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Old 04-14-2012, 10:31 PM   #41
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Default Re: LG L1953S doesn't power on

The easiest will be using small grain of wheat 6v, and 12v lamps connected to the outputs.
http://www.radioshack.com/product/in...ductId=2102814

http://www.radioshack.com/product/in...ductId=2102815

These are good when work on LCD monitor power supply.
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Old 04-15-2012, 11:57 AM   #42
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Default Re: LG L1953S doesn't power on

You want to me use something like that to watch the voltage on outputs without a multimeter or for adding a load and see if it has the same behavior?
I have just notice that the +12V output drops at the same rate regardless I keep it plug or not..I mean if I keep it pluged or I plug it for half a second and unpluged it then it makes no difference.

P.S It takes about 6 minutes to drop from 14V to 1.5. It seems to me like a capacitor discharging without any load at all..
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Old 04-15-2012, 02:31 PM   #43
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Default Re: LG L1953S doesn't power on

We are trying to find out why you do not have the 5v output as you have indicated, but you do have 12v, but since the5 and 12 are generated from the same transformer tehn the only thing that can be due to fault circuits in the 5v rectifier/filter section, open winding for the 5v, etc, so with the lamp, you should be able to see if the 6v lamp will light up for seconds also.
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Old 04-15-2012, 02:52 PM   #44
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Default Re: LG L1953S doesn't power on

How about checking near the transformer - should voltage at D203 be 5v?
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Old 04-15-2012, 02:57 PM   #45
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Default Re: LG L1953S doesn't power on

Since I don't have incandescent lamps I will connect leds with suitable resistors and right polarity..
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Old 04-15-2012, 05:00 PM   #46
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Default Re: LG L1953S doesn't power on

LED will do, another place to monitor to see if the running power supply section that keeps the SMPS IC running by supplying the voltage to VCC pin 7 of the IC, I want you to put LED with 1k resistor in series and attach this to pin 7 of the SMPS IC or across C103, This LED should blink on also, if it does not, then that may explain why the SMPS does not stay running.
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Old 04-16-2012, 11:53 AM   #47
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Default Re: LG L1953S doesn't power on

Well, LED trick is really neat..
On the Vcc pin of controller I have constant voltage, no pulses or anything, led lit steadily..
On +12V I have voltage that slowly drops as measured with multimeter but:
On 5V I have I pulse for half a second or less and then nothing.What is more strange is that if voltage on 12V output doesn't drop to 0 it is exactly the same if I have pluged it or not.I mean the only way to get the 12V output again is to wait for it to drop to 0 and then plug it again or to short circuit output( I quess that discharges capacitors) and plug it again..

Last edited by Joun; 04-16-2012 at 12:25 PM..
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Old 04-16-2012, 12:35 PM   #48
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Default Re: LG L1953S doesn't power on

"On the Vcc pin of controller I have constant voltage, no pulses or anything, led lit steady.." So it stays lit and never goes away, but the 12v and 5v are not working, correct?

Now that does not any sense at all. There are 4 windings on that transformer:
Pin 5&3, the primary drive winding.
Pin 1&2, the SMPS running supply for the VCC once the SMPS IC kick started on pin1.
Pin 6 &8 is for 12v, the winding is in series with the winding 7&10 which is for the 5v power supply.
Take the continuity reading of 6-8, 6-7, 6-9, 6-10.
Since you do have VCC that means the primary drive circuits are working to ptoduce AC voltage on pin 1&2 which feeds R111 FUSIBLE RESISTOR, and then the Rectifier Didoe D102, the output on the Cathode side is then filtered by C103 (33UF/50V).
Just think of it this way Pin 1 of the IC is like a batteries in your car, you need it to start the car, once the car is running, the Alternator (WINDING 1&2 is like the alternator) will take over and run the car (on pin 8), the batteries then can be diconnected (whick it does automatically inside the IC).
Check D202/D203 to see if it is OK.

Last edited by budm; 04-16-2012 at 12:37 PM..
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Old 04-16-2012, 01:32 PM   #49
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Default Re: LG L1953S doesn't power on

I think you didn't observe how the windings on secondary go..There is one winding on 6-8 and another on 7-10. The 6-8 winding is for 12Volts output and the 7-10 is for 5volts.I don't see them in series..(am I missing something? ) I can't see how is possible to have a reading on 6-9 and 6-10 since they are on different windings..
The windings are measuring OK, nothing is open. I have check multiple times D202/D203 and every other semiconductor on PCB,they are fine..Fusible resistor R111 is OK..

Last edited by Joun; 04-16-2012 at 01:37 PM..
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Old 04-16-2012, 01:38 PM   #50
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Default Re: LG L1953S doesn't power on

Well what voltages do you get on the transformer 6-8 and 7-10 ?
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Old 04-16-2012, 01:49 PM   #51
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Default Re: LG L1953S doesn't power on

Pin 7 and 8 are connected together a the junction of R201 (left leg on the SCH), Anode of D202, D203. You have to magnify the the drwaing to see the connecting dot of the junction. It is very common practice to do one winding and tap off the section for the voltages you need.
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Old 04-16-2012, 02:04 PM   #52
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Default Re: LG L1953S doesn't power on

Of course you are right..It was a very small dot and didn't see it.
Quote:
Originally Posted by budm View Post
It is very common practice to do one winding and tap off the section for the voltages you need.
I know that well I am an electrical engineer.. Well all windings aren't open so can I check anything else?
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Old 04-16-2012, 02:13 PM   #53
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Default Re: LG L1953S doesn't power on

Do you have Oscilloscope? The whole thing still does not make sense to me yet since the VCC power supply is working unless there is internal leakage between pin 1 and pin 7 inside the IC so the current is flowing through R108 and comes out on pin7. You can try lifiting one leg of R111 and see if the LED will still stay lit.
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Old 04-16-2012, 02:28 PM   #54
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Default Re: LG L1953S doesn't power on

No, I don't have a scope,maybe it is time to get one..I have some other instrument a have to buy before that(electrical installation tester ) for my "day job"..
Because everything else seem to be OK,is a faulty ic the last alternative?

Well I understand nothing..I have just did what you suggested and remove one leg of r111..
To my BIG surprise, I have constant 12.8Volts output and at 5 volts I measure 4.5 volts with multimeter but I see the leds pulsing with 3-4Hz..
Well I'm waiting for suggestions..I can't work on that now ( I have to go to sleep ) but for the first time I'm getting some positive resuls.Thank you very much..

Last edited by Joun; 04-16-2012 at 02:42 PM..
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Old 04-16-2012, 02:43 PM   #55
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Default Re: LG L1953S doesn't power on

I edit my post as well after reading your edited post, now time for me to think what is happening, so do you have voltage at the VCC pin of the IC, if you do, then there is internal leakage inside the IC (failed electronic MOSFET switch between pin 1 and pin 7) and now it is trying to run off pin 1 supply by R108 (47K) but not enough current to run it properly.
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Last edited by budm; 04-16-2012 at 03:07 PM..
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Old 04-16-2012, 02:44 PM   #56
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Default Re: LG L1953S doesn't power on

I have edit my post..
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Old 04-16-2012, 03:15 PM   #57
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Default Re: LG L1953S doesn't power on

Yes the voltage on Vcc is still 9.8 volts..
So? Is it a faulty ic after all?
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Old 04-16-2012, 03:34 PM   #58
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Default Re: LG L1953S doesn't power on

Yes.
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Old 04-16-2012, 03:39 PM   #59
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Default Re: LG L1953S doesn't power on

OK, I will order one and post results..
Thank you all for your time and patience.
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Old 04-23-2012, 08:51 AM   #60
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Default Re: LG L1953S doesn't power on

Hi i have problem with exactly the same Power board , only i don't have any output voltage nether 5 nor 12v. But R108 after plugging in keeps burning out slowly. I'm running out of 47k om resistors. What could be the problem , im guessing some short circuit but i cant find it. Supply voltage correct 240v. big cap ok. No power on secondary circuit.
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