LG 32LN520B TV power up problem help requested

[dicky96]

Re: LG 32LN520B TV power up problem help requested

OK guys I can settle some of these questions.

As I said I do not have a bench PSU of sufficient voltage to test the LED strips. I do have a scope.

I checked with my scope on the gate of the DD35AA FET that connects the negative side of the LED strips to 0V. It is not being pulsed and has no voltage on it.

I then got my bench PSUs and connected negative to TV ground then connected the positive via a 10k resistor to the gate of the FET and, with the bench PSU set to zero volts I turned the TV on.

Then I slowly brought up the PSU voltage and at around 3V, lo and behold, the Back Light LEDs turned on.

So the 120V supply, the LEDs and the FET are all OK. Hey maybe I even taught someone a useful trick here

One other thing I did check - the red led on the IR/Switch PCB that comes on, dims comes back on, dims and then goes out..... well it is using PWM to do that. So there is a microcontroller running somewhere on the PCB making it do that and I assume it is supposed to tell me something?

The only know definite problem I can see so far - there are no pulses from the IR sensor when I point a remote at it. I know someone said this TV should power up without it (presumably when I press the little standby/power button) but are we quite sure about that?

Rich

[vinceroger69]

Re: LG 32LN520B TV power up problem help requested

here is the owners manual it may help with led/switch etc

[budm]

Re: LG 32LN520B TV power up problem help requested

But do you have Dimming control signal? You do have BL-ON otherwise you will not have the boosted Voltage.
BTW, forcing the Gate of the MOSFET fully on is the same as shorting out the MOSFET so you apply the full Voltage to the LED string WITHOUT current limiting which can damage the good LED (if you have some shorted out LEDs), and if you do have have shorted LED, the LED string will still light up but with lots of current flowing through the good LEDs. You should put DCA meter in series with the LED to see what kind of current flowing through when you force on the MOSFET. It is not good idea to force on the LED without knowing if you have shorted LEDs inside the panel or not, you can easily ruin the good LEDs in process, just have to understand the setup first and what it will do to the circuits.

[dicky96]

Re: LG 32LN520B TV power up problem help requested

Thanks for the user manual, unfortunately it does not mention anything about error codes on the power led like I see the on-dim twice.

@budm. Yes I agree a DCA meter in series with the LEDs would be a good idea when testing as I did. Unfortunately the 10 Amps range on my Fluke doesn't work (not the fuse) and my other meter which does work on current range is still in UK.

That's no real excuse though, I think your warning is quite relevant.

I brought up the bench supply slowly until just the point that the LEDs lit, then turned off after a second or two as I was also concerned that the FET may use pwm to control the LED current, though if I were to put money on it I would bet the converter that steps up the 120V supply is probably a constant current source and the FET is probably just a switch.

I think if I measured the 120V rail at the point the LEDs turn on I would see the voltage fall as the CC circuit kicked in.

Out of interest what is the typical current draw for the backlight LED chain?

Still no nearer to fixing this one though other than proving what is working. Not sure where to go with it next, any suggestions?

Cheers
Rich

[budm]

Re: LG 32LN520B TV power up problem help requested

The Voltage Feeding the Anode is constant, the Cathode return is connected to the D of MOSFE, the current sesing resistor for mantining the constant current is connected to S of the MOSFET, another end of the resistor is connected to GND to complete the circuit.
The PWM control the MOSFET to maintain the constant current, it is constantly ON. You can look up the LED driver IC spec whch do you have the P/N of that IC?

"Out of interest what is the typical current draw for the backlight LED chain? " typical around 300mA, and LG will drive it to the max which about 400mA and burn out the LED fast, LED is 1W.
So is the LED driver IC getting the Dimming control signal?
Do you know if ALL LEDs actually light up and not noticing and on them that are not lit due to shorts?

[vinceroger69]

Re: LG 32LN520B TV power up problem help requested

service manual

[budm]

Re: LG 32LN520B TV power up problem help requested

This is how mad LG drive their LEDs in their panel:
https://www.badcaps.net/forum/showth...ht=LG+47LN5790

[budm]

Re: LG 32LN520B TV power up problem help requested

Service manual, thanks Vince.

[dicky96]

Re: LG 32LN520B TV power up problem help requested

@vinceroger - thank you so much for the service manual it should help a lot

@budm - my sincere apologies, I was wrong. You are quite correct there is a current sense resistor (well actually 4 in parallel) giving 0.1R. I mistakenly thought it was connected to Gnd

OK as requested: LED driver IC is MAP3202
file:///C:/Users/EMINEN~1/AppData/Local/Temp/MAP3202-MagnaChip.pdf

Pin 1 EN is high 3V so yes it is enabled
Pin 11 PWMI is oscillating square wave approx 2ms high 10ms low duty cycle
Pin 9 PWMO is low, not running hence no LED drive.
Pin 10 OVP is varying 2.7V - 2.9V approx

I think it is fair to say the Boost Converter section of MAP3202 is running as we have 120V.

Will check tomorrow if all the LEDs are OK

Thanks for the assistance and pointing out my errors guys

Rich

[budm]

Re: LG 32LN520B TV power up problem help requested

So you have PWm Control signal going into the IC but no PWM out to control the MOSFET.
What is the resistance reading on that PWMO pin and GND?

[dicky96]

Re: LG 32LN520B TV power up problem help requested

Yep I do.

Resistance PWMO to ground is 100K, it drives the FET gate via 15K (measured, not traced the resistor)

Just trying to figure out the block diagram of MAP3202 now, Bud. Looks like OVP pin varying between 2.7-2.9V is correct as this is input to an hysteresis op amp circuit with switch points at those voltages. Most likely as there is no LED load this function is doing it's job and is what is causing the 129-119v cycle on the 120V rail (not my meter loading an high impedance source which of course seemed impossible)

-------------------------

Went to check the voltages on FBN FBP and COMP and would you believe it when i powered the TV up the backlight works. I now have PWMO on the FET gate. Only thing I can think is I had a dry joint somewhere on the MAP3202 chip and pushing on pins to measure resistance made he connection. Possible I checked Pin 9 PWMO with my scope on the PCB trace next to the pin and not the pin itself (though I don't think so)

Anyway based on the dry joint theory I resoldered the pins on MAP3202 and the TV now works even if I tap and shake it a bit...... apart from the fact I still don't have the remote control functioning. I can however select HDMI with the little joystick switch and get a picture from video source.

Funny enough when I was brought this TV I was told it would 'come on but not get a picture' I wonder if they mean they couldn't select the video source as it is going to an RF channel by default, some people are a bit vague in their descriptions. In that case maybe the problem I have been fixing happened in transit?

All in all this has been a very educating experience for me as I said I never even looked at a non-CRT TV before, so thanks very much for all the advice not only did it get me somewhere but I understand a lot more about how these work too.

OK so back to the remote now. I have 0V and 3.3V on the sensor but 1V on the output pin and no pulses.

Remote is flashing if pointed at phone camera, tried another remote from an STB just to see if that would give any pulses wich it did not.

There isn't anything else would cause that but a faulty sensor is there, unless I suppose the possibility whatever the sensor connects to has a faulty input pin? Maybe I should check that.

y.

Rich

[dick_barton]

Re: LG 32LN520B TV power up problem help requested

Interesting fault. Glad you got there in the end.

[dicky96]

Re: LG 32LN520B TV power up problem help requested

Fixed
Wasn't the remote sensor, Took it off again to see the part number and realised I had soldered on the wrong way round

TV and Remote now work fine

Rich

[budm]

Re: LG 32LN520B TV power up problem help requested

Well, that does not make sense due to no backlights since you do have all the control signals.

[drifting]

Re: LG 32LN520B TV power up problem help requested

Hi all,

I have the exactly same TV and the backlight is not working.
I can hear sound as normal and see that the panel is working if using a flashlight pointing to it, but the backlight is off.

So the problem should be the LED backlight power supply. I used my multimeter to check the voltage at LEDs connection pins.
Please see by yourself:
- https://photos.app.goo.gl/xcSrD6NRfaoRQrwHA

At each time the voltage goes up, I can hear a tiny click from the PCB (I think it is from the big transformer).

Does anyone have a hint?

[drifting]

Re: LG 32LN520B TV power up problem help requested

I would really appreciate any help, because it is just a backlight issue... it might be simple.

But, I can always replace the original LED strips with generic ones from Ebay, and use a separate controller

Thank you for any help.

[nomoresonys]

Re: LG 32LN520B TV power up problem help requested

Check all the pins on the connector going from powersupply board to mainboard with multimeter, report all voltages, check in standby which is plugged in turned off, then turn on tv, make note of both conditions. Something like pin 1 ps-on 0v off and 5 v powered on, theres a video how to in the second link there in my signature below.