Samsung PS-42V4S 42" Plasma TV

[Mad Professor]

Hi All.

I would still class myself as a novice when it comes to component level fault finding and repair.

I am trying to repair a Samsung PS-42V4S 42" Plasma TV.

When mains power is applied the standby led comes on, when you try to power on the tv (come out of standby) the standby led flashes, but the display does not light up.

My first port of call was to check all the glass fuses on the PCB, as well as the TH fuses, all are fine.
My second port of call was to start checking all the output voltages.

None of the output voltages from the SMPS had any voltage, so I started working my way back.
I have 340vdc at the switching mosfet for the smps, but it appears that there is no signal to the gate of the mosfet.

I have traced back as far as IC1 the power factor correction ic, and I am not getting the kind of voltages that I would be expecting on some of the pins, and no voltage at all on pins 2&4.

I am thinking that this IC has failed, or that something else is making this IC act funny.

The IC in question is a ML4824 - Power Factor Correction and PWM Controller Combo. <Click for Datasheet.



Thanks for your time.

Best Regards.

[wraper]

I'm pretty sure you are not giving it a "power on" signal and expect something out of it at the same time. And if it does not run and there are no blown fuses/mosfets or shorted output, you should look at electrolytic capacitors.

[Armadillo]

Check R9 and R10.

[wraper]

Check R9 and R10.

He said there is 340V at mosfet, therefore those are fine. and I think you mean R8 and R10.

[Armadillo]

Check R9 and R10.

He said there is 340V at mosfet, therefore those are fine. and I think you mean R8 and R10.

Yes I am suspecting the ground is broken, but I think we are not at all certain where he used as ground for the measurement of the mosfet voltage you said before or after the resistors in question, so I ought not to assume it especially when he said no voltage at pin 2 and 4.

[wraper]

Pins 2 and 4 are not power pins, therefore nothing extraordinary. Space between rectifiers and those resistors is very small, so it's very unlikely to use that as reference in measurment. Moreover, there won't be 340V as capacitors won't have any way to charge. But otherwise it's impossible to measure 340V, voltage would be much lower.

[Armadillo]

Pins 2 and 4 are not power pins, therefore nothing extraordinary. Space between rectifiers and those resistors is very small, so it's very unlikely to use that as reference in measurment. Moreover, there won't be 340V as capacitors won't have any way to charge. But otherwise it's impossible to measure 340V, voltage would be much lower.

But I understand your point, but I am not assuming how he had measured it.
Other than that, would you account why Pin 2 and Pin 4 did not show voltages?

[Armadillo]

Now I have the time to look into it a bit more.

Actually the switching power supply does not need the power factor controller to function, its sole purpose is to improve the Power Factor by timing boosting the voltage.
The reason why the supply didn't switch and there were no pyrotechnic, I reckon must be the "Designed Weak Link" is broken.

If that is being true, even if the Weak link is restored, do not just turn it on without variac or light bulb in series.

[wraper]

It's not a weak link. Don't you see that it's 0.05 ohm 10W in total? It's for current sensing. And there is a fuse which will blow much faster. Max continuous current for those resistors is 14A and will survive 10x higher higher in peak.

[Armadillo]

It's not a weak link. Don't you see that it's 0.05 ohm 10W in total? It's for current sensing. And there is a fuse which will blow much faster. Max continuous current for those resistors is 14A and will survive 10x higher higher in peak.

You can't straight 1 + 1. These things run at elevated temperature and need to be derated. That's must be why its 2, adding surface areas. Have you not seen it failed before?

Can you answer the question on Pin 2 and Pin 4 ? or Jump?

 

Edit: Aside from temperature derating, designers will also include wire brittle ageing derating.

Edit: Not saying that the OP problem is this, but here some photo showing these things do burn out.

[wraper]

It's not a weak link. Don't you see that it's 0.05 ohm 10W in total? It's for current sensing. And there is a fuse which will blow much faster. Max continuous current for those resistors is 14A and will survive 10x higher higher in peak.

You can't straight 1 + 1.

These things run at elevated temperature and need to be derated. That's must be why its 2, adding surface areas. Have you not seen it failed before?

Don't confuse 50 mohm current sense resistors with inrush current limiting resistors of significantly higher resistance. And I already said why they cannot be faulty given measurments made.

[Armadillo]

Don't confuse 50 mohm current sense resistors with inrush current limiting resistors of significantly higher resistance. And I already said why they cannot be faulty given measurments made.

hahahahah LOL, you should focus on the OP subject why no voltage at all for Pin 2 and Pin 4 because your diagnosis solution to bridge the opto does not fit the OP experience and measurement explanation.   
And I don't think you have the answer Right.   

[wraper]

The question is what OP considers as no voltage. Pin 2 is current input and voltage normally won't exceed 0.6V, don't know if OP wrote about pin 4 correctly. But on pin 3 it's guaranteed to be 0V when PSU is not running under load. Without optocoupler being on/shorted, there is no chance this PSU running. No voltage on pin 4 can be explained buy faulty R46/R47 which can happen much more often that faulty R8/R10 and can happen by itself, without power components failing. And please learn about AC electricity  . If R8/R10 was open, maximum voltage he could measure would be 240V (RMS voltge). To measure 340V he needs C5 charged with mains peak voltage.

[Mad Professor]

Thank you for all your replies.

Armadillo: You said

Actually the switching power supply does not need the power factor controller to function, its sole purpose is to improve the Power Factor by timing boosting the voltage.

As said before I am a novice, but please correct me if I am wrong, The ML4824 is a Power Factor Correction and PWM Controller Combo, and as such controls the FPC, and also outputs signal to IC30 (IR2109) Half-Bridge Driver, for the SMPS Mosfets.

I did start doing some more voltage tests before reading your replies, the voltages were measured at the connectors, unless otherwise stated in the notes.


I will look at the past listed in the above posts, and report back.

Thanks again.

[wraper]

Try shorting optocoupler as suggested. This will ensure that PSU is always on. And will help to rule out possible control circuit problems. Also it seems that some of the voltages mentioned on the table are from standby PSU and are not present on attached circuit diagram.

[Armadillo]

The ML4824 is a Power Factor Correction and PWM Controller Combo, and as such controls the FPC, and also outputs signal to IC30 (IR2109) Half-Bridge Driver, for the SMPS Mosfets.

Yes you are right, the PWM part needs to run and do the switching to Q6 and Q8,

When you said, Pin 2 and Pin 4 of ML4824 in particular no voltage, can you elaborate on it. I like to know where did you put the black negative probe. Can you measure it again as follows;

Measure Pin 2 with respect to Pin 10 [Gnd] =        volt
Measure Pin 4 with respect to Pin 10 [Gnd] =        volt

Measure Pin 13 with respect to Pin 10 [Gnd] =      volt

[Mad Professor]

I do not own a variac, and I do not have any suitable incandescent light bulbs to put in series at this point in time. In my home I only use CFL's. And as such I am somewhat reluctant in trying to force the smps to start up.

I have redone the tests again with another DMM.


It does seem that I am now getting voltages on Pin 2 & 4 of IC1, but each time I power cycle the TV the voltage values change.

Thanks again.

[Armadillo]

The question is what OP considers as no voltage. Pin 2 is current input and voltage normally won't exceed 0.6V, don't know if OP wrote about pin 4 correctly.
Without optocoupler being on/shorted, there is no chance this PSU running.
No voltage on pin 4 can be explained buy faulty R46/R47 which can happen much more often that faulty R8/R10 and can happen by itself, without power components failing.

Finally you admitted and look into others view hence your proposal to bridge the opto will not solve the Pin 4 no voltage problem and you know your direction is incomplete and incorrect. And if PSU is not running, why Pin 2 read no voltage as OP said?

And you said it could be faulty R46/R47 and it could be, though unlikely R9/R10, hence so that's why I am suspecting it, like you do NOW. hahahahaahah.

There is no need to bridge the opto just to satisfy you when a simple voltage measurement at the SS pin will suffice.

I think and I suspect and that's part of troubleshooting norms. I may not be correct but that doesn't make you more correct than me. OK I can study AC but please go and learn DC first.     

[Armadillo]

Try shorting optocoupler as suggested. This will ensure that PSU is always on. And will help to rule out possible control circuit problems. Also it seems that some of the voltages mentioned on the table are from standby PSU and are not present on attached circuit diagram.

LOL, I think it is the reverse, do not short to ON.

I think you need more than learn DC, like manners for example.

[Armadillo]

I do not own a variac, and I do not have any suitable incandescent light bulbs to put in series at this point in time. In my home I only use CFL's. And as such I am somewhat reluctant in trying to force the smps to start up.

I have redone the tests again with another DMM.


It does seem that I am now getting voltages on Pin 2 & 4 of IC1, but each time I power cycle the TV the voltage values change.

Thanks again.

It's OK. With the new voltages as measured, Please take the capacitor out of the circuit and either ESR test it or change a new one. I suspect bad capacitor.
[DO NOT SHORT OR BRIDGE THE OPTO AS OTHERS ADVISED. IT'S WRONG]

Edit: When you install the new capacitor back, do note that positive to Pin 5.

[wraper]

LOL, I think it is the reverse, do not short to ON.

Yes, I checked datasheet and it should be open. Then best would be to temporarily remove Q21

[wraper]

What voltage is on pin 1 of IC2? Check voltage on pin 3 of IC2 (shutdown) during standby and power on.

EDIT:
ML4824 needs >1.25V on pin 5 to start. So likely there is something wrong in shut down or protection circuit.

[Mad Professor]

Armadillo: I will replace the highlighted item shortly.

wraper: you say that you would like me to test voltages at IC2 this appears to be for the STD-5v output, if this was faulty surly I would not be getting the STD-5V voltage output, as listed in the above tests.

[wraper]

you say that you would like me to test voltages at IC2 this appears to be for the STD-5v output, if this was faulty surly I would not be getting the STD-5V voltage output, as listed in the above tests.

I was thinking about IC50? (number is not clearly readable) IR2109 by IC2. There is no IC2 on schematic you attached, should be on separate sheet, I guess.

[wraper]

Likely protection circuit is pulling down enable inputs of PFC and PWM controller ICs.