42' plasma TV - Denver KTV-55 - Faulty power supply

[Djihatch]

Hi, I'm back with another problem ;-) I hope that someone will help...

I bought 5 TVs from a guy who thought that they were LCDs (I know a bit more about LCDs). When I was in front of the TV's, they were obviously plasma TVs. They were cheap and my aim is to learn, so I bought them. But now I need your help to fix them !

The PSU is a Samsung one : LJ44-00133 A (I seached a lot and can't find the schematics, if someone can help ;-) ).

The problem :
When I plug the plasma, the led panel is red (which seems to be ok)
When I turn on the plasma the led turns to green, I hear a relay turn on and off during a second but the panel stay green (that is strange because it usualy blinks red).
No way to turn it off and on again using the button : I have to unplug and replug the TV to hear the relay again by pressing the button.

-Fuses are OK.
-All eletrolityc caps are OK (unsoldered and tested).
I checked big FETS onboard and they're not shorted (As they are not easy to unsolder, I wait for your advices for easyer troubleshoot on other components before I unsolder them for accurate test)
- I assume that, as LCD TVs, there is a feedback using optos, etc... So I cheched what happens on transformers that separate hot from cold side (using a scopemeter). On hot side, nothing happens when I hear relays being on during a second. Obviously, the cold part is not supplied, so there is no feedback wich causes the relay to be off after one second (correct me if I'm wrong).
- I checked what happens on the FETS (gate) with a scopemeter and I think there's nothing significant. Maybe the signal is too low for me to catch him, or maybe there's a problem because FETS should receive something significant on their gates.
- I checked volts on connectors. Nothing on 12V and more connectors. I assume that this is "normal" because the cold part is not powered. only Vsb is OK on "low voltage" connectors. Other voltages are unsignificant (ex : 0.5 v where it should be 5 V).

At this point, I'm stuck ;-). I'm pretty sure that something prevent the first fets to let the power go ahead...
I could unsolder all transistors and ceramic caps to check them all. But it will take ages, and that may cause other problems (bads solders as an example).

So, If anyone has and idea about how to reduce the area to check (I'm pretty sure that the problem is in the hot part).
Is there a way to bypass some electronic controled parts (such as FETS) (with non destructive methods, of course !) to troubleshoot that board...

If anyone has and idea ;-)

Thanks by advance.

PS: I appologize for my poor english (I'm french).

[retiredcaps]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

When I was in front of the TV's, they were obviously plasma TVs. They were cheap and my aim is to learn, so I bought them.

If anyone has and idea ;-)

The Coppell TV repair blog has some valuable info on plasma TVs over the last 2 years at

http://blog.coppelltvrepair.com/

[Djihatch]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Thanks a lot. this website seems to be very interesting.

I found a KIA78R15PI (voltage regulator) that gives me 10 V (Vout controled with a scopemeter) instead of 15 V, wich may explain that the NCP1653 controler doesn't drive correctly the FETs.
Does that means that the voltage regulator is bad ? (That sounds weird to me, I thought that a bad voltage regulator should give the same voltage as Vin if shorted, or nothing is open but not another regulated voltage)
Does that mean that it is overloaded ? How can I chack that ?

Thank you.

[PlainBill]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Thanks a lot. this website seems to be very interesting.

I found a KIA78R15PI (voltage regulator) that gives me 10 V (Vout controled with a scopemeter) instead of 15 V, wich may explain that the NCP1653 controler doesn't drive correctly the FETs.
Does that means that the voltage regulator is bad ? (That sounds weird to me, I thought that a bad voltage regulator should give the same voltage as Vin if shorted, or nothing is open but not another regulated voltage)
Does that mean that it is overloaded ? How can I chack that ?

Thank you.

The easiest way to check the current draw would be to unsolder pin 1 (input), then use an ammeter to measure the current the regulator is drawing. (Pin 2 would show the current the regulator is supplying, but pin 1 may be easier to access.) An even simpler method would be to check the temperature of the regulator.

If it radiates heat and leaves a blister on your thumb, it's overheating.

PlainBill

[Djihatch]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Thanks Plainbill

I unsoldered the voltage regulator an put it on a PCB like (I don't know how its called in english : The blank ones used in labs where you can put components and wires without soldering).
So I was free to perform all tests. I can't check the temperature because power is on during 1 second then the relays turn it off. So there is not enough time for this component to heat.
Here are the results, during the second when the power is on :
pin 1 (Vin - Dc input) : 20.3 V - 38 mA
pin 2 (Vout - Dc output) : 9.3 V (instead of 15 V) - 33 mA
pin 3 : Ground
pin 4 (On - Off control) : 6.3 V - 0 mA

If this voltage regulator,is driven by a current, this is obvious that it can't work with 0 mA on pin 4. What sounds weird to me is that there is 9.3 V and 33 mA on pin 2, it should be 0.

This component is still out of the board. Is it possible to put a dummy resistor instead of pin 4 to check if there's a significant current which would mean that pin 4 is open ?

Are they common failures on those kind of PSU that prevent voltage regulators to be set to "On".

Thanks again.

[joshnz]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Pin 4 turns regulator on or off.
voltage greater than 2volts = on
voltage less than .8 volts = off

output voltage is low regulator need capacitors on input and output to give good voltage. Yes you can use a resistor to provide dummy load in your testing setup.

PlainBill was asking you to lift pin 1 and measure amps going in to the regulator from the board.

[PlainBill]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Pin 4 turns regulator on or off.
voltage greater than 2volts = on
voltage less than .8 volts = off

output voltage is low regulator need capacitors on input and output to give good voltage. Yes you can use a resistor to provide dummy load in your testing setup.

PlainBill was asking you to lift pin 1 and measure amps going in to the regulator from the board.

Correct. However, the measurements suggest the regulator is bad. Djihatch can try adding bypass caps at input and output as you suggested which should take only a few minutes and would confirm this.

PlainBill

[Djihatch]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Thanks to both of you.

In fact, the voltage regulator is out of the PCB (for the measurement to be easyer), but the wiring is still the same. I assume that the design of this board is good (with corrects capacitors), so the Vout should be correct too...
for the test, what kind of capacitor should I use ? how should I solder them ? (first between ground and Vin, second between ground and Vout ? 1 unpolarized cap between Vin and Vout ?)

Thanks again

[Djihatch]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

I used a 250 ohm 1 W resistor between Gnd and Vout. I've measured 11.3 v instead of 15. with this resistor, I should have 6 mA at 15 V (Vout). I'm just above load regulation that is written in the datasheet (5 mA to 1 A) so it should work.
2 options : the regulator is bad or Vin supply is bad and doesn't support load. I'd like to put a dummy resistor between Vin and ground but as I have no schematics, and I don't want to burn something.

++

[joshnz]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Here is a example test circuit.

[joshnz]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Are you getting Vin from the tv?

[Djihatch]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

I get 20.3 V on Vin pin. Unfortunately, I don't have the schematics of this PSU, so I don't know if it's correct. I assume that if it's more than 15v, it should be regulated.

Thanks

[joshnz]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

yes should be regulated.
The regulator is designed to provide 15volts at 1 amp with a vin of 15.5 volts or greater up to 30 volts.

Looks like it is faulty.

[Djihatch]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

thanks,

I've built the test circuit but I have no regulated supply at home. I'll grab one at work tomorow and let you know.
Have a nice day (or good night ;-)

[joshnz]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

oh i see what you ment the input to the regulator does not need to be regulated

[joshnz]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

The output of the psu will be fed into the regulator to provide a regulated supply that is why that regulator is the to take the 20 volts to make nice and smooth 15volts.

[Djihatch]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Hi,

I've built a test circuit for the voltage regulator... And it has exploded !!! I have to find a new one (I found some of these in china but they cost 20 $ + 15 $ shipping !!!).

Let me know if you have cheaper solution...

Thanks.

[PlainBill]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Hi,

I've built a test circuit for the voltage regulator... And it has exploded !!! I have to find a new one (I found some of these in china but they cost 20 $ + 15 $ shipping !!!).

Let me know if you have cheaper solution...

Thanks.

Pin 4 of the KIA78R15P switches it on and off. Was that function used, or was the control pin held high by a resistor?

Forget that!! The equivalent part is available from Fairchild as KA78R15C.

The good news (if you live in the USA) - it is available from Mouser as item number 512-KA78R15CTU.

The better news is they have 12,706 in stock.

The best news is they cost $0.95 each.

The bad news is shipping will cost about $5.00

And this is a good opportunity to suggest you add your location to your profile. It makes it easier to suggest a meaningful supplier.

PlainBill

[Djihatch]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Thanks Plainbill,

I added location in my profile ;-), unfortunately, I live in France, so shipping will be increased for me.
How did you do to find an equivalent ? I searched for hours and no way...
At least, you suggested me and equivalent. I hope that it will be easier to find even in France.
As I'm curious, I used a 3 pin equivalent voltage regulator, and it worked for 20 minutes (it could work longer, but now that I'm sure that this voltage regulator is the only faulty component, there's no need to test it more).
this regulator is not controled by an IC. It's switched on with relays and resistors. I'm not good enough in electronic to be sure that there's no need for this voltage regulator to be controled. I think that if the designer decided to use a 4 pin regulator instead of a 3 pin (and cheaper !) regulator, that's for a good reason.
I'll try to find a cheap controller available in France and I'll let you know.

Thanks again.

[PCBONEZ]

Re: 42' plasma TV - Denver KTV-55 - Faulty power supply

Check here:
http://www.rs-components.com/index.html