Getting started with troubleshooting [Samsung 245BW]

[retiredcaps]

Re: Getting started with troubleshooting [Samsung 245BW]

It still shows continuity.

If it didn't show continuity, you can always run a wire to something that is in the circuit. And since you already mapped it out earlier, it would be easy to do.

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

That's fine don't worry. Been there done that many times. You can always use a piece of wire in case you mess it up permanently. Get yourself a solder sucker and some desoldering braid and you won't lift a pad again ever.

Hahahaha, I have both, and I still lifted the pad on this one. Guess I need more practice on the bigger joints.

I am actually surprised I got this far before doing something like that, I usually screw up much earlier on. Knock on wood - I'm not out of the woods yet...

Parts are on order - Thanks again guys for all the help!

[Th3_uN1Qu3]

Re: Getting started with troubleshooting [Samsung 245BW]

That's fine don't worry. Been there done that many times. You can always use a piece of wire in case you mess it up permanently. Get yourself a solder sucker and some desoldering braid and you won't lift a pad again ever.

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

I just checked the continuity of that lifted pad by touching the pad with one lead and another component along the trace further down. It still shows continuity. Hopefully that's a good sign?

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

Correct. They are shorted. Desolder them and re-verify they have same readings.

Yup, they have the same readings out of circuit as well. Unfortunately, I may have messed up. Removing these were a lot tougher than capacitors and resistors, and I lifted the trace up slightly on one of the legs of one of the MOSFETs. I attached a picture - though it's pretty blurry. Does that mean this board is done for, or is there something I can do to repair it?

Since they are "live" (at mains potential), there will likely be a rubbery thermal pad between them and the heatsink. Just re-use that. Also make sure that you don't lose the plastic washer on the screw if that's the case. Power semiconductors don't need as good thermal transfer as a CPU does, you can just use cheap white silicon paste if you want to, but in most cases that pad is good enough by itself. Arctic Silver is likely to be conductive (even if they say it isn't).

Thanks, good to know. Hopefully I didn't break my board and I'll put this info to use...

[Th3_uN1Qu3]

Re: Getting started with troubleshooting [Samsung 245BW]

Since these are connected to heatsinks, I assume I'll need some thermal paste? I have some leftover Arctic Silver that I used on my CPU heatsink. Will that be sufficient?

Since they are "live" (at mains potential), there will likely be a rubbery thermal pad between them and the heatsink. Just re-use that. Also make sure that you don't lose the plastic washer on the screw if that's the case. Power semiconductors don't need as good thermal transfer as a CPU does, you can just use cheap white silicon paste if you want to, but in most cases that pad is good enough by itself. Arctic Silver is likely to be conductive (even if they say it isn't).

[retiredcaps]

Re: Getting started with troubleshooting [Samsung 245BW]

Actually, on closer inspection, some of those MOSFETs have diode symbols on them.

Those are probably schottky diodes.

I tested QM802 and QM803 and here's what I got:

Black on S1, Red on D1 - .4 ohms
Black on S1, Red on G1 - 1.6 ohms
Black on D1, Red on G1 - 1.6 ohms

Black on S2, Red on D2 - .4 ohms
Black on S2, Red on G2 - 1.3 ohms
Black on D2, Red on G2 - 1.3 ohms

So it looks like these are bad?

Correct. They are shorted. Desolder them and re-verify they have same readings.

A diode only allows current in one direction. Since these are shorted they are causing the RM801 resistor to blow.

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

Actually, nevermind. Sadly, my local electronic shop doesn't have them, so I'll have to order it online. I wasn't able to find it on DigiKey (probably because I didn't know the right catagory to narrow it down to), but I found the exact replacement on mouser.com:

http://www.mouser.com/ProductDetail/...R9QRetsMpF3E9k

Since these are connected to heatsinks, I assume I'll need some thermal paste? I have some leftover Arctic Silver that I used on my CPU heatsink. Will that be sufficient?

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

Thanks for the confirmation Th3_uN1Qu3. So, are MOSFETs like resistors - where if all the specs match, they're all basically the same? Or are they like capacitors where I also need to watch for brand and a series?

[Th3_uN1Qu3]

Re: Getting started with troubleshooting [Samsung 245BW]

WOW! Look at all those other MOSFETs that were hiding. Actually, on closer inspection, some of those MOSFETs have diode symbols on them.

If it looks like a duck and quacks like a duck, it's a duck. If it has a diode symbol and tests like a diode, then it is a diode. They are most likely double diodes with common cathode on the center pin. Check those for shorts too. Normally, a secondary side short should make the power supply fail "gracefully" ie it shuts down, but you never know. Use the diode setting of your multimeter. Black lead always goes to cathode, in this case center leg. Since it is a double diode, move the red lead between the outer legs and get readings on both. They should read between 1000 and 100. If they read lower than 100, reverse red and black leads and see what you get. A diode must only conduct one way so if you get a reading the other way round it's bad.

There's also a MOSFET on the large heatsink on the far right of the board, lower right hand corner marked ICM851. It's 5-pins

That's a power supply controller with integrated MOSFET switch. Probably for the standby supply. Since you have standby voltage then that part is OK. Good for you - they are very difficult to get a hold of and a cumbersome adaptation would have been needed if the original part could not be located.

I tested QM802 and QM803 and here's what I got:

Black on S1, Red on D1 - .4 ohms
Black on S1, Red on G1 - 1.6 ohms
Black on D1, Red on G1 - 1.6 ohms

Black on S2, Red on D2 - .4 ohms
Black on S2, Red on G2 - 1.3 ohms
Black on D2, Red on G2 - 1.3 ohms

So it looks like these are bad?

Uh huh.

I don't have an antistatic wrist strap.

Neither do i. This is a non-issue nowadays with most semiconductors. If you read a datasheet, you will notice that they have been tested for ESD (electrostatic discharge). Nothing bad will happen to them if you handle them, unless you happen to be rubbing your feet on the carpet or something.

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

First of all, congrats for the great writeup and thorough documentation. Even i don't do that sometimes.

Thanks, I figure if I'm getting free advice, the least I can do to put is put in the legwork to describe my problem and what I've done.

That MOSFET seems to be okay, but you are testing the wrong one. The ones on the heatsinks are what you should be worried about. Trace the path from the resistor that blows up and you will arrive at the cause of failure.

Test QM802 and QM803 for a short.

WOW! Look at all those other MOSFETs that were hiding. Actually, on closer inspection, some of those MOSFETs have diode symbols on them. I'm guessing those aren't tested the same way as QM802 and QM803? There's also a MOSFET on the large heatsink on the far right of the board, lower right hand corner marked ICM851. It's 5-pins, and I can't quite make out the manufacturer. I think it's SK, or SIK, and 2008HFE. I couldn't find a match after a quick Google.

I tested QM802 and QM803 and here's what I got:

Black on S1, Red on D1 - .4 ohms
Black on S1, Red on G1 - 1.6 ohms
Black on D1, Red on G1 - 1.6 ohms

Black on S2, Red on D2 - .4 ohms
Black on S2, Red on G2 - 1.3 ohms
Black on D2, Red on G2 - 1.3 ohms

So it looks like these are bad? Is there anything special I need to do to remove them and replace them, or is it just a matter of desoldering and resoldering? While reading up on MOSFETs, I found this forum thread, and this post in particular caught my attention:

http://www.edaboard.com/thread125626.html#post549583

I don't have an antistatic wrist strap. Should I get one when I buy new MOSFETs?

[retiredcaps]

Re: Getting started with troubleshooting [Samsung 245BW]

Test QM802 and QM803 for a short.

[Th3_uN1Qu3]

Re: Getting started with troubleshooting [Samsung 245BW]

First of all, congrats for the great writeup and thorough documentation. Even i don't do that sometimes.

That MOSFET seems to be okay, but you are testing the wrong one. The ones on the heatsinks are what you should be worried about. Trace the path from the resistor that blows up and you will arrive at the cause of failure.

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

How did you test the Mosfets ?????That sort of resistance value ie 0.22ohms,is regularly used a the Drain resistor in the Mosfet,and does exactly what you have noticed,in that it burns out,if the Mosfet has gone short circuit.
This may help if that is the problem:-
http://www.4qdtec.com/mostest.html

I used this section from RetiredCaps' write-up:

https://www.badcaps.net/forum/showpo...0&postcount=13

Mosfets
---------

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test a mosfet (Q, IC designation) "in circuit" by (power off and unplugged)

a) black on pin 1- red on pin 2 - record ohms
b) black on pin 1- red on pin 3 - record ohms
c) black on pin 2- red on pin 3 - record ohms

If any ohm reading is less than 30 ohm you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

Some mosfets are more than 3 pins. To test those, identify the part number and search for its datasheet. Once you find the datasheet, the pins will be designated source (S), gate (G), and drain (D). It will probably be documented as S1, S2, G1, G2, D1, D2.

Simply test

a) black on pin S1- red on pin G1 - record ohms
b) black on pin S1- red on pin D1 - record ohms
c) black on pin G1- red on pin D1 - record ohms

Repeat for the "2" pins. That is S2-G2, S2-D2, G2-D2.

Note: a shorted mosfet would likely cause a very brief flash of the backlight or no backlight.

Here is a case study.

https://www.badcaps.net/forum/showthread.php?t=8598

[Side Note: After reading that case study in RetiredCap's post, I see that BM801 on my board looks similar to the inverter fuse in the case study. Can anyone confirm?]

I googled "MOSFET" to get an idea of what it looked like, and I was only able to find one MOSFET, which I marked in the attached picture (burned-resistor-top.jpg). If there are others, please let me know and I'll test those as well. The one MOSFET I found doesn't seem to be anywhere near the burned resistor, but I suppose that doesn't really mean anything.

Using RetiredCaps' MOSFET testing method, this was my result:

a) black on pin 1- red on pin 2 - 105K ohms
b) black on pin 1- red on pin 3 - 30K ohms
c) black on pin 2- red on pin 3 - 30K ohms

Since none of them were lower than 30 ohms, I took this to mean that the MOSFET was ok?

When I test the MOSFET using the link Rtech provided:

So: connect the meter negative to the MOSFET's source. This is arrowed in the drawing above which shows the most popular TO220 MOSFETs.

First touch the meter positive on to the gate.

Now move the positive meter probe to the drain. You should get a low reading. The MOSFET's gate capacitance has been charged up by the meter and the device is turned on.

With the meter positive still connected to the drain, touch a finger between source and gate (and drain if you wish, it matters not). The gate will be discharged through your finger and the meter reading should go high, indicating a non-conducting device.

1. Black on source, red on gate - Diode test reads "700"
2. Black on source, red on drain - Diode test reads "out of range"
3. Black on source, red on drain, finger touching between source and gate - "out of range"

Step 3 was pretty hard to do, and I'm pretty sure my finger touched all 3 pins. Do the results above mean a working or non-working MOSFET?

As RetiredCaps asked, I used my continuity meter and did my best to trace the components near the leads of the burned resistor (burned-resistor-trace.jpg). Since I faded out the underlying circuit board so the trace and labels would be clearer, I also attached the same picture of the bottom side of the power board, without all the tracing (burned-resistor-bottom.jpg) and no fading - just as a reference.

The red and purple lines originate from the RM801 resistor leads. The blue line is from the MOSFET, which is off of the picture to the bottom right. I can take a picture of the area around the MOSFET and trace that, if that will help (or is necessary).

In black, I labeled a couple capacitors, a diode, the fuses, and the legs of what I believe is a heatsink (labeled HS_2 on the board). There were a couple other components I didn't recognize, so I labeled those as well (in black) and then referenced them in burned-resistor-top.jpg

Please let me know if I did that right, or if it's unclear, or if there's anything else you'd like me to do. Thanks again, guys!

[Th3_uN1Qu3]

Re: Getting started with troubleshooting [Samsung 245BW]

Yep, Rtech hit the nail on the head. If the 0.22 ohm resistor blows up then the MOSFET switch that drives the main supply is shorted. Replace it.

[Rtech]

Re: Getting started with troubleshooting [Samsung 245BW]

How did you test the Mosfets ?????That sort of resistance value ie 0.22ohms,is regularly used a the Drain resistor in the Mosfet,and does exactly what you have noticed,in that it burns out,if the Mosfet has gone short circuit.
This may help if that is the problem:-
http://www.4qdtec.com/mostest.html

[retiredcaps]

Re: Getting started with troubleshooting [Samsung 245BW]

BTW, you should scrape off all that glue that has turned yellow on the RM801. That glue was white from the factory, but has turned color and might be conductive.

[retiredcaps]

Re: Getting started with troubleshooting [Samsung 245BW]

This is what I mean ...

https://www.badcaps.net/forum/showpo...7&postcount=86

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

There is where it helps to use the continuity feature of your multimeter.

Start with one leg of the resistor pad and move to the next component. Put your black probe on the resistor pad and red on the next component. If your multimeter beeps, then you have identified part of the circuit path.

I suggest taking a close up photo/pic of the resistor and the surrounding components and draw out the path for us. Identify each component in the path.

Ah, ok, sounds good, I'll get right on that! A couple questions though for the diagram. I assume high resistance resistors will show "not continuous", and I should test the resistance between them to confirm there isn't anything wrong w/ the resistor? And I assume the same is true for high capacitance capacitors?

[retiredcaps]

Re: Getting started with troubleshooting [Samsung 245BW]

I'm trying to follow the traces on the board and work my way back towards the power supply to figure out what along the path could be causing this resistor to blow, but I'm getting lost along the way.

There is where it helps to use the continuity feature of your multimeter.

Start with one leg of the resistor pad and move to the next component. Put your black probe on the resistor pad and red on the next component. If your multimeter beeps, then you have identified part of the circuit path.

I suggest taking a close up photo/pic of the resistor and the surrounding components and draw out the path for us. Identify each component in the path.