Samsung UE32F4510 Red led blinking

[vinceroger69]

Measuring the voltage on the 3 large diodes (left side of transformer) I get 5.2V and 17v. It seems that the power on "fails"! Could it be a protection cancelling the power on procedure?

failed leds might cause protection circuit too kick in (did you have leds disconected when you jumped the two pins to test psu)

[electrocuted]

No, I had the leds connected. Let me try again and see.

[yokoono]

https://www.youtube.com/watch?v=azUHkApvQUI
https://www.youtube.com/watch?v=S_ZHOHUzxwg

[electrocuted]

https://www.youtube.com/watch?v=azUHkApvQUI
https://www.youtube.com/watch?v=S_ZHOHUzxwg

Thanks, I've seen the video but this is not my case! Diodes test ok and they have voltage on their cathode. Tried to turn on power supply with leds disconnected, same zero voltages accross connector.

[Davi.p]

check the voltage on every axial diode on secondary, there will be, i guess, 5,2v, 12v, 17v , then check the path of the power on signal, first a resistor, then probably a prefinal transistor, then those power mosfet smd solder side.. eventually remove old solder and put new..

[vinceroger69]

This service manual might help.

[electrocuted]

As far as I see for the first two transistors it seems ok. To change the voltage on these, power on input needs connection to ground (0V) and not A5V.

This service manual might help.

This is quite similar but not the same model. I can't find the exact same diagram for the power supply. This is the closest one I got:

[electrocuted]

It's difficult without diagram but I see two power mosfets (D4186) to switch on B5V and B13V. The B13V output is used to turn on B5V but both fets gates are 0 Volts! Inputs on these read 5.2V and 24V(!). When I connect a 24V led bulb the voltage drops to 15.5 and the bulb lights pretty low. Then when grounding ps_on it goes even more dim! I will continue tracking...

[Davi.p]

Put an 1A approx. load to the A5V and see if it drops.. if yes replace the shunt regulator 431 similar.. check also the 400v big cap..

[electrocuted]

Put an 1A approx. load to the A5V and see if it drops.. if yes replace the shunt regulator 431 similar.. check also the 400v big cap..

Tried it and voltage stays solid 5.2V. The strange thing is when I put the load, the voltages B5V and B12V are there on the connector! (ps_on unconnected) In fact the more amperage on the A5V line, the more voltage on B12V.
A5V B12V

0.5A 7V
>1A 13V

[Davi.p]

so this means the feedback circuit is normally connected only to 5v line and you can't load only the 12v..

I see two power mosfets (D4186)

this is wrong name, D stays for diode, i see 2 power diodes coming in to power enabling mosfet QM853 (smd) , check it if good and follow its path to the connector..

[electrocuted]

Sure, that's correct but how did the ps turn on without using the ps_on line?

[Davi.p]

in Samsung led tv there is an internal pullup resistor that keeps high the ps-on with no main board, in your last photo the qm853, its gate connects to the bottom network, must check every resistor/diode/transistor for short/open, must also check ps-on V at unconnected PSB.

[electrocuted]

I tried with ps_on to ground with same result. Seems it has no effect on the operation of the power supply, only a load on A5V "can" switch it on. I will recheck nearby items around QM853. ps_on unconnected measures 3.3V.

[Davi.p]

if ps-on is high, qm853 gate must be low, i guess is P channel, or not? before any other test you can jumper D-S of mosfet and plug everything, then plug tv to mains and see if it works..

[electrocuted]

N-Channel MOSFET - AOD4186

[electrocuted]

I'll try that tonight because I have removed the power board from tv. If the led array is defective will it affect the inverter circuit?

[electrocuted]

Done, the backlight comes on with the cable to the main board disconnected! If it is connected nothing happens. A strange high pitch noise is heard from power supply...

[nomoresonys]

Make sure all solder connections are good on bottom of board, search for bad solder joint images if needed.

[nomoresonys]

Try to isolate the location of noise, maybe try a straw or rubber hose.