Samsung LE32B350F1W BackLight problem

[HellasTechn1]

Hello everyone !

I am back with a new riddle for us to solve.

Ihave this Samsung LE32B350F1W television that a colleague brought to me to either fix it or scrap it.

This tv set will turn on "can hear the turn on sound" but the backlight will turn on for one secod and then turn off. The tv remains in the on state.

At first i thought that it could be a bad ccfl lamp so i removed the power board and with my ccfl tester i have turned on all four ccfl lamps and turns out they are all good.

The psu is this one BN44-00289A

I measured voltage across the main filter cap and i get 350V dc. That is pretty low as far as i remember. Should be around 380 or more with TV turned on right ?

So could anyone guide me what to do next ?

Could it be a bad PFC chip ? i dont have a replacement for that right now...

[vinceroger69]

Re: Samsung LE32B350F1W BackLight problem

maybe worth a read of this thread as its the same psu and has a link too the schematic also maybe worth you checking the resistance on the invertor transformer winding's
https://www.badcaps.net/forum/showthread.php?t=25196

[dskall]

Re: Samsung LE32B350F1W BackLight problem

There should be feedback resistors on your pfc chip. Make sure they are reading correct

[HellasTechn1]

Re: Samsung LE32B350F1W BackLight problem

maybe worth a read of this thread as its the same psu and has a link too the schematic also maybe worth you checking the resistance on the invertor transformer winding's
https://www.badcaps.net/forum/showthread.php?t=25196

Thank you I will.

There should be feedback resistors on your pfc chip. Make sure they are reading correct

To be honest i have no idea what to measure. Could you please guide me further ?

[budm]

Re: Samsung LE32B350F1W BackLight problem

https://www.badcaps.net/forum/attach...4&d=1356896986
Check the PFC diode DP802 (?) to make sure it is not shorted or has bad leakage resistance.

[vinceroger69]

Re: Samsung LE32B350F1W BackLight problem

just found the specs on the invertor transformer, The secondary resistance is about 1.0K between S1 and S2(S3 and S4). The primary resistance is about 0.3 ohm between P1 and P2.

[HellasTechn1]

Re: Samsung LE32B350F1W BackLight problem

https://www.badcaps.net/forum/attach...4&d=1356896986
Check the PFC diode DP802 (?) to make sure it is not shorted or has bad leakage resistance.

The Dp802 removed from the circuit (by removing the jumper smd's) on my DMM set to diode test, reads 0 reverse biased and forward biased would slowly rise up to 0.513 - 0.515.

I also get strange readings from ZDI802.With DMM set to diode test, when on circuit it will read 0.032 both forward and reverse biased.
When removed it reads 0.692 forward biased and 0 reverse biased
for the zener by forward biase i mean (negative off dmm to stripe on body and positive to other side). when removed i read no short accross its pads.
P.S. i am going to replace it anyway since i happen to have one and i will see what happens. Though i suspect it is the rest of the circuit that produces the weird readings.

[HellasTechn1]

Re: Samsung LE32B350F1W BackLight problem

I replaced DP802 and the problem is still there.
I noticed thogh that Voltage across the main filter capacitor now when TV is on, is 383Vdc
and when off, it goes down to 310.

I will report the otuput readings of the PSU because i think they are slightly low.

CN802
PIN No.-------DESCRIPTION--------------DMM READING
16.----------------GND
15.----------------GND
14.--------------DET_5V-----------------------3,01V
13.---------------E-PWM-----------------------5,05V
12.----------INVERTER ON/OFF--------------12,95V
11.----------------13V------------------------12,95V
10.----------------13V------------------------12,95V
9.-----------------GND
8.------------------5,3V------------------------5,17V
7.------------------5,3V------------------------5,17V
6.------------------GND
5.------------------GND
4.------------------13V------------------------12,95V
3.------------------GND
2.------------------5,3V-------------------------5,17V
1.-----------POWER ON/OFF-------------------2,15V

So DET_5V is way out and 5.3V also not good. I am not sure about power on/off pin, i think that should be 3.3

[vinceroger69]

Re: Samsung LE32B350F1W BackLight problem

disconnect CN802 that goes to the main board (this will force the PS_ON and BL_ON line to go HI, if the board is OK, the backlights should turns on when you plug the TV into the AC outlet.

[HellasTechn1]

Re: Samsung LE32B350F1W BackLight problem

Good advice, Though i have already tried that and with mainboard disconnected, backlight comes on for a second and goes off again. Same symptom. That indicates a bad PSU.

[vinceroger69]

Re: Samsung LE32B350F1W BackLight problem

did you measure the invertor transformer windings?

[HellasTechn1]

Re: Samsung LE32B350F1W BackLight problem

Yes, i forgot to mention that. 1kohm each secondary winding and 0.something on primary windings of both transformers. So they are good.
To be honest i think a bad transformer is rarely the case.

[HellasTechn1]

Re: Samsung LE32B350F1W BackLight problem

I think that the low voltage on DET_5V suggest problem with inverter or lamps... Thats probably why lamps go off but again how can i say for sure... Is there a way to test the CCFL controller chip ?

[vinceroger69]

Re: Samsung LE32B350F1W BackLight problem

if you read that other thread i linked im sure it tells you how too test the controller ic or disable the det line

[budm]

Re: Samsung LE32B350F1W BackLight problem

DET_5V should be <1V when TV is in standby and it should be >2.5V when the inverter circuit is running.

[budm]

Re: Samsung LE32B350F1W BackLight problem

I replaced DP802 and the problem is still there.
I noticed thogh that Voltage across the main filter capacitor now when TV is on, is 383Vdc
and when off, it goes down to 310.

I will report the otuput readings of the PSU because i think they are slightly low.

CN802
PIN No.-------DESCRIPTION--------------DMM READING
16.----------------GND
15.----------------GND
14.--------------DET_5V-----------------------3,01V
13.---------------E-PWM-----------------------5,05V
12.----------INVERTER ON/OFF--------------12,95V
11.----------------13V------------------------12,95V
10.----------------13V------------------------12,95V
9.-----------------GND
8.------------------5,3V------------------------5,17V
7.------------------5,3V------------------------5,17V
6.------------------GND
5.------------------GND
4.------------------13V------------------------12,95V
3.------------------GND
2.------------------5,3V-------------------------5,17V
1.-----------POWER ON/OFF-------------------2,15V

So DET_5V is way out and 5.3V also not good. I am not sure about power on/off pin, i think that should be 3.3

So now the PFC Voltage is correct. Does it stay at that level when the backlights come on?
Det_5V is just the error signal, LOW = <1V, HIGH = > 2.5V.
That 12.95VDC control signal cannot be right, it cannot be that high.

[HellasTechn1]

Re: Samsung LE32B350F1W BackLight problem

I am performing new measurements as we speak. posting back in a minute.

[HellasTechn1]

Re: Samsung LE32B350F1W BackLight problem

I wrote down the readings wrong....

DET_5V = -0.009V
E-PWM = 3.01V
INVERTER ON/OFF = 5V

My mistake was that i counted down from after pin 16 but i forgot that the first gnd pin was the one i had the test lead on ! how dum i am...

I can confirm that the main filter cap has a stable 383Vdc before and after ccfl shutdown.

also i have tested all diodes i could find and found none shorted. I have tested most smt diodes also and are good also. the tl431 vregs are also good.

[HellasTechn1]

Re: Samsung LE32B350F1W BackLight problem

Now this is interesting !

i was looking at the OZ9938 datasheet and i thought to myself, why dont i try to disable protection just to see if the ccfl's will keep on !
So i shorted pin 3 between the resistor and the capacitor to pin 16 and it worked.
The ccfl's winn not go off.

That confuses me further because the lamps are good, i have tested them one by one with a ccfl tester and they are all glowing.

[HellasTechn1]

Re: Samsung LE32B350F1W BackLight problem

I have tested the ccfl's again one by one and found out that one of them is running a little noisy... Yesterday when i tested them the room was not that quiet so i could not hear it. I will dissasemble the panel later and replace it to see what happens.