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Old 21st October 2009, 03:31 PM   #1
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Default Checking capacitors

Hi

I was wondering if someone could offer a little advice?

My office uses a large TFT display for presentations. The monitor powers up but there is only a very faint imagine displayed on the screen. I have removed the switch mode power supply from the case and I think the main reservoir cap may have gone. I don't have a capacitance meter but I have attached a DMM set to ohms, and it doesn't appear to charge. The top of the capacitor hasn't bulged but the plastic top has cracked, probably do to heat.

Click the image to open in full size.

Is there anything else I can try?

Thanks

Richard

Last edited by Tripmaster; 21st October 2009 at 03:52 PM.
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Old 21st October 2009, 04:15 PM   #2
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Starting from a discharged state, once your leads are attached, the ohmmeter should increase from zero ohms to infinity in a certain period of time, which would be a ballpark estimate of capacitance once you know the characteristic of your meter. If it doesn't behave like this at all, chances are it's smoked.

However, an ohmmeter doesn't even tickle a 450V capacitor, so you're better off, now that it's been removed, of just replacing it and be done. This capacitor runs between $3 and $5.
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Old 21st October 2009, 04:25 PM   #3
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Quote:
Originally Posted by zigzagflux View Post
Starting from a discharged state, once your leads are attached, the ohmmeter should increase from zero ohms to infinity in a certain period of time, which would be a ballpark estimate of capacitance once you know the characteristic of your meter. If it doesn't behave like this at all, chances are it's smoked.

However, an ohmmeter doesn't even tickle a 450V capacitor, so you're better off, now that it's been removed, of just replacing it and be done. This capacitor runs between $3 and $5.
Thanks for that

Can I attached a battery to the cap to enable a greater charge ?
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Old 21st October 2009, 09:08 PM   #4
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Wouldn't recommend it. If your DMM doesn't do the trick and you don't have a capacitance meter, just replace it.

Were you dealing with a $30 capacitor, maybe some other options to test would be warranted.
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Old 21st October 2009, 10:21 PM   #5
trd1587 is offline trd1587  Canada
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If you are to play in electronics alot i Suggest you buy yourself a capacitor meter. You can find some under 100$ on ebay.

Regards....Marc
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Old 26th October 2009, 07:54 AM   #6
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I hooked up a light bulb tester after changing the cap and the bulb glows brightly, and flickers in time with the switching power supply.

I didn't use the bulb tester before changing the cap so I am unsure if the bulb would have flashed in the same way.

Has anyone connected a light bulb tester to a SMPS, if so what are its characteristics? I'm use to the bulb glowing for a second due to inrush current, then going out completely if no short is found within the circuit.

Regards

Richard
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Old 26th October 2009, 11:10 PM   #7
dangus is offline dangus  Canada
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Is there any light coming from the screen? If not, it could be the backlighting inverter. Those fail. Try googling the model number and hopefully you're not the first to deal with this. If there's no help, check for a fuse near the high voltage inverters, or pull the driver transformers and check them for shorts or opens. It could also be due to bad power supply caps that feed the backlighting. Bulging tops usually means bad.
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Old 26th October 2009, 11:21 PM   #8
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Quote:
Originally Posted by dangus View Post
Is there any light coming from the screen? If not, it could be the backlighting inverter. Those fail. Try googling the model number and hopefully you're not the first to deal with this. If there's no help, check for a fuse near the high voltage inverters, or pull the driver transformers and check them for shorts or opens. It could also be due to bad power supply caps that feed the backlighting. Bulging tops usually means bad.
Hi Dangus

Thanks for your reply!

Yes the screen is black and I can just about make out a faint picture. I inspected all of the components and couldn't see anything untoward. I change the SMPS reservoir cap because the plastic top has started to break up and I thought it might not be supplying enough charge.

I don't have any experience with SMPS and was a little unsure whether to power the SMPS without a light bulb tester, just in case there's a short.

Do you have a bulb tester and SMPS handy?
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Old 27th October 2009, 01:10 AM   #9
star882 is offline star882  United States
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Quote:
Originally Posted by trd1587 View Post
If you are to play in electronics alot i Suggest you buy yourself a capacitor meter. You can find some under 100$ on ebay.

Regards....Marc
For power supply capacitors, just charge the capacitor, leave a voltmeter connected to it, and connect a resistor across it (value such that the expected time constant is at least a few seconds) while timing for it to discharge to about 37% of the original voltage. Even some of the largest capacitors can be tested using this method.

For testing ESR, connect it through a resistor to a signal source (such as an audio amplifier) and use a frequency high enough that capacitive reactance is negligible. Then measure voltage across the resistor to find current and measure voltage across the capacitor to calculate ESR.
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Old 27th October 2009, 01:58 AM   #10
dangus is offline dangus  Canada
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So, it sounds like the backlighting inverter. Sometimes those are separate modules that can be replaced easier than repairing them.
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