Samsung UH50EH5300FXZA, help with a transformer?

[budm]

Re: Samsung UH50EH5300FXZA, help with a transformer?

"Is it really OK to ground the PS_on pin with the voltage being low and unstable?" Yes, that will stop the rest of the switched power supplies to come on so only the standby voltage will be present.

[DXseekerMO]

Re: Samsung UH50EH5300FXZA, help with a transformer?

OK, that is good. The circuit is switching but it is not switching right.

It's ok, So what is the voltage you get on CM853, you might want to check Cm809 and Cm814
In standby the voltage on CM853 could be around 8 volts

I was just thinking that without the main connected, the power supply might be trying to turn itself on due to pull up resistors on the PS_ON pin. You could ground the PS_ON pin and see if A5V is stable

I see you have 160v on the main filter so the pfc is off at this time

Caps CM809 and CM814 have proper capacitance out of circuit.

On CM853 there is 3.5 to 4.2 volts, alternating from lower to higher about every couple seconds. The rhythmic fashion of the voltage change reminds me of something trying to start.

Is it really OK to ground the PS_on pin with the voltage being low and unstable?

[R_J]

Re: Samsung UH50EH5300FXZA, help with a transformer?

It's ok, So what is the voltage you get on CM853, you might want to check Cm809 and Cm814
In standby the voltage on CM853 could be around 8 volts

I was just thinking that without the main connected, the power supply might be trying to turn itself on due to pull up resistors on the PS_ON pin. You could ground the PS_ON pin and see if A5V is stable

I see you have 160v on the main filter so the pfc is off at this time

[budm]

Re: Samsung UH50EH5300FXZA, help with a transformer?

OK, that is good. The circuit is switching but it is not switching right.

[DXseekerMO]

Re: Samsung UH50EH5300FXZA, help with a transformer?

0.027uF 5% Polypropylene type. It is commonly failed cap in that section of the circuit.
SIC cap looks like 598B series.

Capacitance checks at 0.0264uF

[R_J]

Re: Samsung UH50EH5300FXZA, help with a transformer?

That FA5760N will be operating in burst mode (standby) so the only voltage that should be close would be the A5V, the voltage off the transformer will be low likely around 8 volts

[budm]

Re: Samsung UH50EH5300FXZA, help with a transformer?

0.027uF 5% Polypropylene type. It is commonly failed cap in that section of the circuit.
SIC cap looks like 598B series.

[DXseekerMO]

Re: Samsung UH50EH5300FXZA, help with a transformer?

How about the capacitance of that coupling/DC blocking cap CM803s?

I'll have to get it off of the board and check it. Do you know what the prescribed capacitance is for that cap? 0.027uF?

[budm]

Re: Samsung UH50EH5300FXZA, help with a transformer?

How about the capacitance of that coupling/DC blocking cap CM803s?

[DXseekerMO]

Re: Samsung UH50EH5300FXZA, help with a transformer?

I don't think so, Why? If the ic was bad I don't think you would be getting any voltage at all on the 5VA standby, Did you check the parts on the gate of the shorted fet?

Checked, here's what I found:

CM803 has about 16Kohms in one direction and open in the other. I didn't see a bulge on the case anywhere.

All items off the gates of QM801CS and QM802CS appear to be of the appropriate resistance and not shorted, save for resistance across CM810 which seems low at 101ohms?

[R_J]

Re: Samsung UH50EH5300FXZA, help with a transformer?

I don't think so, Why? If the ic was bad I don't think you would be getting any voltage at all on the 5VA standby, Did you check the parts on the gate of the shorted fet?

[DXseekerMO]

Re: Samsung UH50EH5300FXZA, help with a transformer?

Is there a solder pad on the bottom of FE5760? I can't read Japanese so I can't tell by the datasheet.

[DXseekerMO]

Re: Samsung UH50EH5300FXZA, help with a transformer?

No, its likely ok, it should be close to 0Ω

Cool.

[R_J]

Re: Samsung UH50EH5300FXZA, help with a transformer?

No, its likely ok, it should be close to 0Ω

[DXseekerMO]

Re: Samsung UH50EH5300FXZA, help with a transformer?

There is a resistor, RP821 off the (-) of the bridge (between LP802CS coil and JP811) check that it is ok

Indicates zero resistance on ohms scale. Looks like this one has to come out of circuit for an accurate assessment?

[DXseekerMO]

Re: Samsung UH50EH5300FXZA, help with a transformer?

If QM801CS and QM802CS were shorted, I would check the resistors/diodes on there gates, when those fets short they can blow the gate driver parts also.
I would also check that white square cap CM803s they like to go open (usually they bulge)
It does look like it is trying to work if you have some voltage on the A5V

"I would also check that white square cap CM803s they like to go open (usually they bulge)"
+1
The GATE drive outputs can also be damaged when MOSFET shorted out.

CM803 has about 16Kohms in one direction and open in the other. I didn't see a bulge on the case anywhere.

All items off the gates of QM801CS and QM802CS appear to be of the appropriate resistance and not shorted, save for resistance across CM810 which seems low at 101ohms?

[R_J]

Re: Samsung UH50EH5300FXZA, help with a transformer?

There is a resistor, RP821 off the (-) of the bridge (between LP802CS coil and JP811) check that it is ok

[DXseekerMO]

Re: Samsung UH50EH5300FXZA, help with a transformer?

So right now of you run the power supply board by itself, is the A5V steady?
Your report does not make any sense:
second is unplugged from the main board.
12 A5V……1.8 to 4.4 unstable…….. unknown/0v
BTW, that 16-IC is the SMPS IC, the LED driver circuit (boost converter circuits are in the cold side, not in the hot side).
The power transformer is being driven otherwise you will not have any Voltage on the cold side, but it is not stable right now.
BTW, I still cannot see the location of the so called sparks on the board, and that board also has many through hole rivets which are known for cold solder joints.

"I installed MOSFETS QM801CS and QM802CS" were any of them tested as shorted? To blow 5A main fuse, something was drawing lots of current.

At this time A5V is low and not steady, from .9v up to 3.3v and swings wildly.

I explained in my update that the voltages on the first row are with CNM803 hooked up to the main board and the second row is with connector to main board at CNM803 unplugged. By the time I got around to checking for voltage at the even numbered pins for CNM803 all the voltage had disappeared at the connector.

The sparks from the board you are referring to were seen during the initial diagnostic. I piggybacked a fuse on to the blown main fuse and plugged it in. The sparks were seen inside of transformer LP802CS until the main fuse popped again.

MOSFET QM802CS was short. If I remember right QM801CS was OK but I replaced it anyhow.

[budm]

Re: Samsung UH50EH5300FXZA, help with a transformer?

"I would also check that white square cap CM803s they like to go open (usually they bulge)"
+1
The GATE drive outputs can also be damaged when MOSFET shorted out.

[R_J]

Re: Samsung UH50EH5300FXZA, help with a transformer?

If QM801CS and QM802CS were shorted, I would check the resistors/diodes on there gates, when those fets short they can blow the gate driver parts also.
I would also check that white square cap CM803s they like to go open (usually they bulge)
It does look like it is trying to work if you have some voltage on the A5V