EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: CapacitorJack on April 19, 2013, 07:53:16 pm
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Hello everyone! My first post here. I have only dabbled with a few electronics projects over the years and still consider myself a beginner.
I recently was given a free 40" Mitsubishi LCD tv and I decided to check the capacitors to see if they were at fault. The tv was manufactured around 2007 I believe. I have a BK Precision Capacitor sorting meter and I decided to start with the largest capacitors.
I took out the four Nichicon 820 mF (200 v) caps and tested them. They tested as follows. 729 mF, 739 mF, 759 mF and 766 mF. None of them or any others in the LCD are ballooning or leaking. It's my understanding that capacitors can be bad without showing any visible signs.
What I am wondering is if having all these capacitors showing low readings, can that be a cause of the problem? This tv is showing verticle lines intermittently which I have read can be caused by bad caps. These large caps go for about $17 a piece so I am a little hesitant to just start replacing them when the problem could be due to a problem with the LCD panel as well.
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Wait, 820 mF? Not uF, mF (820000 uF)?? Are you sure? Note that occasionally they're labelled MFD, and that actually means uF |O
A typical electrolytic capacitor can be as low as 20% (so 656 if labelled 820). The larger, the crapper, and 820mF is huge. (820uF is not.) Also, capacitance measurements will rarely tell you anything useful for electrolytics; they tend not to fail in ways that alter their capacitance (short of spraying their guts onto the PCB). You need to measure ESR or dissipation factor.
Still - are you sure it's not 820 uF? And if it is, you really ought to be able to get them cheaper than $17.
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Panasonic 820uF, 200V, 105°C, $4 (http://www.mouser.com/ProductDetail/Panasonic-Electronic-Components/ECO-S2DB821CA/?qs=sGAEpiMZZMtZ1n0r9vR22QqzopUJ6WFXl6sJ%2fFc1b84%3d)
CDE 820uF, 200V, 85°C, $3.41 (http://www.mouser.com/ProductDetail/Cornell-Dubilier/SLPX821M200A9P3/?qs=sGAEpiMZZMtZ1n0r9vR22XwFQlyFcob6wMuZXXDtZak%3d)
These aren't low ESR, but with that 200V rating I doubt they are in a position where you'd need low ESR.
After a quick check, I don't think 820mF 200V caps even exist, so I'm pretty sure they're really 820uF.
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What I am wondering is if having all these capacitors showing low readings, can that be a cause of the problem? This tv is showing verticle lines intermittently which I have read can be caused by bad caps.
I'm going to take a wild guess at which caps these are (rectifier capacitors on the line input) and say they're probably not the problem - they don't receive a lot of abuse, so they're not likely to fail, and if they do fail, the whole TV will quit working. There are other capacitors for you to check, though I can't really think of a way to describe where to look for them without seeing a photo of the board.
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Thanks for the responses, I guess I didn't have to mention I am a beginner. You are all right, it is actually 820uF. I will try to be more careful.
I have attached the pics of the boards. If you need closer pics of anything I will post them. Thanks for the links to cheaper parts.
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Pic of the 4 I removed.
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You can bet your bottom dollar that these caps will never fail (at least not before we do). Check the other electrolytics that are 25, 35 or 50V. And except capacitance, normally you also check for ESR. There are tables that show the permited range of ESR depending capacitance and rated voltage. Start from the capacitors near the output, the biggest ones. The most common failures I have seen in SMPSs are
1. Electrolityc output capacitors
2. Fusable resistor + input diode
3. Shorted input MOV + fuse
All other failures are relatively more rare.
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i replace bulging caps quite often and they always read within spec capacity wise.
You need an esr meter to check for dead caps when theres no visible signs
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Great. thanks for the tip. I'll check those and post back the results. I guess there's no other way to check esr?
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It sounds like there are bad connections with the LCD drivers. (Had a cheap tablet fail like that, fortunately while it was still under warranty.) Try reseating the LCD connections.
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Vertical lines have absolutely nothing to do with dead caps.
Is the vertical lines on the OSD too ? if yes, you probably have a bad TAB-Bond connection on the panel. Have a look here: 52" Sony Bravia Ghost Image Fix (https://www.youtube.com/watch?v=zW998RTAq-0#)
// Per.
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you probably have a bad TAB-Bond connection on the panel. Have a look here: 52" Sony Bravia Ghost Image Fix (https://www.youtube.com/watch?v=zW998RTAq-0#)
Or here ;)
how to fix a plasma tv with a baseball bat and a wall (https://www.youtube.com/watch?v=0ykPsV2voYU#)
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Does your BK meter have a D of DF reading ? That is much more usefull as ESR. A D of, let say, 0.8 is bad, you do not have to know more. An ESR of 0.8 Ohm can be good, or very bad. You have to know frequency and capacitance to judge it, or better a datasheet.
ESR can be easy measured with a sqope and a squarewave generator. See here: http://www.pa4tim.nl/?p=3775 (http://www.pa4tim.nl/?p=3775)
Most common failures:
- DC leakage
- capacitance loss
- DF increase (and the related ESR)
About your monitor, best way to find these problems is a scope, but be carefull, high voltage and be double carefull probing mains (use diff probes, an isolation transformer or a good insurance)