LG 32LD550 won't turn on - HELP!

[yrn11]

Re: LG 32LD550 won't turn on - HELP!

We need a schematic at this point I think, I found 3 for your model on ElectroTanya but for different variants. It should be written probably on a sticker inside somewhere which one yours is.

Should say 32LD550-XX where XX may be UB or some such code, We probably need to know which one to get the right schematic. (I say probably, because sometimes they are all exactly the same and the code may only refer to something like the case colour or design which has nothing to do with the electrical design)

Product code is - 32LD550-TA.AMFWLH, can you share the service manual?

[Agent24]

Re: LG 32LD550 won't turn on - HELP!

This looks like the one then:

For the Inverter control line, start with the circuit top left of page 19.
The pin they call "INV ON" pin 18 of P401 is where the inverter control signal should come from. Controlled by Q405. I would start checking there if that signal is missing.

You should probably check the secondary power rails on the mainboard first though, it could be that one of them is down or low.

[yrn11]

Re: LG 32LD550 won't turn on - HELP!

This looks like the one then:

See attached.
what should I look for?

[Agent24]

Re: LG 32LD550 won't turn on - HELP!

See attached.
what should I look for?

See edited post above - that's where I would start anyway. Budm might have some other suggestions too.

[budm]

Re: LG 32LD550 won't turn on - HELP!

Trace the INV ON pin to see where the signal is stopped from going HIGH, there are switched transistor and etc, in that signal path that you need to check. You know how to read the schematic in details and understand how transistor works, right?

[Caleb]

Re: LG 32LD550 won't turn on - HELP!

I had one of this series, (46ld550) that initially presented as a dead set. I reflowed the main processor and got back on/off functionality and backlights but it never would boot up properly, still havent located a cost effective replacement board. In looking around this series seems to have a lot of issues with failing main boards.

[yrn11]

Re: LG 32LD550 won't turn on - HELP!

Trace the INV ON pin to see where the signal is stopped from going HIGH, there are switched transistor and etc, in that signal path that you need to check. You know how to read the schematic in details and understand how transistor works, right?

Well I'm a bit lacking on how to read a schematic map, but I took the readings, Negative probe was on chassis - (see pic1 for schematic)
Q405 - 0.04, 0.67, 0
R415 - 0.04, 0.04
R418 - 0.04, 0
R419 - 3.3, 0.04
R421 - 3.4, 0.67
R425 - 0.04, 0.94
R426 - 3.4, 3.4
R427 - 3.4, 0

I looked up "INV_CTL" and only found that is connected to R1064 (see pic2) - didn't took readings there yet.

[Agent24]

Re: LG 32LD550 won't turn on - HELP!

Your readings suggest (I cannot be certain as you did not write down which pin or side of each component you measured each voltage on) that the transistor Q405 is turned ON from the current through R426 and R421, which means that the collector will be pulled to ground and so will INV_CTL line to the PSU, which makes sense if it is currently at close to 0v.

What should happen is that IC1002 will bring its Pin 10 low through R1064, this will turn Q405 OFF, and the INV_CTL line to the PSU would then rise to 3.3v powered from the +3.3V_Normal rail via R419.

I get the impression that either R1064 is open circuit, or IC1002 is not bringing Pin 10 low for some reason (it could be one of a variety of things).

Check the voltage you get on R1064 and go from there.

[budm]

Re: LG 32LD550 won't turn on - HELP!

What DCV do you get between the Emittter and Base of Q405 (INVERTER CONTROL LINE SWITCH) when TV is OFF and when TV is turned ON?

[yrn11]

Re: LG 32LD550 won't turn on - HELP!

Well I took some readings again for all components while now each one of the probes was on each side of the component (is this the right way to check?).

R1064 - Standby - 0,0
R1064 - ON - 3.4, 3.4 --> seems like this is getting out the voltage right.

What DCV do you get between the Emittter and Base of Q405 (INVERTER CONTROL LINE SWITCH) when TV is OFF and when TV is turned ON?

Q405 - Standby & ON (I'm not sure which is the emitter and which is the collector) - 0.67

R415 - 0
R418 - 0.03
R419 - 3.3
R421 - 2.7
R425 - 0.9
R426 - 0
R427 - 3.4

Attached pics of the circuits

[budm]

Re: LG 32LD550 won't turn on - HELP!

Ok, this might be easier if you look at the diagram 1 that your post 27, check the DCV on each end of that resistor R421 when TV is OFF and when TV is ON.
i.e
left side TV OFF = 0.60VDC, TV ON = 0.10VDC
Right side TV OFF = XXXX VDC, TV ON = XXXX VDC.

[yrn11]

Re: LG 32LD550 won't turn on - HELP!

Ok, this might be easier if you look at the diagram 1 that your post 27, check the DCV on each end of that resistor R421 when TV is OFF and when TV is ON.
i.e
left side TV OFF = 0.60VDC, TV ON = 0.10VDC
Right side TV OFF = XXXX VDC, TV ON = XXXX VDC.

R421:
1st time connecting to AC:
Left = 3.4, Right = 0.64

ON -
Left = 3.41, Right = 0.68

StandBy -
Left = 0.02, Right = 0.02

- Why is there a voltage after connecting it for the first time to AC?

[budm]

Re: LG 32LD550 won't turn on - HELP!

OK, Your reading does not make sense to me.
See the picture of the Q405 with E, C, and B marking for you.
Plug the TV in turn it on and wait a few minutes and then turn the TV OFF then take the DCV reading using chassis for the meter Black probe.
Report the readings:
TV is turned off: B= XXX VDC, C= XXX VDC
TV POWER SWITCH is activated: B = XXX VDC, C = XXX VDC.

Notes: TV may do the auto power on when it is first plug into the AC outlet.

[yrn11]

Re: LG 32LD550 won't turn on - HELP!

OK, Your reading does not make sense to me.
See the picture of the Q405 with E, C, and B marking for you.
Plug the TV in turn it on and wait a few minutes and then turn the TV OFF then take the DCV reading using chassis for the meter Black probe.
Report the readings:
TV is turned off: B= XXX VDC, C= XXX VDC
TV POWER SWITCH is activated: B = XXX VDC, C = XXX VDC.

Notes: TV may do the auto power on when it is first plug into the AC outlet.

Hi,

Here are the readings -
OFF: B = 0.02, C = 0
ON: B = 0.67, C = 0.04.

[Agent24]

Re: LG 32LD550 won't turn on - HELP!

Hi,

Here are the readings -
OFF: B = 0.02, C = 0
ON: B = 0.67, C = 0.04.

I wonder if R419 is open circuit? Without base voltage to Q405, it should be off, and then I'd think the collector should be at 3.3v supplied through R419.

With power OFF and TV unplugged, can you measure the resistance of R419? (Should be 1K Ohms as per schematic)

If that is OK, can you check resistance between C and E of Q405 - perhaps it is shorted.

Well I took some readings again for all components while now each one of the probes was on each side of the component (is this the right way to check?).

R1064 - Standby - 0,0
R1064 - ON - 3.4, 3.4 --> seems like this is getting out the voltage right.

No, you need to measure each side of the resistor with respect to ground.

That means, you would connect your negative meter probe to ground\chassis and check each side of R1064 with the positive lead.

Then write down the voltage from each terminal of R1064, noting which terminal goes to what in the circuit. For example:

R1064 side connected to IC1002 - 0v
R1064 side connected to R421 - 3.4v etc etc.

If you don't know which end of R1064 connects to R421 for example, then, with the TV OFF and unplugged, you need to do continuity (diode or ohms mode) check between each end of R1064 and R421.


So, you would put say your meter into Ohms mode, connect one probe to one end of R1064. Then connect the other end to one end of R421.

If at that point you have connected your probes to the terminals which are connected to each other, then you would get a reading of close to zero ohms. If you had the correct end of R1064 but the other end of R421 you would get 10K ohms, since you would be measuring the resistance of R421 then.

If you got nothing from one end of R1064 between either end of R421, you'd probably connected to the R1064 terminal that was connected to IC1002 instead.

By checking the resistance between each terminal of R1064 and each of R421, you should find one combination that produces a reading of close to zero ohms or continuously sounds the buzzer. These would be the two terminals linked together.

I hope that makes sense....?

[budm]

Re: LG 32LD550 won't turn on - HELP!

"Here are the readings -
OFF: B = 0.02, C = 0
ON: B = 0.67, C = 0.04."

The BASE/EMITTER junction is being bias ON and OFF but there is no change on the Collector at all, the processor OUTPUT is OK and able to change state.

'OFF: B = 0.02, C = 0' You should have >2.5V at the Collector since there is NO BIAS to turn on the transistor.
So check the E-C resistance, check the resistor connected to the Collector.

I will have to look at the diagram to make sure that there is another circuit to lock-out the Inverter on output control line. I can see that the ERROR_OUT pin 24 of the connector is tied to the Collector of that INVR switched Transistor Q405 through 100 Ohms resistor, if that ERROR_OUT pin is stuck LOW, then it will not allow the Collector of Q405 to swing HIGH toward the 3.3V supply. I will have to download the service manual to look at it in details.
At this point you need to check the Q405 Collector resistor and the E-C resistance first.

[budm]

Re: LG 32LD550 won't turn on - HELP!

It looks like the Power supply board is the one that generates (but I am not sure since we do not have the diagram of the power supply board) the ERROR_OUT signal, NO-ERROR pin 24 should be HIGH, <1V = LOW = in standby mode, or if the power supply detects error when TV is turned on so it keeps the line LOW to keep the Inverter from coming on.
That error signal is fed to IC101 pin 41 on page 17 of the service manual.
There is a wire on that pin 24 of the connector, right?
Which one of these is your power supply board?
http://www.shopjimmy.com/catalogsear.../?q=LG+32LD550

[budm]

Re: LG 32LD550 won't turn on - HELP!

OK I trace out the power supply connector P201 ERROR PIN, that pin is traced to pin 11 (Error mode output pi (open DRAIN) of the SMD PWM IC BD9270. Please verify I am correct by measuring the resistance between the ERROR pin of the power supply connector and the pin 11 of the IC, it should show <1 Ohm, but the you can have bad E-C of Q405 that is keeping the INVR-ON LOW.
So there is something wrong in the inverter/backlights circuits, but I am hoping that is just a bad transistor.

[yrn11]

Re: LG 32LD550 won't turn on - HELP!

I wonder if R419 is open circuit? Without base voltage to Q405, it should be off, and then I'd think the collector should be at 3.3v supplied through R419.

With power OFF and TV unplugged, can you measure the resistance of R419? (Should be 1K Ohms as per schematic)

If that is OK, can you check resistance between C and E of Q405 - perhaps it is shorted.

R419 resistance - is very close to 1k Ohms, it's about 0.9k Ohms.
Q405 C - E resistance - is also very close to 1k Ohms, 0.95k Ohms.

No, you need to measure each side of the resistor with respect to ground.

That means, you would connect your negative meter probe to ground\chassis and check each side of R1064 with the positive lead.

Then write down the voltage from each terminal of R1064, noting which terminal goes to what in the circuit. For example:

R1064 side connected to IC1002 - 0v
R1064 side connected to R421 - 3.4v etc etc.

If you don't know which end of R1064 connects to R421 for example, then, with the TV OFF and unplugged, you need to do continuity (diode or ohms mode) check between each end of R1064 and R421.


So, you would put say your meter into Ohms mode, connect one probe to one end of R1064. Then connect the other end to one end of R421.

If at that point you have connected your probes to the terminals which are connected to each other, then you would get a reading of close to zero ohms. If you had the correct end of R1064 but the other end of R421 you would get 10K ohms, since you would be measuring the resistance of R421 then.

If you got nothing from one end of R1064 between either end of R421, you'd probably connected to the R1064 terminal that was connected to IC1002 instead.

By checking the resistance between each terminal of R1064 and each of R421, you should find one combination that produces a reading of close to zero ohms or continuously sounds the buzzer. These would be the two terminals linked together.

I hope that makes sense....?

Well I've checked R1064, both sides shows the same numbers.
TV OFF = 0v at both sides.
TV ON = 3.4 at both sides.

[yrn11]

Re: LG 32LD550 won't turn on - HELP!

OK I trace out the power supply connector P201 ERROR PIN, that pin is traced to pin 11 (Error mode output pi (open DRAIN) of the SMD PWM IC BD9270. Please verify I am correct by measuring the resistance between the ERROR pin of the power supply connector and the pin 11 of the IC, it should show <1 Ohm, but the you can have bad E-C of Q405 that is keeping the INVR-ON LOW.
So there is something wrong in the inverter/backlights circuits, but I am hoping that is just a bad transistor.

I kind of lost you on the way, I've checked Q405 & R419 resistance as Agent24 said -
R419 resistance - is very close to 1k Ohms, it's about 0.9k Ohms.
Q405 C - E resistance - is also very close to 1k Ohms, 0.95k Ohms.

But I didn't quite followed you, between which elements should I check the resistance? Error pin and?
Are you talking about IC1002?, because I couldn't find in the service manual any IC BD9270 (where should I look for it?).