Samsung Syncmaster 172N with fried resistors under T1 - No backlight

[EGuevarae]

I have two pieces in my bench right now. One was powering up fine, but the friend that handled them to me (I'm the new electronics-know-it-all of the block.... go figure...) said that it will go to a black screen after half an hour. The other one did that immediately. I opened them and both had CapXons (1000uf 10v and 470uf 25v) bulged. I changed them and they powered up just fine. About ten minutes later, BOTH OF THEM WENT BLACK. I could hear a hiss from both of them, so I turned them off. A visual inspection showed heat around T1, but nothing was wrong until I looked down the board : three resistors of 10k in series were burned in one and there were not even present on the other (they were there before, I SAW THEM!!!!).
If I want to replace them, can I do it with just one 30k resistor? What Wattage will be adequate?
The last picture shows a trace of the path before and after the three resistors (yellow). Can I just solder a 30k resistor between the two points (red)?

[PCBONEZ]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

Not sure, can't see it that well.
Looks like they might be across different taps on that transformer.
If they are then you need the three.

You need to trace and draw out circuit and figure out why they burned.

[Toasty]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

Where are D20 & D21 ?? Overheat and desolder themselves?

Is this circuit "mirrored" on the other end so you can get some voltages?

I'm betting on 120-200 volts across all 3 resistors to cause this. You could *temporarily* use a 30K, 2 watt resistor to bridge that burnt section to test the unit.

>>they were there before, I SAW THEM!!!!<<
If the others were there, did they disintegrate or fall of into the case?

Boy, you sure can find 'em! LOL

Toast

[EGuevarae]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

Where are D20 & D21 ?? Overheat and desoldered themselves?

Not present on any of them

Is this circuit "mirrored" on the other end so you can get some voltages?

No. Just this one have (had) the resistors. The other one doesn't

*********************************************************
I suggested the 33/30K resistor because a while ago I had a "conversation" over email with a guy with the same problem, who was asking for a board of an "AS-IS Samsung 172N" I listed on eBay that can't be repaired (same situation of this two) and here is the email that mentions that resistor:

I'll check it out. My 33K ohm repair with holes burned in the board is working so far. The other one that looked better is a lost cause. I used the good repaired board on the LCD and the LCD is bad with vertical lines on the right side and bad backlight tubes. Also a bad "exit" switch on the control panel. So that unit is trash. The only thing good is the cabinet. But it was only $25 and the other repaired one was free because I got a refund from Dealtree because they sent me thr wrong monitor.

Attached pics are from the other unit. That one have another burnt component, as you can see.

[EGuevarae]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

Not sure, can't see it that well.
Looks like they might be across different taps on that transformer.
If they are then you need the three.

You need to trace and draw out circuit and figure out why they burned.

The blue lines are the "path" from the pins of one side to the pins on the other side of T1. The pink Line goes (via a jumper on the other side) across and ends up in the same pin, the one without the blue caps on the exit, on the other transformer.
The brown path is the new proposed points for the 30/33k resistor. Or it can be between the first R and the last one, just to preserve the original V path.

What do I need to check before the resistor bank to try to figure out what went wrong?

[PCBONEZ]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

I see the xfmr pins now. Was afraid some were between resistors.

I would look for a short on secondary winding.
Short on that side will pull more current through primary where those resistors are.

[Toasty]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

By the looks of this, the only things on the output of the transformer are the blue caps and the backlight tubes.

Check the blue high voltage caps right next to the transformer. As in the other unit, they may be the source of the problem. Unit #2 has a fried one (obvious) and possibly others, and Unit #1 could have one or more that are starting to fail. They are most likely the source of the overload and resistor burn out.

?? What are those caps ratings ??

No. Just this one have (had) the resistors. The other one doesn't

I meant at the other end of the same board. You have the same transformer and caps. How about the same resistors?

Toast

[EGuevarae]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

I meant at the other end of the same board. You have the same transformer and caps. How about the same resistors?

Toast

I mean the same board, other end too.
It only has resistors in one side. Here's a (not so good) picture of the board, of the inverter side. All the components are mirrored except those three resistors. The caps say:

27J
3KV

[Toasty]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

A bit more glare please, my tan is fading....

A better pic would make this easier to explain, but those 3 resistors are tied to the same point on both transformers. Trace it, you'll see.

[EGuevarae]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

A bit more glare please, my tan is fading....

A better pic would make this easier to explain, but those 3 resistors are tied to the same point on both transformers. Trace it, you'll see.

Sorry, I forgot to remove my welding mask when I took the pic, and with that thing on, it looked crystal perfect

[EGuevarae]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight



The transformer is shorted.
From left to right, pins 1~3 are together (in circuit), as well as 5~7 (again, in circuit). The wire starts in pin 3 and ends in pin 6, and I read no resistance between them. On the other side (the secondary?) I read in one, 480 ohms between pins and no connection between the sides. On the (suspected) bad, I get 480 ohms between pins, and between sides - shorted, isn't it?

[Toasty]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

The caps say:
27J
3KV

That's 27 pF ±5% @ 3,000 volts

[Toasty]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

The transformer is shorted.
From left to right, pins 1~3 are together (in circuit), as well as 5~7 (again, in circuit). The wire starts in pin 3 and ends in pin 6, and I read no resistance between them. On the other side (the secondary?) I read in one, 480 ohms between pins and no connection between the sides. On the (suspected) bad, I get 480 ohms between pins, and between sides - shorted, isn't it?

>The transformer is shorted.<
I doubt it.

>as well as 5~7<
Actually 6&7 - 4&5 are no connection. Pin 3 to 6 is correct for primary. (see attached transformer image)

Check all the blue caps with ohmmeter on a high resistance range. They should all read zero. You will have to pierce or remove the white goo to get to their connections. You may also have to desolder one of the legs on the exposed connections to isolate them from the rest of the circuit. Make sure you reseal them before you put it back in service. About 2,000 volts there on open circuit, and 500-700 volts normal.

I think your "between sides" reading is because somehow you're reading the other transformer through a shorted component. Again the "glare picture" is not helping. If you can desolder the transformer outputs leads so as to isolate them from the circuit, I think you'll find them open to the rest of the circuit, and just 480 ohms across them.

I've traced the circuit between the 2 transformers and from the fried resistors so it's easier to see. See the other 2 attached pics.

[PCBONEZ]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

Is circled chip burnt?

[Krankshaft]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

Usually they like to bubble up it could just be some excess flux on there.

I am curious as well.

[Toasty]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

See the other pics. The one of the whole board, with the glare. That mark does not appear.

There's a nice plugin for Firefox called Image Zoom. Really helpful, for us elderly.

http://imagezoom.yellowgorilla.net/

http://imagezoom.yellowgorilla.net/help/

[PCBONEZ]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

I thought photos were of two different but same part boards.

[EGuevarae]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

>The transformer is shorted.<
I doubt it.

Can you please tell me how I could be sure of it not being shorted?
It will be of great help. I'm just guessing here, and any ideas should be appreciated. If I'm in an error, let me know.

>as well as 5~7<
Actually 6&7 - 4&5 are no connection. Pin 3 to 6 is correct for primary. (see attached transformer image)

Yes, you are right, it's 6 & 7, in circuit.

Check all the blue caps with ohmmeter on a high resistance range. They should all read zero. You will have to pierce or remove the white goo to get to their connections. You may also have to desolder one of the legs on the exposed connections to isolate them from the rest of the circuit.

Removed them all (4 on each side). All read 0 ohms on the 200K range of my DMM. I haven't resoldered them back.

... and 500-700 volts normal.

I checked another one and the reading was 590v

I think your "between sides" reading is because somehow you're reading the other transformer through a shorted component.Again the "glare picture" is not helping. If you can desolder the transformer outputs leads so as to isolate them from the circuit, I think you'll find them open to the rest of the circuit, and just 480 ohms across them.

I knew you will say that, so I removed both transformers to test them. Actually, easier than I imagined. The readings are from the Ts off circuit. Each read different. The one "shorted" is from the side with the burnt resistors.

[EGuevarae]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

Is circled chip burnt?

No. It's just hot glue or something like that covering the pins. It is in good condition at the naked eye.

[EGuevarae]

Re: Samsung Syncmaster 172N with fried resistors under T1 - No backlight

Here's another picture, with no glare. I hope this helps you to help me (still one LCD left to mess with ...).


The readings between pink pins (primary) is 0 ohms. Between the blue ones is 480 ohms. On the one I say is shorted, the readings between one pink and one blue is 480.