out and stays out.

Had a look at the little IR sensor?? chip with my scope. got four pins from the left marked on the pcb G O V G. Seems fairly obvious ti me that would be Gnd, Out, Supply, Gnd

I had a quick look on my scope

Got 0V on the G pins, 3V on the V pin and 1V on the O pin. I pressed buttons on the remote and reasonably expected to see some pulses on the O pin. I didn't (unless they are too fast or small to see), Odd that... I thought I would see something. Checked the remote by pointing it at my phone camera - it flashes.

Any suggestions how I diagnose further, for example what voltage rails should I see on the PSU (got 24V in, 120V and there obviously must be a 3V from somewhere).

Does the led that flashes and dims twice signify something useful?

Cheers for your advice
Rich

[dicky96]

Re: LG 32LN520B TV power up problem help requested

Sorry forgot the pics

[dick_barton]

Re: LG 32LN520B TV power up problem help requested

It is an IR receiver module as you suspected and certainly looks as though it is correctly soldered back in place. You should be able to see pulses on your oscilloscope at the O pin when the remote is pointed towards it.
Is there a part number on the IR module?

Some well focused photo's of your boards would be useful.

[dskall]

Re: LG 32LN520B TV power up problem help requested

Need good clear sraight on pictures of back of tv with cover off and each board.

Need voltages from connector that goes from power supply board to main board. Will need pin#-pin label-Pin voltage in standby-Pin voltage when pushing power button and picture of connector on power supply board.
Format:
Pin# - Pin Label - Pin Voltage standby - Pin voltage power on
1 - XXXXXXX - XXv - XXXv
2 - XXXXXXX - XXXv - XXXv
3 - XXXXXXX - XXXv - XXXv
4 - etc.

[Dgtech]

Re: LG 32LN520B TV power up problem help requested

Your IR board (EAX65356601) I saw on ebay for about $10, free shipping. I think it showed that he'll ship it worldwide.

[dicky96]

Re: LG 32LN520B TV power up problem help requested

Hi Guys

Thanks for your replies. Only three PCBs in this TV that I can see

The main one has PSU and most everything else. There is the little PCB with the standby switch (actually this switch also moves left/right/up/down plus press inwards) then a narrow PCB across the bottom of the screen panel

The little IR sensor did have a part number or SMD code but I would have to desolder it again to see the number. Not a big problem if I really need to.

Pic of main PCB attached. If these pics not clear enough will get my Canon DSLR out rather than use my phone cam, but I have noticed when I attach pics here they are greatly compressed in file size so I guess that means they lose a lot of resolution?

Rich

[dskall]

Re: LG 32LN520B TV power up problem help requested

Well never done a power/mainboard. There must be test points for voltages and turn on voltage. Picture to far away to see any.

[budm]

Re: LG 32LN520B TV power up problem help requested

That transformer and the SMD power MOSFET and the SMD Diode the LED driver power supply, it boost the 24V DC incoming DCV to around 100VDC or so that is why it has 33uf 160 VDC cap for filter in the circuit.
There are lot of buck converter circuits (I see at least 4 inductors for the circuits, not the four inductors by the speaker connector)on that main board to drop down the 24 VDC to run the logic circuits.
I believe the TV should function without that IR RECEIVER MODULE in place, so you have to start probing Voltages on the board, without diagram it is going to be tough.

[dick_barton]

Re: LG 32LN520B TV power up problem help requested

There was a similar board on this forum the other day. Have a look on the underside of the board for voltage test points. 3.3V, 5V, etc

[dicky96]

Re: LG 32LN520B TV power up problem help requested

Thanks guys for the help so far.

Special Thanks budm for that breakdown of the pcb sections

given that info I did some more checking - please refer to attached pic.

PSU above the pcmcia slot is the USB supply. There is 5.15V on the USB connector V+ so that seems to be working OK.

LED backlight PSU - I have 120V on each of the 33uF caps so that looks OK

SMPS above the scart socket - IC is a TPS5433.


Has 24V on Vin, 4.16V on EN and PH is ackling away quite nicely at high frequency, around 30V peak-peak. The two caps to right of the inductor just above the SCART have 3.3V on them. This looks OK to me.

The bottom PSU, the IC Is an AP7173
https://cdn.badcaps-static.com/pdfs/...405400f8d7.pdf

Has 3.3V on Vin and 1.55V on Vout. Measured R1 and R2 (according to AP7173 datasheet) they both read 2.5K so that should give an output of 3.3V / 2 which is.... hey guess what! 1.55V! All is OK here then.

The connector and wires (bottom right) goes to the little pcb with the IR sensor and switch. Pin 3 of the connector has 3.3V on it and when I push the standby/power switch it changes to 0.55V. That seems quite reasonable to me.

The connector to top right goes to the backlight LEDs. It has 129-119V on one pin (it seems to discharge when I put my Fluke DMM on here and when it drops to ~ 119V it jumps back to 129V and repeats. The other pin to the backlight LED has 85V-79V on it and follows the same discharge pattern on my DMM.

The component below the backlight connector is I think a FET (marked DD35AA and an F logo which may be Ferranti?)

Can't find a datasheet for this device, however it has (from the top)
0V in pin 1
the same 85-79V discharging on pin 2
and 0V on pin 3 (which connects to GND)
Which says to me a MosFET that is switched off which explains why no Backlight LEDs illuminated.

Also that discharging 120V seems a bit odd, like it must be a very high impedance source to do that with a DMM and yet it can't be otherwise it could never supply the LED current Just checked it goes direct to the 33uF caps which do the same discharge thing.

OK so that's as far as I am with this, I'm not a newbie with electronics but really not sure what to test for next. Everything I check seems to be OK but something is quite blatantly wrong somewhere as it won't even try to power up.

This is my first attempt with an LED TV - they do seem harder figure out than the old CRT ones even though there is less stuff in them!! I do hope that is not typical of these things

[dick_barton]

Re: LG 32LN520B TV power up problem help requested

What's the led panel details?

[dicky96]

Re: LG 32LN520B TV power up problem help requested

Oh forgot to mention. I traced the O pin for the IR sensor directly to pin 7 of the connector on main PCB.

The one thing I can definitely say is I don't see any pulses on this pin when I point the remote at it. I even tried another remote from an STB to see if that would cause any pulses on this pin. Nope, Nada, Nothing, Zilch. Just 1V steady.

[dicky96]

Re: LG 32LN520B TV power up problem help requested

@dick_barton. Sorry I don't know what you mean by LED Panel details, please elaborate mate. And sorry I missed you previous post before posting myself.

[dick_barton]

Re: LG 32LN520B TV power up problem help requested

The back metal panel of the LCD set normally has a sticker with the panel details on it. Samsung, AOU , LG etc. and model number

[dicky96]

Re: LG 32LN520B TV power up problem help requested

@dick_barton
Had a look on reverse of the PCB - no components or markings at all.

There is a sticker on one of the bars that support the LEDs

See pics, these any help?

Cheers
Rich.

[dick_barton]

Re: LG 32LN520B TV power up problem help requested

Thanks for the information which proves useful.
There are 3 strips of 7 leds per strip = 21 leds.
If 3V/leds = 63V and if 6V/leds 126V.

You say in post #11 "the voltage is 129V when first switched on" so the leds should flash and then the voltage falls turning the leds off.

I suspect that if the 3 strips are connected in series (42V/strip) that you have a led(s) fault.

If you have access to a current limited power supply then I would test each strip.

[dicky96]

Re: LG 32LN520B TV power up problem help requested

Hi mate. No I said when I first put my DMM probe on the LED power supply pin it reads 129V.... it then drops to approx 119V over a coup[le of seconds then suddenly jumps back to 129V, and repeats.

I assumed the reason the LEDs do not illuminate is because there is what looks like FET marked DD35AA (can't find datasheet) connecting the other LED power lead to ground, or it should if the FET has any gate voltage which it doesn't.

I do have a CC PSU but it will only go to 30V. However if I connected a suitable value resistor (I don't know... 10K or someting?) from the 3.3V rail to the DD35AA FET gate pin wouldn't that turn the backlight LEDs on thus testing them, and the 120V PSU?

Rich

[budm]

Re: LG 32LN520B TV power up problem help requested

"The connector to top right goes to the backlight LEDs. It has 129-119V on one pin (it seems to discharge when I put my Fluke DMM on here and when it drops to ~ 119V it jumps back to 129V and repeats. The other pin to the backlight LED has 85V-79V on it and follows the same discharge pattern on my DMM"
You have one LED string.
Sop you have about 120V feeding the Anode of he LED string, the Cathode side of the LED string shows about 85V, that means the Vdrops on the LED string is 120V- 85V = 44V which cannot be right since you have 21 LEDS.
Did you check each one of the LED yet?

[dick_barton]

Re: LG 32LN520B TV power up problem help requested

Just my thoughts. Are you sure that the gate of the mosfet is not being pulsed. It does seem as though the backlight boost voltage is switching on (129V) but not lighting the leds. So either no current flow because of an open circuit led or failure of the led driver to drive the mosfet on/off is causing the set to restart. Do you have access to an oscilloscope?