Samsung T22b300 power supply problem

[sempre]

Hi I'm trying to repair the Samsung lt22b300 power supply. I replaced the srtw6052 chip that was shorted together with the blown fuse and a blown 222 1kv capacitor.

Now if I check the output voltages they all seem ok, but as soon as I connect a load, led or mainboard, all the outputs become oscillating (backlight flashes).

To rule out problems with the LEDs or the main board, I tried to replace the power supply board (PD23A0Q) with a borrowed one from another working TV and everything works regularly.

I checked all the diodes and non-smd capacitors both with the curve trace and by disassembling them and I have not encountered any problems. On the soldering side, I checked the smd diodes in the area of the strw integrated circuit and I also found nothing there. Please I would like to know if someone has solved a similar fault and if you can advise me how to proceed.

Thank you
Edoardo

[dskall]

Re: Samsung T22b300 power supply problem

Did you also check the main cap?

[sempre]

Re: Samsung T22b300 power supply problem

Yes, thanks for the reply, I checked that too.

[R_J]

Re: Samsung T22b300 power supply problem

Replace CM807 and see if that fixes it. Also Remove one end of RM803 (0R35Ω2w) and check DM803 in case it is shorted.

[sempre]

Re: Samsung T22b300 power supply problem

Hello,
Thanks for the reply.
I first replaced rm803, (I used a 0.39ohm) because it seemed slightly cracked, the measured value was still correct.
When I removed the resistor, I could check the diode dm803 (with the curve tracer) and it's ok.
I have powered the board and connected only the leds and there is always the same problem.
Then I replaced the 47uF capacitor next to the strw ic and the 1200uF and 100uF 100V capacitors.
Nothing has changed.
I tried to attach an animated gif but I couldn't (now is attached as a zip file).
Any other ideas on how I can proceed?

Greetings,
Edoardo

[dskall]

Re: Samsung T22b300 power supply problem

What voltage are you getting on main cap?

[sempre]

Re: Samsung T22b300 power supply problem

300v

[dskall]

Re: Samsung T22b300 power supply problem

Does this board have PFC? Without pictures and voltage readings it's hard to help.

[sempre]

Re: Samsung T22b300 power supply problem

Thank you for your reply.
In this board there are mainly 5v and 13v for the main board, and 58v for the leds. When there is nothing connected I find the voltages at the right value. When I connect a load, they oscillate and (the 5v goes down to 0 then back to 5).
I redid the reading on the 100uF 450V capacitor and there are between 295v and 297v. I noticed that these values drop by a couple of volts when I connect the main board and the leds, the reading fluctuates a little.

[dskall]

Re: Samsung T22b300 power supply problem

So you replaced icm901s? You have checked all caps. resistors and diodes in that area? What is your ac voltage into the power board?

[sempre]

Re: Samsung T22b300 power supply problem

I replaced icm901s, the 220pF 1kV capacitor and a fuse, all shorted or blown.
Next I checked and replaced rm803 and two capacitors one on 5V another on 58V.
The TV is powered at 220vac

[lotas]

Re: Samsung T22b300 power supply problem

I redid the reading on the 100uF 450V capacitor and there are between 295v and 297v. I noticed that these values drop by a couple of volts when I connect the main board and the leds, the reading fluctuates a little.

PFC does not work, in operating mode it should be about 400v.

[nomoresonys]

Re: Samsung T22b300 power supply problem

pfc driver bad maybe?

[R_J]

Re: Samsung T22b300 power supply problem

There is no PFC circuit on this board.

[R_J]

Re: Samsung T22b300 power supply problem

Thank you for your reply.
In this board there are mainly 5v and 13v for the main board, and 58v for the leds. When there is nothing connected I find the voltages at the right value. When I connect a load, they oscillate and (the 5v goes down to 0 then back to 5).
I redid the reading on the 100uF 450V capacitor and there are between 295v and 297v. I noticed that these values drop by a couple of volts when I connect the main board and the leds, the reading fluctuates a little.

There are different 5 volt circuits, A5V and B5V. Is the A5V also fluctuating? Also check the 13v supply on DM852 cathode, is it changing?

[lotas]

Re: Samsung T22b300 power supply problem

Here is a piece of the diagram

[sempre]

Re: Samsung T22b300 power supply problem

I checked rm802 (close to the transformer output) and it is open! Tomorrow I try to replace it, I have to look for it on some other board.
Thanks everyone for helping me!

[dskall]

Re: Samsung T22b300 power supply problem

I see no PFC. If you are running 220vac. Voltage seems a little low. I think it should be around 310v

[R_J]

Re: Samsung T22b300 power supply problem

I checked rm802 (close to the transformer output) and it is open! Tomorrow I try to replace it, I have to look for it on some other board.
Thanks everyone for helping me!

That would do it.

[dskall]

Re: Samsung T22b300 power supply problem

I checked rm802 (close to the transformer output) and it is open! Tomorrow I try to replace it, I have to look for it on some other board.
Thanks everyone for helping me!

I think that resistor feeds the vcc to srtw6052