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Bad Caps?

oakdust

Feb 27, 2010
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I tested the caps in a TV power supply with an ESR metter and they all tested good but I did find a bad 33 ohm resiter and Mosfet. I replaced them, the TV worked for about 30 seconds and shut off. Now it just clicks when you try to turn it on (origninal problem). Can the caps be bad with a good ESR?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Check the mosfet and the resistor again. Chances are that the actual problem is not them, but they are killed by it. You need to look further.
 

oakdust

Feb 27, 2010
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I am sure the Mosfet and resistor are dead again, has anyone had a cap with a good esr but end up to be bad? Any other ideas other than caps?
 

NickS

Apr 6, 2010
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You said it measured good with an ESR meter. I have never used one but how do you know the ESR was in the correct range(MFR spec?). Have you used an LCR to measure the capacitance of the cap?

I have always been able to find my bad caps by first short testing then capacitance testing(out of circuit).

Can you give more detail about the circuit surrounding the MOSFET?
 

oakdust

Feb 27, 2010
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Hey Nick, That is a good question, I checked some known good caps and got the same range. I haven't removed the cap to test it, I am leaning towards a bad HOT but haven't had time to test it yet. I can send a copy of the schematic and top view of the board if you like. Both are from the service manual
 

(*steve*)

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Good idea. Scan the appropriate part of the schematic (we'll tell you if we need more).

Is there any particular reason you're so focussed on the capacitors?
 

oakdust

Feb 27, 2010
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No not really a reson to be focused on the caps other than allot of things i have read says the caps are normally the problem. And since I am trying to teach myself electronics I guess I followed the herd.
 

(*steve*)

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Heh -- if you have a hammer, then every problem looks like a nail.

Unless there's evidence of the problem being bad capacitors, don't fixate on it. You have an ESR meter. They are *very* good at finding "bad capacitor" problems. The other tool is your eyes -- are any capacitors suffering from bulging tops, leakage, or have been pushed over by pressure from within? If the answer to these are NO, look for something else.

Generally the classic "bad cap" problem is limited to applications where a low ESR is a requirement. These are places where there is a high ripple current and you find 105 deg rated caps (although it's not universal -- especially with poor design).

I've *rarely* seen them outside of circuits handling power at moderate frequency (switchmode regulators are the classic). That's not to say they don't fail elsewhere,

So having covered that, *AND* the fact that you've checked them with an ESR meter means that we can probably eliminate that as a cause for the moment.

If you have a mosfet and a resistor that seem to be overheating and being destroyed, then you need to look at both the load and the gate drive. The fact that the resistor (is it in series with the mosfet's drain or source?) is affected seems to me to point to a problem in the load rather than the drive to the gate.

If the gate wasn't being driven sufficiently hard to turn the mosfet on or off totally, then the mosfet could overheat and die. If it ends up as a short circuit, then I would not imagine the resistor would be affected, unless it is not rated for a continuous load (and the circuit diagram should reveal that).

If the load is short circuited (or the supply rail is high, or any manner of other conditions) then the mosfet could overheat, and then (or simultaneously) so might the resistor.

All of this is speculation until we see the circuit though. But it's an indication of how you might be thinking if confronted with this fault.

edit:

It would be useful to know what failure mode both the mosfet and the resistor exhibited. Was the mosfet shorted from drain to source? Was the resistor open circuit? (I presume we'll find the type of mosfet and the value/rating of the resistor from the circuit diagram (otherwise those too please).
 
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oakdust

Feb 27, 2010
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I think there is a zip file attached here. Well maybe not,, how do add a file, I get the upload page and it says file uploaded but now I can't find it.
The resistor is 330 ohm 2 watt metal oxide, I am not sure about the mosfet,, Q601# MOS FET 2SK3563 QFWZ02SK3563
Thanks for the help. I read allot but seem to be learning slow. Also, thanks for the info on the caps, that answered my question that if the ESR is good the cap mostlikely is too. Now if you could help me attach the files, I can say I learned two things today. I should add it hase never blown a fuse, no black spots on the board or componets
 
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oakdust

Feb 27, 2010
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My file is in the attachment key as a pdf, how do I add it to the forum? I have clicked the attachment icon downloaded the file and can't seem to get it past the attachment key. What am I doing wrong
 

Resqueline

Jul 31, 2009
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Is the file over 97.7 KB? If not, then are you maybe clicking on the red X in the corner to close the uploader window instead of clicking on "close this window"?
The filename should appear somewhere in the message composer window after doing the latter.
 

NickS

Apr 6, 2010
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Interesting problem. Are you sure the document is a pdf? If you are trying to attach an unsupported file type then you get an error message in the attachment window that looks similar to my attachment Capture.png.

If you have successfully attached a file then it should show up below the text field of your (pre submitted) post as shown in my attachment Capture2.png

If everything is in order and it just will not show up try taking a screenshot, cropping out the excess and saving it off as a picture file(jpg, png...). Then attach the picture.

Hope this helps
 

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NickS

Apr 6, 2010
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That good but I was hoping for a schematic so I could see how they are connected.
 

NickS

Apr 6, 2010
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how about a screenshot of the portion of circuit in question(cap, fet, res and nearby elements)?
 

oakdust

Feb 27, 2010
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I checked the HOT with a transistor tester and it tested bad. I checked all the diodes around it and they are fine. I'll replace it and see what happens, I'll get some pics of the circuit board up later tomorrow. Thanks everyone for all your help and patients:D
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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The resistor is 330 ohm 2 watt metal oxide,

And it looks burnt and crispy?

I am not sure about the mosfet,, Q601# MOS FET 2SK3563 QFWZ02SK3563

OK, that's a 500V 5A TO-220 N channel mosfet.

And it looks like you're playing around in a switchmode power supply from the looks of that transistor.

The list of components is not helpful. We really need the schematic of the power supply.

Post it at imageshack and then give us a link if you need to.

Be warned, fixing switchmode power supplies is non-trivial for a whole stack of reasons and looking at the voltage rating of that mosfet, you're likely to have rectified mains nearby with a capacitor that has the potential to kill you. At the very least it will scare the pants off you if you get a belt from it (and I speak from experience -- not the death part!).
 
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