Getting started with troubleshooting [Samsung 245BW]

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

Ok, some good news and some potentially bad news -

The good news is that I removed the resistor and it appears to be open - tested it with the "20M" setting on my DMM and now it shows out of range.

Can I pick up this resistor at my local electronic supply shop (they sell capacitors, resistors)? Or do I need one with special ratings, like the capacitors, and order it from Digikey?

The potentially bad news is that when I went to remove the solder with my desoldering braid, I noticed I left behind some black residue (picture attached). I tried gently cleaning it with a Q-tip, but it wasn't coming off. If so, is there anything I can do to repair it? I'm asking because I'd like to know if there's anything else I need to pick up other than a new resistor.

[jetadm123]

Re: Getting started with troubleshooting [Samsung 245BW]

Ok, some good news and some potentially bad news -

The good news is that I removed the resistor and it appears to be open - tested it with the "20M" setting on my DMM and now it shows out of range.

Can I pick up this resistor at my local electronic supply shop (they sell capacitors, resistors)? Or do I need one with special ratings, like the capacitors, and order it from Digikey?

The potentially bad news is that when I went to remove the solder with my desoldering braid, I noticed I left behind some black residue (picture attached). I tried gently cleaning it with a Q-tip, but it wasn't coming off. If so, is there anything I can do to repair it? I'm asking because I'd like to know if there's anything else I need to pick up other than a new resistor.

No harm in trying to find it at the local supply. Radio Shack would not carry it. If you live near a Fry's, then you might have a chance. 0.22 ohm is not a standard value. Be sure to take the old resistor as a sample as it's hard to tell from the photo what wattage it is. Maybe 1W?

The black stuff is probably burnt rosin from the braid. Q-tip dipped in alcohol should work to clean it off.

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

Be sure to take the old resistor as a sample as it's hard to tell from the photo what wattage it is. Maybe 1W?

The black stuff is probably burnt rosin from the braid. Q-tip dipped in alcohol should work to clean it off.

What markings should I look for to determine the wattage of the resistor?

While the electronic supply store in my town has been great with giving me advice on how to solder, when I asked what brands/types of capacitors they carried, they said "it didn't matter they are all the same". Out of curiosity, I bought a couple capacitors for my 204B monitor repair, and they were Nichicon VZs. Needless to say, I ended up buying the right ones from Digikey and used those instead.

Basically, I'm not completely comfortable deferring completing to their judgement on this.

Also, is rubbing alcohol what you meant?

[jetadm123]

Re: Getting started with troubleshooting [Samsung 245BW]

What markings should I look for to determine the wattage of the resistor?

While the electronic supply store in my town has been great with giving me advice on how to solder, when I asked what brands/types of capacitors they carried, they said "it didn't matter they are all the same". Out of curiosity, I bought a couple capacitors for my 204B monitor repair, and they were Nichicon VZs. Needless to say, I ended up buying the right ones from Digikey and used those instead.

Basically, I'm not completely comfortable deferring completing to their judgement on this.

Also, is rubbing alcohol what you meant?

For resistors of this size, it's usually the actual physical size of the resistor that determines the wattage. The higher the wattage, the larger the resistor. Measure it and see if you can match it to something on digikey.

Yes, I meant rubbing alcohol. Some members like the 90/10 stuff, but i get along fine with the 70/30. It just takes a bit longer to dry.

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

For resistors of this size, it's usually the actual physical size of the resistor that determines the wattage. The higher the wattage, the larger the resistor. Measure it and see if you can match it to something on digikey.

Yes, I meant rubbing alcohol. Some members like the 90/10 stuff, but i get along fine with the 70/30. It just takes a bit longer to dry.

Hmm, well, from what I can tell it's either .5W or 1.0W. Here are the dimensions I got from the calipers I have:

Diameter (measured at the fattest end): 3.5mm
Length: 9.0mm (or just a little shy of 9)

And it seems like either of these could be right:
RSMF1JBR220-ND
RSF12JBR220-ND

So, with capacitors, it's safe to go up with a higher voltage. Is there a similar rule with resistors? Would I be playing it safe by getting either higher or lower wattage than the original resistor, or is it critical that it be the exact same wattage?

[jetadm123]

Re: Getting started with troubleshooting [Samsung 245BW]

You never want to go lower wattage than the original. However, higher wattage is okay. The 1W you chose is out of stock. The only one I see is:

0.22ACCT-ND

Good news is that they're 37 cents each. Bad news is that you have to purchase a minimum of 10.

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

I just picked up two 0.22 ohm 0.5W resistors from the store ($1.50). They look to be the same size as my original resistor, and the 1W versions were a bit big. The lady mentioned something to me that didnt really register until i left. She said i could go up to 1W and insulate the leads.

On my original resistor there is a brownish paint(?) that is coating the leads that were exposed on the topside. Is that insulating paint? Will i need some for when i put the new one in?

[jetadm123]

Re: Getting started with troubleshooting [Samsung 245BW]

I just picked up two 0.22 ohm 0.5W resistors from the store ($1.50). They look to be the same size as my original resistor, and the 1W versions were a bit big. The lady mentioned something to me that didnt really register until i left. She said i could go up to 1W and insulate the leads.

On my original resistor there is a brownish paint(?) that is coating the leads that were exposed on the topside. Is that insulating paint? Will i need some for when i put the new one in?

Glad to see that you were able to find the resistor locally. The brown stuff you're referring to is an adhesive used to hold the resistor in place during the soldering process. No need to replace.

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

Sigh, well that wasn't good.

I put in the new resistor, and turned on the power supply (by itself with nothing else connected) and within seconds, POP! The new resistor popped. I'm guessing that means I should've gone with the 1W resistor? I also noticed that the original resistor was ceramic coated, and the resistors I purchased were metal (or at least had a glossier look). Is this an issue?

I removed the RM801 resistor again, and I'm not sure if I'm paranoid or what, but I noticed that a component right next to it had a little black on the lead. I've attached a picture of it, it's labeled BM801. It doesn't look like the other diodes or jumpers on the board... diodes seem to start with "DM", "DB" or DP" and the jumpers start with "J". Just trying to figure out what it is so I can determine how to check if it's damaged.

Also, after the RM801 resistor popped, I decided to check to see if any of the other resistors were damaged and rechecked all their resistances. They all checked out fine, except I noticed something interesting with resistors RB801 and RB802 (the ones to the NE of the SMPS chip). Depending on which "direction" I measured the resistance, I got different values. I attached a picture with the leads labelled.

RB802:
Black lead on "A", Red lead on "B": 18.8 kOhms
Red lead on "A", Black lead on "B": 98.0 kOhms

RB801:
Black lead on "C", Red lead on "D": 99.1 kOhms
Red lead on "C", Black lead on "D": 88.5 kOhms

I retested all the other resistors and confirmed they have the same resistance (within tolerance) regardless of "direction".

I'm not sure how I missed this before... regardless, I'm not sure what this means. Does this mean that RB801 and RB802 are also bad?

Glad to see that you were able to find the resistor locally. The brown stuff you're referring to is an adhesive used to hold the resistor in place during the soldering process. No need to replace.

I don't mean the gunk on the resistor and the board. I mean it looks like there's a brown "sheathing" on the lead itself. I attached a picture of RM801 again, with the part I'm referring to circled in red. Is that heatshrink insulation?

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

Well, I got curious and I pulled RB801 and RB802 and tested them out of circuit. They both test just fine in both directions when out of circuit so I'm guessing those aren't an issue.

I'm going to go pick up a higher Watt 0.22 ohm resistor today and put that in tonight and see if that helps. If that one blows too... I guess that means it's something else entirely?

[nerdbot]

Re: Getting started with troubleshooting [Samsung 245BW]

Well, I guess I am insane - I tried the same thing, but expected a different result. I put in the 1W 0.22 ohm resistor, and that popped instantly as well. 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.

I took a look at RetiredCaps thread again, and I tested everything that I could find on the board (diodes, transistors, MOSFETs, resistors) and they all seem to be ok. I'm all out of ideas at this point, any other suggestions you guys could provide would be much appreciated. Thanks!

[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.

[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]

This is what I mean ...

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

[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.

[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

[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.

[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]

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.

[retiredcaps]

Re: Getting started with troubleshooting [Samsung 245BW]

Test QM802 and QM803 for a short.