Samsung LA32B350F no backlight

[beechman35]

Hi Guys,
First post so be gentle with me. Samsung 32inch LCD with no backlighting. Power supply board is a BN44-00289B.

Plugging in AC supply, the stby light is lit solid. Power on the TV and Stby light flashes 5 times and then extinguishes with audio welcome tone from speakers. Watched the screen in a darkened room and no flashes from CCFL's. Used the flashlight trick and can see menu when selected. LCD panel appears good with no cracks visible.

Measuring the voltages on the power supply connector I see the following:

Description TV in stby TV power on
PWR on/off 0.01 0.766
STBY 5.3 5.16 5.16
STBY 13 13.1 12.9
5.3 5.16 5.16
13 M 13.1 12.9
Inv on/off 0.02 5.12
E-pwm 0.0 2.87
DET_5 0.0 0.16
Sync 0.0 0.0

Volts on Main cap 331.0 326.0

No bulging caps noticeable on PS board. Diodes checked in circuit to look for shorts as per previous threads and nothing noted. DP 805 part number checked and it is a 31GFL6L. Pulled the diode and tested and it appears no leakage.
Any thoughts and direction would be greatly appreciated.

Regards

Rob S

[budm]

Re: Samsung LA32B350F no backlight

"PWR on/off 0.01 0.766" This one does not look right.
Check the DC Voltage right at the TWO LEGS of that yellow main filter cap to see what you have, it should be >360VDC.
Do not touch any thing with your hands or body, you are working in the primary hot deadly side of the circuits.

[CapLeaker]

Re: Samsung LA32B350F no backlight

And a nice pic of the main board please.

[beechman35]

Re: Samsung LA32B350F no backlight

Back again,

Voltage on the yellow cap (listed as main cap in first post) 331 VDC standby mode and 326 in power on mode. Confirmed the power on/off switches to 0.766 measured at pin 1 of plug CN802 with all cables connected.

Pic of main board attached.

Regards
Rob

[budm]

Re: Samsung LA32B350F no backlight

OK, that verify the board is not getting high enough PS_ON (PWR ON/OFF)signal (>2.5VDC) to turn on the PFC Voltage booster.
So what we can do is first remove the PS_ON wire from the connector, then use 100~1K resistor 1/4W between the PS_ON pin and the STBY 5.3V, you can solder the resistor in place to try it out, then plug the TV into the outlets, if the inverter is OK, you should see backlights come on, and if other section of the main board and the T-CON boards are OK then you should see picture on the screen.

[beechman35]

Re: Samsung LA32B350F no backlight

Ok, disconnected the wire from pin 1 of CN802 and worked out which jumpers on the power supply board are connected to 5v stby and pwr on/off so I could monitor all voltages. Supplied 240VAC and measured open wire from main board and no volts. Stby pwr at 5.3. Measured link wire that feeds pwr on/off to power supply, no volts. Turn TV on with remote and get power on sound and then measured voltage at wire coming from main board for pwr on/off and measure 4.88 volt(on/off wire disconnected). Measure link wire on power supply board without the 1k resistor connected and get 0.76 (same voltage as with wiring connected). Connect 1 k resistor from 5.3 Stby pwr to pwr on/off link and nothing changes (ie nill ops of back lighting), however, measured the voltage now applied to the pwr on/off line being fed from the 5.3 V stby power via the 1 k resistor and only measure 1.55V, still too low to fire up the power supply obviously.

Something is pulling the pwr on/off line low in my mind.

Any thoughts?

Rob

[budm]

Re: Samsung LA32B350F no backlight

All you have to do is to connect the resistor bewteen 5VSTBY PIN and the PWR ON/OFF PIN on the power supply board without having the wire going back to the main board because we do not want to back feed this force on Voltage back to the main board, plug the TV, and check the DC Voltage on the main filter cap to see if you get >360VDC right at the two legs of the main filter cap, that is what I am looking for (PFC boosted voltage).

[gajds]

Re: Samsung LA32B350F no backlight

hello, sorry for joining this post, i just need help and i cant post a topic.

It is about our samsung led tv series 4 (UA32d4000n),

At first when the tv is turned on (samsung logo is blue) and tv is functioning well, after few minutes it is turning red.
Then i re-plug again samsung logo just turned into red and also the pictures of the tv.

I tried already the factory reset but it only last for few minutes then turned again to red.

Does the service reset is the same with the factory reset?

Thank You

[beechman35]

Re: Samsung LA32B350F no backlight

Hi Budm
Connected resistor as you describe with the wire to the main board disconnected. Volts at the main filter cap are 331 VDC.

[budm]

Re: Samsung LA32B350F no backlight

OK, the PFC Voltage booster circuit is not working, the backlights inverter circuit requires that Voltage to work.
Check those 3 big diodes (black diodes with white band at one end) close to the main filter cap.

[beechman35]

Re: Samsung LA32B350F no backlight

Hi Bud, Diodes checked and look good as diode check in circuit. we did pull DP805 and check out of circuit for leakage and appears good. Also confirming that the diode is the 31GF6, not the MUR360.

[mykeford]

Re: Samsung LA32B350F no backlight

Hello all, i am new to this community and i am not skilled in repair of computer or lcd television repair. i was given a samsung ln32b360c5dxza because it had sound but no picture. i confirmed that there is no picture, only sound. could anybody give me a starting point for troubleshooting this issue.

[budm]

Re: Samsung LA32B350F no backlight

@mykeford; Please start a new thread with the make and model and provide pictures.
Your samsung ln32b360c5dxza is ot applied to this thread.
Also search the thread using your model numbers to see if there are other tread with the same model as yours.

[mykeford]

Re: Samsung LA32B350F no backlight

@Budm: Thank you Budm, i will do as you suggested . at this point i am unable to start a new thread, (site rules) as soon as i am able then i will. thank you for your time.
myke

[budm]

Re: Samsung LA32B350F no backlight

Hopefully you will be able to start a new thread so we can see the pictures of your boards and go from there as to where the Voltage test points will be.

[budm]

Re: Samsung LA32B350F no backlight

@beechman35, Please check the Voltage at the Emitter and Primary circuit GND of QB801, at the Base and Primary circuit GND. You may have leaky ZB801 or bad PFC IC controller.

[beechman35]

Re: Samsung LA32B350F no backlight

Hi Bud, First off, let me thank you for your efforts on this. As we all know sometimes it just might not be worth fixing something on a commercial basis, but now I just want to know what the hell is wrong for my own personal benefit. OK, the results. PS board installed, all wiring intact. turn on TV, get the welcome audio and then measure the voltages. Collector of QB801 has 18.2 volts, base has 0.5 volts and emitter has0.6 volts. PS board removed and Zb801 checked in circuit and no apparent leakage. Removed QB801, tests normal so refitted. Removed RP823 (zero ohm) resister and powered board up again, no change in voltage at emitter.
Looks to me to be around the dual op amp UB802 if its output to QB802 is telling it to limit base voltage to QB801.

Can you briefly describe the purpose/ops of this circuit, I am assuming its looking at output of the PFC and regulating same?

Thanks again

Regards

Rob

[budm]

Re: Samsung LA32B350F no backlight

OK, I need more Voltage and resistance testing. Right now the QB801 is not turned to supply the Voltage to run PFC IC or the dual op-amp UB802 (and latching circuits (QB802, 803) which monitors the PFC output Voltage to see if it goes too high and also detect UV (Under Voltage) INVERTER circuit.
But since there is no Voltage out put of the 15V linear regulator to run the UB802 is not functioning until you get 15 at the Emitter of QB801.

[beechman35]

Re: Samsung LA32B350F no backlight

Hi Bud,
Voltages checked and as per attached PDF. Checked the resistor values and they look OK. The Zener appears ok as an in circuit test and if nothing else comes to mind, I will pull it and check the voltage drop.

[budm]

Re: Samsung LA32B350F no backlight

Change Z801 15V Zener. Testing this Zener with DMM may not be valid, If you have Variable power supply and resistor, then you can use that to Verify for sure that the Zener is OK.
Do you have ANY zener diode around?