samsung LE40N87BDX - power supply tranformer

[jimmiscom]

hello. i have the famous samsung clicking on/off problem. no bulging capacitors , no cold joints or any visible problem. i replaced all the capacitors on the cold side, i replaced the eeprom with new one but none of them solve the problem.
im getting steady 5.2 standby volt but because of the clicking problem i could not measure the other voltages so i disconected the mainboard and i jumber the standby pin with ps_on pin . that trick turned on the psu and i managed to measure all the other lines voltages
at line of 12v i was getting about 10.5v
at line of 13v i was geting same.. 10.5v (seems strange to me)
all other lines 5.4v and 5.2v(standby) was ok.
After searching why i was getting low voltage i noticed that from the tranformer (TM801S) i was getting no volts from pin 10 and also was shorting with the ground pins. i desoldered the tranformed and checked again pin 10 and still shorting with the ground pins 8,9 .
Anyone knows if this is normal ? i have attached photo of the transformer and a part of the schematic that shows the tranformer TM801S.
thank you

[R_J]

Re: samsung LE40N87BDX - power supply tranformer

The transformer is fine, you are measuring the dc resistance of the wire in the transformer. What is the BN44- number of the power supply board

[budm]

Re: samsung LE40N87BDX - power supply tranformer

The DC resistance of the transformer windings will be very low, they are normal for DCR to be low, you need ringer to test transformer, I doubt that it is bad. The 12V and the 13v are not regulated, and there is no feedback from those output to be used to regulate those two outputs, the circuit regulate by sensing the 5V output which is used by the main board to run the Logic circuits, the 12v/13V are for audio which is not critical so it doe not need tightly regulated supply. So did you verify that those filter caps for the 12V/13V do not have high ESR and still have correct capacitance.

[jimmiscom]

Re: samsung LE40N87BDX - power supply tranformer

The DC resistance of the transformer windings will be very low, they are normal for DCR to be low, you need ringer to test transformer, I doubt that it is bad. The 12V and the 13v are not regulated, and there is no feedback from those output to be used to regulate those two outputs, the circuit regulate by sensing the 5V output which is used by the main board to run the Logic circuits, the 12v/13V are for audio which is not critical so it doe not need tightly regulated supply. So did you verify that those filter caps for the 12V/13V do not have high ESR and still have correct capacitance.

thank you for info and sorry for late reply.
i thought that maybe was the transfomer but as per your information must be ok.
Since the low voltage problem is difficult to find myself i though to gave up. So i soldered the transformer back to the psu so i can check that everything was the same as before desoldering the tranformer but when i pluged the cable to the tv.... BOOM..!! one resistor 0.3ohm (RM808) exploded. I replaced it but same again ..exploded. i checked the back side of the psu to see if anything else was burned (while the cable was on the psu) and i saw sparks from smd resistor (RM807).
So again your help. can you check the attached schematic (page1) and suggest why the two resistors explode ? (RM808 & smd RM807) .
thanks

[R_J]

Re: samsung LE40N87BDX - power supply tranformer

I suspect, Qm801 will be shorted and UM801s could also be bad, I would also check the diodes and resistors in that section, like rm802, dm801,dm802,zm802 etc.
Did fm802 fuse blow also?

[jimmiscom]

Re: samsung LE40N87BDX - power supply tranformer

I suspect, Qm801 will be shorted and UM801s could also be bad, I would also check the diodes and resistors in that section, like rm802, dm801,dm802,zm802 etc.
Did fm802 fuse blow also?

all fuses included fm802 is ok. i will check Qm801 , section resistors and diodes but any idea how to check um801s ?

[R_J]

Re: samsung LE40N87BDX - power supply tranformer

You can't really check the ic, but if the qm801 is shorted it might have damaged the um801s ic.
(if drain shorts to gate it can damage drive ic)
Also if qm801 shorted (or was turned on) and rm808 opened, that would place a high voltage on rm807 and feed it into the ic, that could explain why it burnt up
If the fet is ok it could be that CM803 shorted, that may be all thats wrong, if it is bad replace it with a 2kv version

[jimmiscom]

Re: samsung LE40N87BDX - power supply tranformer

i think qm801 is shorthed . i desoldered it and test it with multimeter.
both drain and source is shorted when i touced it with red and black probes and same result when i reverse the probes.
Drain + gate are not shorting. do you think that the ic um801s is ok?
and what else you suggest to check.

[R_J]

Re: samsung LE40N87BDX - power supply tranformer

with qm801 shorted, it is possible the ic is ok, but since rm807 is burnt I would not trust the ic.
Also check that cm803 (blue disc) make sure it is not cracked, they are common to go bad on some of these samsung power supplies

[jimmiscom]

Re: samsung LE40N87BDX - power supply tranformer

with qm801 shorted, it is possible the ic is ok, but since rm807 is burnt I would not trust the ic.
Also check that cm803 (blue disc) make sure it is not cracked, they are common to go bad on some of these samsung power supplies

cm803 seems ok.
I dont have time today to desolder the other components on same section to check them, so i will try tommorow.
Do you think that this problem related with the clicking on/off problem that i have? because before i started soldering and desoldering components the psu was just turning on/off, no blowing issues.

[jimmiscom]

Re: samsung LE40N87BDX - power supply tranformer

i cannot find in my local market mofset qm801 (type 6N80) but i have one JCS7HN65C from another chinese brand tv (not remeber the brand)
can you check attached datasheet and tell me if it safe to use it ?

[jimmiscom]

Re: samsung LE40N87BDX - power supply tranformer

hello
can anyone check if i can replace 6N80 mosfet with JCS7HN65C
i cannot find 6N80 at my local store

[jimmiscom]

Re: samsung LE40N87BDX - power supply tranformer

anyway i replaced the shorted mosfet with JCS7HN65C the burnt 0.3ohm resistor with 2 x 1ohm paralel resistor and the burnt RM807 220ohm smd resistor with another 220ohm smaller smd resistor. i pluged again cable but RM807 burnt again. is it possible the ic UM801S to caused the burnt ?

[R_J]

Re: samsung LE40N87BDX - power supply tranformer

Yes it is very possible. You could check to see if CM801 is shorted but my money is on a bad ic. also if RM807 burnt it must have had a lot of current flowing through it, did the fet short again and open the RM808 resistor.
The Isense line of the ic has an internal 10k followed by a diode but the ic could still be bad as it max voltage on that line is 3 volts

[jimmiscom]

Re: samsung LE40N87BDX - power supply tranformer

cm801 is not shorted, i measure about 83ohm. it must be the ic, when i measured pins 3,4,5 seems shorted (0 ohm) but i took the measure while on circuit. i dont have time to desolder it now.
if i desolder the ic , then replace the burnt components (resistors and mosfet) and plug the cable again do you think that any component will explode again ?
and If nothing happens wihout the ic on the circuit do you think that the problem finally is the ic ? what do you think.. can i try it ? or i will make more damage to the board?

[R_J]

Re: samsung LE40N87BDX - power supply tranformer

Pull the ic and check the other components, I would not try it without the ic, without the ic to drive the gate of the fet, the fet could be turned on and that would apply the pfc voltage across rm808, result... Boom

[jimmiscom]

Re: samsung LE40N87BDX - power supply tranformer

i will pull out the ic mayby tomorrow but since i didnt find the ic on my local store i have to order online but it will take long time to have it on my hand.
i will inform you once i get it and replace it.