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Old 5th October 2008, 04:12 AM   #1
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Default Repairing SMPS

Hi,
Anyone here has the experience repairing a switch mode power supply? Kind to share where to check for faults & what is the common components failure?
The incoming AC (230V) is OK, no components burn, visually everything looks fine. Fuse is OK too.
Thanks!
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Old 5th October 2008, 05:12 AM   #2
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I have repaired a number of switching power supplies in various electronic items. I see a lot of bad capacitors. Many of these are made with cheap caps and they seem to fail often.

You should be able to easily check the diodes and transistor for anything shorted. and there is a small opto-couple there that could have failed. But i would bet if you just "shotgunned" all the caps, that it will come back to life.


Zc
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Old 5th October 2008, 02:48 PM   #3
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Initially I thought is the cap (the one circled in red) gone bad, found no reading. I de-soldered it & the measurement is OK (47uF).
My electronic knowledge is poor, which one is the small opto-couple?
Thanks!
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Old 5th October 2008, 02:50 PM   #4
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Just because the cap measures good doesn't mean the leakage value is good too! IC2 is the optocoupler. above and to the left in your phone. near the end of the white tag. small IC with 4 pins
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Old 6th October 2008, 01:26 AM   #5
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Thanks for the info. How do I measure this IC for fault?
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Old 9th October 2008, 04:59 PM   #6
Mooly is offline Mooly  United Kingdom
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Most common failure points on SMPS are electrolyitics. The value WILL read OK, however the ESR will be very high. Very very common failure mode.
Diodes can fail, you cannot use ordinary types to replace, they must be high speed soft recovery types.
It would be normal practice to replace all electroylitics on a PSU like this, they are very highly stressed, that's not a cop out , just sound practice.
Some PSU's also need a load on them to function correctly.
The IC may well be faulty, maybe caused by those caps

Edit, check also for "start up resistors" that are O/C. They would be on the primary side usually in the 200 to 500 K range.
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Old 10th October 2008, 08:55 AM   #7
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Thanks for the reply.
I guess i'll try to replace all the caps & diodes first & see whether it works.
It could well be the MOSFET (the component on the right of the red circle) too, someone told me that.
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Old 10th October 2008, 06:05 PM   #8
Mooly is offline Mooly  United Kingdom
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Chopper transistors -- Mosfet or Bjt, 99.99% of the time fail short, usual taking any low value resistors round about, with them.
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