The DVD/VHS combo player where I'm living stopped working about a week ago, and I just took the time to have a good look at it. I checked the mains supply first, that's OK, so then I took the lid off the case looking for anything obvious and irreparable but couldn't find anything. Then I located a fuse on the power supply board; took it out and tested it with a home made test instrument (9V battery, 12V light bulb and a paper clip), and the fuse is not blown. Then I notices some white "stuff" below a 400V 100μF electrolytic capacitor. I don't know what it looks like when one of these fails; could this white stuff be electrolyte that has leaked out?
Capacitor looks fine , sorry , that's not your problem . If the top of the capacitor is bulging out , you know where the problem is .
Cheers ,
Rens
Cheers ,
Rens
A common fault with switch mode power supplies is that the filter caps on the secondary side develop high internal resistance called ESR but do not always display external symptoms like bulging tops. Easy to find if you have an ESR meter but if not replacement of all usually fixes them.
I must have been very lucky , I repaired at least a dozen switched power supplies , Set Top Boxes , DVD's , LCD monitors etc . just changed the bulged caps and all back in business again .
Cheers ,
Rens
Most of the switcher failures I see start with shorted rectifiers on the secondary side. After that, I see blown switching transistors. And yes, the occasional bulged cap.
In my climate(high humidity) an smps fails due to a transformer failure. This then blows the switching component and copper from pcb starts evaporating. Result coper and black marks around. After all this has happened, the fuse will blow.
Gajanan Phadte
Gajanan Phadte
I have just finished re-capacitoring a switched mode power supply for a satellite receiver.
I switched a mains powered FM radio on yesterday and the whole of the FM band was wiped out by a raspy mains hum. Initially I thought the radio was faulty but it exhibited the same symptoms on batteries when inside the house but was OK outside. I then noticed the standby light on the satellite receiver was flickering slightly and it wouldn't power up. The FM interference disappeared when I turned the satellite receiver off at the mains. The power supply had one capacitor bulging slightly and one leaking slightly around the bulging top. I had a refurb. kit at hand to replace all 15 capacitors in the power supply so I removed and replaced them all. The measured ESR on 10 out of the 15 was well above the expected value, some being 2 or 3 times more but were not bulging at all. The 100u 400V reservoir capacitor (the one with a slight bulge) was open circuit, with no measurable capacitance apart from strays, which accounted for the mains born interference picked up on the radio.
It goes to show that aluminum electrolytic capacitors deteriorate with age in the hot environment of a switched mode power supply and do not always show signs of bulging or leakage. Some of the ones removed were rated at 85C, the replacement ones were all 105C so they should last longer. They were all the cheap and cheerful CapXon made ones which are notorious for early failures. The replacement ones are all Panasonic which have a much better reliability record.
I switched a mains powered FM radio on yesterday and the whole of the FM band was wiped out by a raspy mains hum. Initially I thought the radio was faulty but it exhibited the same symptoms on batteries when inside the house but was OK outside. I then noticed the standby light on the satellite receiver was flickering slightly and it wouldn't power up. The FM interference disappeared when I turned the satellite receiver off at the mains. The power supply had one capacitor bulging slightly and one leaking slightly around the bulging top. I had a refurb. kit at hand to replace all 15 capacitors in the power supply so I removed and replaced them all. The measured ESR on 10 out of the 15 was well above the expected value, some being 2 or 3 times more but were not bulging at all. The 100u 400V reservoir capacitor (the one with a slight bulge) was open circuit, with no measurable capacitance apart from strays, which accounted for the mains born interference picked up on the radio.
It goes to show that aluminum electrolytic capacitors deteriorate with age in the hot environment of a switched mode power supply and do not always show signs of bulging or leakage. Some of the ones removed were rated at 85C, the replacement ones were all 105C so they should last longer. They were all the cheap and cheerful CapXon made ones which are notorious for early failures. The replacement ones are all Panasonic which have a much better reliability record.
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