LG LCD power supply

[momaka]

Re: LG LCD power supply

The VCC is fed by the 5VSTBY which he had found the the 5VSTBY output was showing shorted circuit (post 21) so I start helping him locate the shorts, we fond that the MOSFET for the buck regulator were shorted, then he changed the IC and now the 5VSTBY is back but the 3,3 V is at only 1.4V.

Right, I got that part.

I can see that the circuit on his board indeed matches the application circuit in the datasheet quite well. What I am really questioning, however, is if U200 is now getting 5V on pin 2 (VCC) and more than 9V on pin 6 (Vc), or if there is something in the way stopping these. Because if that's the case, then that could well prevent the buck regulator circuit from working.

https://www.badcaps.net/forum/attach...8&d=1463421872

Yes, great drawing! I've been using it myself to help me with the last couple of posts.

[budm]

Re: LG LCD power supply

The circuit can be turned ON/OFF per spec sheet:

https://www.badcaps.net/forum/showpo...&postcount=107
The circuit is so simple and not many parts so it should work, unless the output is being heavily load down which is why I would think running the board by itself should do something.
I did not try to trace the 3.3V_ON line to see if it goes back to the U200 or not, that may be the next thing I need to do.
I wish I have the board in front of me, tracing the board from pictures is very tiring.

[vinceroger69]

Re: LG LCD power supply

Thanks for the input guys i will ask my relative if i can borrow there psu out of there 37in lg set (same psu i recapped works 100% ok in there set and mine) and take some voltage readings for you from a working board without the 3.3 on pin jumped with a resistor and see if the other 3.4 volts run ok any other tests i need too do whilst i have the psu as not sure how long i can borrow it for.

Edit the 3.3 on pin on these boards always reads low 2.00 volts usually there are numerous threads stating this im not sure why it reads this low but it does i will take voltage readings of this pin whilst in my relatives set and also my set just to confirm but on badcaps site looking at this power supply all threads this pin seems low.

[vinceroger69]

Re: LG LCD power supply

ive ran some tests with both good and bad psus connected in the sets

p201 3.4 on good psu in standby 0.04 good psu on 2.05
bad psu in standby 0.04 bad psu on 2.05

p202 3.4v good psu in standby 0.31 good psu on 2.87
bad psu in standby 0.31 bad tv on 2.22

p203 3.4 v good psu in standby 0.31 good psu on 2.87
bad psu in standby 0.31 bad tv on 2.22

so with the good psu all functions work as in tuning and sound with the bad psu no sound or tuning the 3.4 volt diffrence is 0.65 volts which is stopping the set from working dont ask me how?
All the other voltages on the board match the pin outs spot on no diffrence on ac dect or err pins either its a odd fault as why wont the 3.4 volt supply run when running the psu on the bech but as soon as its in the set theres a voltage on the pins? i may solder a resistor on the good psu and see if the 3.4 volt is on the good board out of the set.

Edit just linked 5v stby to on pin with 1 k resistor all voltages fine (except 3.4 on this is good psu) linked also 3.4 on to gnd with 1 k resitor still no 3.4 volts on good psu but set works fine when all boards conneted to tv.

[budm]

Re: LG LCD power supply

So even the good power supply you are not getting 3.3V but only 2.87V? And it is not present unless all the connectors are in place and it cannot be forced on with 1K between 5VSTBY and the 3.3V_ON pin, that is really strange.
You should also check the DCV of all the pins of U200 on the good board.

[vinceroger69]

Re: LG LCD power supply

ive been doing 3.3v on and ground should i try it too 5 v standby? (Ive no idea why i was linking 3.3v on to ground) but hey im still learning so one little hicup isnt too bad but sorry any confusion caused.
yes good psu only 2.87v triple checked and only if all boards connected in the set
i will check all voltages on u200 and report findings.

[vinceroger69]

Re: LG LCD power supply

ok with 3.3von to 5v stby on bad board linked with 1k i get 2.20 volts will do same test now with good psu

[vinceroger69]

Re: LG LCD power supply

ok on good psu with linked as above the voltage is 2.34 volts just going too test u200 now

[vinceroger69]

Re: LG LCD power supply

u200 good working psu
pin
1 0.85
2 5.00
3 0.99
4 0
5 2.67
6 9.17
7 2.36
8 2.86

[vinceroger69]

Re: LG LCD power supply

u200 on bad psu
1 0.78
2 5.12
3 1.88
4 0
5 3.64
6 9.80
7 2.00
8 2.92

[vinceroger69]

Re: LG LCD power supply

q204 on working board 5.12 in 2.38v out
q204 on bad board 5.12 in 2.21 out

[budm]

Re: LG LCD power supply

I just notice one thing, I thought you only get 1.1~1.4V on the bad board, but now you do have 2.21V?
Also this 3.3V buck converter is and switched power supply, looking at the DCV of the bad PSU, U200 pin 3 is at 1.88VDC, good PSU pin 3 is at 0.99V, pin 5 also has quite different dcv.
So I would like to find out is that which pin change state when the 3.3V is forced on and off.
The bottom line though is that both PS are not putting out 3.3V like they should and to me that will cause the TV not to function properly.

[vinceroger69]

Re: LG LCD power supply

no idea about why i had 1.1 1.4 volts but once changed u200 again this is the correct voltages ive given you now.

so just to confirm you want me to remove the 1k between 3.3 on and 5v stanby and measure u200 again? thanks budm

[vinceroger69]

Re: LG LCD power supply

u200 readings bad psu no forced on 3.3v
No Voltages at all on u200

[budm]

Re: LG LCD power supply

no idea about why i had 1.1 1.4 volts but once changed u200 again this is the correct voltages ive given you now.

so just to confirm you want me to remove the 1k between 3.3 on and 5v stanby and measure u200 again? thanks budm

It may have been bad soldering or bad new IC then.
I want the reading on the pins of U200 with the forced on resistor and without the forced on resistor to figure out how this U200 being turned on or off by the 3.3_on pin.
But we still need to figure out why the 3.3V is so low, it may be due to those two Voltage divider resistors for setting the output Voltage.

[vinceroger69]

Re: LG LCD power supply

just removed the resistor on the 3.3 v on pin, no voltage at all on u200 ive just tried two meters to confirm. but if i link 3.3von to gnd i get readings on u200?

[momaka]

Re: LG LCD power supply

I did not try to trace the 3.3V_ON line to see if it goes back to the U200 or not, that may be the next thing I need to do.

It does.
It first goes through a resistive divider formed by SMD resistors R202 and R203.
Then there is some logic signal going on with SMD transistors Q200 and Q201 and SMD resistors R200 and R201.

In particular, Q200 appears to control the Vcc of U200.

But I guess that is not important now, because:

u200 on bad psu
1 0.78
2 5.12
3 1.88
4 0
5 3.64
6 9.80
7 2.00
8 2.92

This verifies that U200 is getting proper Vcc and Vc voltages even on the bad board.

Speaking of those readings above, the only thing I see different between the good and bad boards is the slight higher voltages on pins 3 and 5 of the bad board. Pins 3 and 5 are the Lower Gate and High Gate drives for the switching MOSFETs (Q204 an Q205) respectively.

Now, that could be due to the multimeter not picking up high-frequency pulsed DC properly (since that is what the MOSFETs' Gate signals are). Or it could also mean that U200 is driving Q204 and Q205 much harder on the bad board, perhaps because there is something loading down the 3.4V line. But what?

Edit just linked 5v stby to on pin with 1 k resistor all voltages fine (except 3.4 on this is good psu) linked also 3.4 on to gnd with 1 k resitor still no 3.4 volts on good psu but set works fine when all boards conneted to tv.

As I mentioned in post #114 and #117, SMD resistor R202 has high enough of a resistance that you don't actually need to use a 1 KOhm resistor when pulling 3.4V_ON low or high.

Not to say that you never should - in many cases it IS a good idea. For this PSU, it is not needed, though.

ok with 3.3von to 5v stby on bad board linked with 1k i get 2.20 volts will do same test now with good psu

... so it works, i.e. switches on (or appears to, it seems ). That's good.

ive ran some tests with both good and bad psus connected in the sets

p201 3.4 on good psu in standby 0.04 good psu on 2.05
bad psu in standby 0.04 bad psu on 2.05

p202 3.4v good psu in standby 0.31 good psu on 2.87
bad psu in standby 0.31 bad tv on 2.22

p203 3.4 v good psu in standby 0.31 good psu on 2.87
bad psu in standby 0.31 bad tv on 2.22

so with the good psu all functions work as in tuning and sound with the bad psu no sound or tuning the 3.4 volt diffrence is 0.65 volts which is stopping the set from working dont ask me how?

Well, that 0.65V difference could be just enough to make the logic board not function properly. After all, the PSU board says it provides 3.4V. Less than 3 Volts is quite a stray from that.

Now, what is puzzling is that first P201 test, where you got 2.05 Volts both for the good and bad boards. Was the TV still working? If the voltages are the same, I don't see why the "bad" board would have caused problems then. Seems the results would be inconsistent... unless you measured or wrote down those test voltages incorrectly. (But I doubt that)

In any case, I am really curious to see what would happen if you were to supply an external 3.3V source on the 3.4V line (such as from a PC ATX PSU) on the bad board. Would the TV still work then? If yes, then that would verify that the low voltage on the 3.4V line is causing the TV to malfunction.

(Of course, if you do try that test, make sure to disconnect/remove jumper J23 and inductor L201 to disable the 3.4V buck regulator circuit on the PSU board from interfering with the 3.3V line of the PC ATX PSU.)

The bottom line though is that both PS are not putting out 3.3V like they should and to me that will cause the TV not to function properly.

Exactly what I was thinking too.

[vinceroger69]

Re: LG LCD power supply

Now, what is puzzling is that first P201 test, where you got 2.05 Volts both for the good and bad boards. Was the TV still working? If the voltages are the same, I don't see why the "bad" board would have caused problems then. Seems the results would be inconsistent... unless you measured or wrote down those test voltages incorrectly. (But I doubt that)

Answer to above yes tv starts up with back lights but no channels as in no tuner voltage to even search for channels no firmware version listed of the main board and no sound so yes board part works even with the low voltages on the good working psu everything works fine will self scan change channels i can read firware version etc sounds great no noticable issues.

[momaka]

Re: LG LCD power supply

I want the reading on the pins of U200 with the forced on resistor and without the forced on resistor to figure out how this U200 being turned on or off by the 3.3_on pin.

Through voltage controll on Vcc pin. See post #137 above.
I can also try to draw and scan a diagram for that tonight if needed.

But we still need to figure out why the 3.3V is so low, it may be due to those two Voltage divider resistors for setting the output Voltage.

Just checked that. The resistive divider formed by SMD resistors R219 and R205 (upper and lower resistors in the divider) appears to be set fine.
With a theoretical 3.4V line, the voltage at the pin 1 of U200 (FB) is:
3.4 x [1000 / (1800 + 1000)] = 3.4 x 0.357 = 1.21 V.
The reference voltage for the IRU3037 IC (U200) is 1.25V.

Thus, the output should be pretty close to that stated 3.4V.

**edit**
The only thing that I see that the designer could have set wrong is that RC network on pin 7 of U200 (the compensation pin) to get such low output voltages with a load.

[budm]

Re: LG LCD power supply

Through voltage controll on Vcc pin. See post #137 above.
I can also try to draw and scan a diagram for that tonight if needed.


Just checked that. The resistive divider formed by SMD resistors R219 and R205 (upper and lower resistors in the divider) appears to be set fine.
With a theoretical 3.4V line, the voltage at the pin 1 of U200 (FB) is:
3.4 x [1000 / (1800 + 1000)] = 3.4 x 0.357 = 1.21 V.
The reference voltage for the IRU3037 IC (U200) is 1.25V.

Thus, the output should be pretty close to that stated 3.4V.

**edit**
The only thing that I see that the designer could have set wrong is that RC network on pin 7 of U200 (the compensation pin) to get such low output voltages with a load.

He should verify that those two resistors have the correct values as printed on top of the resistors and also look to see if the FB Voltage is as expected.
U200 is working at this point just not getting the correct output Voltage.
If he is getting 1.21 V with out put of 2.8V then it has to be bad divider net work. The filter cap for the 3.3V is good for sure?