LG Flatron L1750S - Stopped to work

[rf.pereira]

Hi,

My old LG L1750s stopped working some time ago: it doesn't turn on anymore (even the power LED) and only emits a noise that I'll explain in a moment.

I oppened it to try to figure out the problem, but I didn't found any bad capacitor or something "strange" visible. Even so, I started to follow the steps done in this thread, but changing the capacitors didn't solved my problem.

But I observed something unusual: when plugged, the monitor emits a clicking noise that stops for a little time if I disconnect the mainboard from the PSU. Along with that, the output voltage of the pins in CW201 decreases a lot when the mainboard is plugged again: from ~4.9V to ~2.3V and from ~13V to ~7.5V.

Some data:
- C101 working normaly;
- jumping pins 1 and 5 in U101 shows normal behavor;
- changed 7 capacitors: C206, C202, C204, C205, C203, C105 and C103;
- some pins in CW201 have ~4.8V and others ~13V (unplugged mainboard);
- I couldn't identify which component is emitting the noise;
- PSU model: AIP-0108 REV:02 (Lien Chang)


Thoughts on where to continue?

[piernov]

You didn't replace C207 (I guess, the green one)? You should try to replace it too.
Also you should check for a short to ground on the motherboard power connector's pins.

[rf.pereira]

Originally Posted by piernov

You didn't replace C207 (I guess, the green one)? You should try to replace it too.
Also you should check for a short to ground on the motherboard power connector's pins.

Thank you for your replay.
Yes, I didn't change that one because I don't have the exact same model (470uF - 25V). But I have one that is 470uF - 16V, will it do the job?

Sorry, but how do I check for a short on the motherboard?

And how about the noise? It isn't normal, is it?


Thanks.

[piernov]

No it won't do the job, you can only replace with the same or higher voltage rating. You can use a similar or higher capacity(µF) rating, not too high though. Another 25V 680µF would do the job, or even a 1000µF.

To check for short, put your multimeter in resistance mode (200ohms scale if it's not auto-ranging) put red probe on ground and black probe on each of the pins of the power connector (on the motherboard, not power supply board) and write down the value.

The clicking noise could be the PSU trying to turn on. It could mean there is an issue with the motherboard or backlight circuit.

[rf.pereira]

Ok, I will change C207 using a 680uF 25V.

If I did it correctly (I attached a photo), the results were (starting to count from the closest pin to ground):

pin1- 1
pin2- 1
pin3- 1
pin4- 1
pin5- 00.7*
pin6- 1
pin7- 1
pin8- 00.8*
pin9- 00.8*
pin10- 1
pin11- 1

* - shorted

Thanks.

[piernov]

pin 5, 8 and 9 are probably ground points so it should be fine. To make sure of that, you can check on the power supply board side, you should be getting the same results.

Are the voltages on the connector steady when the mainboard is plugged in?

[rf.pereira]

Originally Posted by piernov

pin 5, 8 and 9 are probably ground points so it should be fine. To make sure of that, you can check on the power supply board side, you should be getting the same results.

Without any power on the PSU, I measured in it:

pin1 to pin4- 1
pin5- 00.5
pin6- 116.7
pin7- 115.9
pin8- 00.5
pin9- 00.5
pin10- 116.7
pin11- 116.7

Originally Posted by piernov

Are the voltages on the connector steady when the mainboard is plugged in?

Now, with the PSU (have yet not changed the C207) and mainboard connected and turning on, I measured in the PSU:

pin1- 1.81 to 2.01V
pin2- 1.72 to 1.91V
pin3- 1.78 to 2V
pin4- 0V
pin5- 0V
pin6- 1.83 to 2.03V
pin7- 1.8 to 2V
pin8- 0V
pin9- 0V
pin10- 7.83V
pin11- 7.83V

This oscillattion isn't normal, is it? And pin 4 has no output voltage, unexpected too...

[rf.pereira]

Hi,

After changed the C207, with the PSU and mainboard connected:

pin1- 0V
pin2- 2.37V
pin3- 0.02V
pin4- 0V
pin5- 0V
pin6- 5.02V
pin7- 5.02V
pin8- 0V
pin9- 0V
pin10- 14.54V
pin11- 14.55V

The clicking noise desappeared, but pin1 to 4 still with a strange behavor... and the monitor still not turn on.

[piernov]

Ok so now you've got a nice 5V rail (pin 6/7). However the 12V doesn't look good, 14.5V instead of 12V is relatively high. I'm not sure why it would behave like that.

Service manual and PSU schematics attached.
Service manual page 29, connector J705 seems to be your power supply connector.
So your pin 4 (NC) is not connected, nothing to worry about.
Pin 2 (DIM) should be a PWM signal so I think that's OK.
Pin 3 (INV) should be 5V for the backlight to turn on.
Not sure about pin 1 (LAMP)

Can you measure voltage on the component right to the power connector on the mainboard? (U801 I guess)

PS: your pin numbering is reversed compared to the service manual.

[rf.pereira]

Thank you for your feedback.

U801 (KIA 78D33F) -

5.02V (IN) 
3.35V (OUT)

Could C207 capacitor (680uF instead of a 470uF) causing this 14.5V?

I had a problem with C105 (lifted pad) but I think I managed to solve. But I'll recheck my solders.

[rf.pereira]

Any idea?

Is there a way to confirm if the PSU is working properly? Maybe some website that shows the correct voltages to each pin?

[rf.pereira]

Originally Posted by piernov

Ok so now you've got a nice 5V rail (pin 6/7). However the 12V doesn't look good, 14.5V instead of 12V is relatively high. I'm not sure why it would behave like that.

Without the mainboard conected, measuring the pins (using the correct numbers):

pin#: voltage
-------------
p11: -0.03V
p10: 0V
p09: 0V
p08: 0V
p07: 0V
p06: 4.9V
p05: 4.9V
p04: 0V
p03: 0V
p02: 13.6V
p01: 13.6V

Could be this difference observed between this values and the ones from the post #8 the indicative of something or is just the normal behavior?

[piernov]

It looks ok, except the 13.6V but it might be normal. I know that some PSU have 14V on the 12V rail and it's perfectly fine. Or you have another bad capacitor on the 12V rail.

Anyway, check voltage on pins AVDD, VDDC, VDDP, HWRESET of U201 (see page 26 of the service manual I posted above).
Check that you've got 3.3V on the power button and that it goes to 0V when you press it.
Check voltage on VCC and RST pins of U501 (p. 27).
Check voltage on pin 1 of the panel connector (p. 29 J704?)

[rf.pereira]

---- U201 ----
AVDD (pin28)- 3.31V
VDDC (pin50)- 1.75V
VDDP (pin47)- 3.36V
HWRESET (pin96)- 1.71V

---- Power button ----
5V on the top 2 legs;
When pressed, goes to 0V.

---- U501 ----
VDD (pin8)- 5.00V
RST (pin7)- 5.00V

---- J704 -----
Couldn't find it.

The connector that goes to panel (LCD) (J702):
0V in all of the pins.

[piernov]

Check voltage on pin 1 and 3 of Q502.

[rf.pereira]

Q502 - kia7042ap
4.29V (first and third pins)
5.03V (first and second pins)

[piernov]

0.74V between 3rd pin and ground then? Q502 could be bad. Try to remove Q502 and put a capacitor between base of Q503 (pin 1) and ground, 33µF/6.3V at least I would say, then check if the monitor turns on.
If it doesn't, check voltage on RST pin again.
If it does, replace Q502, the capacitor is just a temporary workaround to check if the rest of the circuit would work.

[Th3_uN1Qu3]

That high 12v is normal for a monitor when the inverter is not running. They have pretty lax tolerances. Do you get a green light when you push the power button or do you get nothing?

[piernov]

@Th3_uN1Qu3 thanks for the info about the 12V, didn't know that even though I've already seen it. His µC is stuck in reset so he must not get any response at all. This is an active high reset btw, I already did that mistake too…

[Th3_uN1Qu3]

Originally Posted by piernov

@Th3_uN1Qu3 thanks for the info about the 12V

There's a reason they put 25V caps on it you know.

To expand a bit on this, what happens is that the feedback in a monitor PSU is taken from the 5V rail alone, it is not even group regulated like cheap computer PSUs are, the output filter inductors for 5V and 12V are separate.

So the 12V rail is not actually regulated at all. In practice this works fine because the inverter is regulated (thus a variation of the 12V rail will not affect brightness) and the load distribution is chosen so that under normal operating conditions the 12V rail will be reasonably close to that value. When the inverter is not running (no load on 12v), it will rise fairly high, 13-14 volts is not uncommon and nothing to worry about.