Sceptre X20G NAGA-II no power

[aftonr]

Hello all, as described in my title I have a sceptre X20G Naga-II that has no power, no lights are coming on, on the panel buttons or screen. I have trouble shooted this monitor with no luck so far, what I have done so far is replaced a switching transistor (Q11) that was shorted, replaced all capacitors on ps board because I don’t have a esr meter I went ahead and just replaced all of them on the ps board, no caps were bulging before replacement, I also reflowed solder joints on the transformers. Checked the fuses f1 and f2 on the ps board with fluke 115 DMM both fuses were good. I’m stumped and don’t know where to go next with this project, also I can’t find a service manual for this monitor anywhere. I marked the voltage readings I’m getting from the pins that feed the main board hoping someone can tell me if anything looks off. Oh I also forgot to mention I marked Zener diode (ZD102) and Signal Diode (D103) on the bottom view of ps board with Permanent marker because I was getting a reading of .041 on the signal diode and .136 on the Zener Diode I Placed the negative or black lead on the cathode side of the diode when I got these readings, DMM set to diode check, but I dont know how to get the values to replace these components, any help would be appreciated. Thanks in advance!

[sabre504]

Re: Sceptre X20G NAGA-II no power

HI have you checked for voltage on the large Filter ?

In case you dont know but sure you do test this in dc mode Red + black -
and you should see about 165-170 volts

[PlainBill]

Re: Sceptre X20G NAGA-II no power

Lecture 1. When troubleshooting it is important to take a methodical approach. Recognizing symptoms can help avoid wasting time and money.

Lecture 2. You have given us an incredible amount of detail, and some very good pictures. One is missing and another leaves a lot to be desired, however.

Preliminary results. Some of the voltages are unusual (9.1V), others are common (5.3V, 12.9V). What is missing seem to be the control signals to the inverter. The power supply / inverter seems to be working properly, there are problems on the signal card.

Your tasks.

On the picture of the power supply / inverter there are two heat sinks above the transformers (large yellow objects) and two more immediately to the right. The first two are for the power transistors, the second pair are for the output diodes. What are the part numbers of the components mounted on the heat sinks mid-right on the board?

I've marked up the picture of the signal board. There are 5 LDO regulators circled in yellow, I've numbered the pins on one of them. For each of these, report the following information - part number, voltage on pins 1, 2, and 3 (use a mounting screw as the ground point).

There are two switching regulators circled in blue. What are the part numbers of these?

I'd like a picture of the monitor showing all the boards in place and the cables linking them.

PlainBill

[aftonr]

Re: Sceptre X20G NAGA-II no power

Thanks for all the information! I will complete the tasks you laid out for me to do as soon as I get off work this afternoon and will post the results. thanks again.

[aftonr]

Re: Sceptre X20G NAGA-II no power

Got my tasks completed. The part numbers mid right of the ps board are as follows I also marked on the picture where D107 and U105 are located.
D107= PEC 503
ER1002FCT
U105= L7805CV
WCC1A0346
MOROCCO
I also have the part number from the other heatsink mounted parts if needed later, I did not want to flood you with information you did not need or ask for.

The LDO regulator part number is the same for all 5 components
part# 4DH 17-33. And the readings are
U9= (1) 0 (2) 3.3V (3) 5.3V
U22= (1) 0 (2) 3.3V (3) 5.3V
U27= (1) 0 (2) 3.3V (3) 4.9V
U20= (1) 0 (2) 3.3V (3) 5.3V
U21= (1) 0 (2) 3.3V (3) 5.3V

Switching regulator part numbers are both the same.
JM46AB
LM25765
-ADJ P+

Hope this helps please let me know if you need any more information. Also keep in mind I'm taking these pics with my iphone, the wife lost the charger for our good cam . if you need better pics I will buy a new charger just let me know.

[aftonr]

Re: Sceptre X20G NAGA-II no power

HI have you checked for voltage on the large Filter ?

In case you dont know but sure you do test this in dc mode Red + black -
and you should see about 165-170 volts

thanks for the reply I checked the filter and I'm getting 217V

[retiredcaps]

Re: Sceptre X20G NAGA-II no power

The LDO regulator part number is the same for all 5 components
part# 4DH 17-33. And the readings are
U9= (1) 0 (2) 3.3V (3) 5.3V
U22= (1) 0 (2) 3.3V (3) 5.3V
U27= (1) 0 (2) 3.3V (3) 4.9V
U20= (1) 0 (2) 3.3V (3) 5.3V
U21= (1) 0 (2) 3.3V (3) 5.3V

Note the suffix in the part number denotes the correct output voltage. So all your regulators above are good.

Switching regulator part numbers are both the same.
JM46AB
LM25765
-ADJ P+

Datasheet is at

http://pdf1.alldatasheet.com/datashe.../LM2576-5.html

Now measure the DC voltage of each of the 5 pins with respect to ground. I normally put my black probe on a screw on the logic board for ground. Report all readings for all chips.

[retiredcaps]

Re: Sceptre X20G NAGA-II no power

thanks for the reply I checked the filter and I'm getting 217V

217V???

Are you measuring DC voltage across the legs of the large filter capacitor? Black probe on negative and red probe on positive leg of the capacitor?

Warning this is high voltage.

[aftonr]

Re: Sceptre X20G NAGA-II no power

217V???

Are you measuring DC voltage across the legs of the large filter capacitor? Black probe on negative and red probe on positive leg of the capacitor?

Warning this is high voltage.

Actually I was using a ground screw and got that reading. I rechecked it with your direction and got 394.4 V DC

[aftonr]

Re: Sceptre X20G NAGA-II no power

Now measure the DC voltage of each of the 5 pins with respect to ground. I normally put my black probe on a screw on the logic board for ground. Report all readings for all chips.

Here are the results attached pic with how I labled the pins.

Q4= (1) 8.84 V
(2) 2.5 V
(3) 0
(4) 1.22 V
(5) 0

Q11= (1) 12.52 V
(2) 5.0 V
(3) 0
(4) 1.22 V
(5) 0

[retiredcaps]

Re: Sceptre X20G NAGA-II no power

Actually I was using a ground screw and got that reading. I rechecked it with your direction and got 394.4 V DC

If you are in Missouri USA, then this implies that this monitor uses some sort of PFC.

http://en.wikipedia.org/wiki/Power_factor

Normally, we expect 120AC x 1.414 = 170V DC. With PFC, that DC voltage gets boosted up well over 300V DC.

[retiredcaps]

Re: Sceptre X20G NAGA-II no power

Wait a minute, the actual part number is LM2576S, not ML25765 (now that I see the close up photo of Q11). Correct datasheet is at

http://pdf1.alldatasheet.com/datashe...H/LM2576S.html

With the -ADJ in the part number, I *think* Q4 and Q11 are good based on the output voltages on pin 2.

[aftonr]

Re: Sceptre X20G NAGA-II no power

Here is the close up of Q4

[aftonr]

Re: Sceptre X20G NAGA-II no power

thanks for the replies and links its good information I'm always up to learn something new. do you know of anything else I can check..

[PlainBill]

Re: Sceptre X20G NAGA-II no power

This monitor does have a PFC front end which seems to be working.

The voltages on the LDO regulators are all fine, the buck regulators are also reasonable, but my question is why is there no 1.8 volts?

Other than hoping I could see something with a better picture of the signal board, I can't think of any further steps.

PlainBill

[aftonr]

Re: Sceptre X20G NAGA-II no power

Here is a few pics of the signal board you requested I gave an overview shot and two close up pics of the top and bottom half of the board.

[aftonr]

Re: Sceptre X20G NAGA-II no power

also I noticed that the green LED "circled red" is the only light that comes on and it is a very faint light that blinks slowly. the other LED next to it does not even come on. I didn't know if this would tell you something. The 2 wires that feed this board go to the ps.

[aftonr]

Re: Sceptre X20G NAGA-II no power

This monitor does have a PFC front end which seems to be working.

The voltages on the LDO regulators are all fine, the buck regulators are also reasonable, but my question is why is there no 1.8 volts?

Other than hoping I could see something with a better picture of the signal board, I can't think of any further steps.

PlainBill

I just wanted to update my progress with this monitor. I gave up on this project and put it back together to store it away. and after I was done putting it back together I thought I would plug it in to see if it would work just wishful thinking but to my surprize it came on ... I dont know why it wouldn't come on when I plugged it in during tests? but I'm not going to complain it is just very puzzling to me. I'm thinking the switching transistor I replaced must have been the problem but why I couldnt get it to respond when the casing was off is a mystery to me..

[retiredcaps]

Re: Sceptre X20G NAGA-II no power

but to my surprize it came on

You are 2nd person within the last 36 hours to experience that.

https://www.badcaps.net/forum/showthread.php?t=19354

[aftonr]

Re: Sceptre X20G NAGA-II no power

this may be a stupid question but there was a plate that was screwed on across the 2 heatsinks on the ps board I had this off during tests I just assumed it was there for thermal reasons.. am I right about this or was this some kind of ground??