04/09/2015 at 19:07 •
Having got the charger working with the earlier listed replacements, I decided to open it up for one (hopefully!) final time to get some of the pictures I attached in this project. As well, since I had it open, it was a good opportunity to replace rest of the capacitors with new ones. The backside of the charger has printing "200134" which I take to mean it was manufactured in 34th week of 2001, indicating the capacitors could be almost 15 years old - if true, it's held up pretty well, despite there being much critique about Ansmann quality over the web (perhaps it only took a turn for the worse after).
Not to mention I'm not really sure what was the triggering fault in this one. Could have been one of the primary side caps drying out, but could just as well have been one of the diodes breaking due to power surge or similar. Regardless, where there's a bad cap there's usually several, so I set out to replace all of them, and added the electrolytic capacitors inside into the component list. I didn't want to make more substitutions (besides the line capacitor), so I put in another order for missing capacitors from eBay, and settled in for a long wait...
And that's how I got ripped off by the Chinese on eBay for the first time :) I had put out an order to happy-shop09 (Though they're shipping in Newfrog gadgets envelopes) for a capacitor assortment supposed to have up to 10 pieces of 25 capacitors each. After about a months wait it arrived, only for me to discover about one third of the capacitors are 0.1uF 50V pieces. In particular, low voltage & high capacitance pieces were all missing. If I had been shipping just for filling for junk-box that might've been acceptable, but since I was looking for the specific values listed, and getting new ones from eBay will be another month...
Since I want to get this one out of the table and move on to the other projects I have open, I replaced the missing 16V220uF capacitor with a 25V220uF capacitor that I had laying around. Getting it to fit required bending some of the components out of the way, but no biggie. And the replacement capacitor had an effective series resistance of about 0.44 ohms, which is definitely too high for a 25V capacitor (though datasheet seems to specify up to 1.2 ohms for these KMG miniature caps!). On the other hand it replaced a 16V220uF cap with 0.58 ohms series resistance, and as said is larger so will dissipate more power and hopefully not heat as much, so maybe it's an improvement nonetheless.
The 35V220uF capacitor measured 0.23 ohms, and was replaced with equal piece with 0.08 ohms series resistance, and 10V1000uF measured 0.16 ohms, replaced with a new 0.05 ohms capacitor of same values. All 105 degreess C of course. My worst case ESR table gives about two times the ESR of the replacement caps for those pieces, these measuring over triple the replacement caps were definitely due to be replaced even if they hadn't totally failed yet. For the replacement pieces I measured ESR direct without use/pre-conditioning, it might have changed a bit from long shelving time.
As a final note, and to get to the "hacking" in the hack-a-day, I had to hack a piece of plastic off the separating lip in the cover piece to fit in the physically larger line capacitor (450V10uF). If ever having to do this, be very, very careful as it's still necessary to maintain separation and safety distance between the line voltage and low voltage sides, and that plastic lip serves to enforce it. As you may be able to see from the pics, I only cut little off the corner to get the cap just fit. The insulation paper wrapping the line voltage side was originally doubled over under the circuit board, while some images and layout suggest the flap in it is meant to slip through the matching slit in the circuit board to add to that isolation so I bent it up through the slit as well.
And that should finish this repair, hopefully for another 15 years or so :)
03/07/2015 at 21:04 •
On the first pic is an assortment of some of the parts involved. Despite the loud report of the power supply's failure, the hole at the middle of the cap (upright position near middle) is barely visible and easy to miss, although the slight outward bend of the top plate is more reliable indicator of a failed cap. After removal, from the leaked and burned electrolyte at the bottom in the second picture, the failure becomes much more obvious. It's a 10uF 450v 105C piece, about 21x13 millimeters.
Finding a suitable replacement part is often an artform of its own, because people don't usually keep components of every possible value and size hanging around. On the far right is the first cap I tried, which I had to solder dangerous extra leads to because it was snap-in type and so big it wouldn't fit in the original location at all. The 33uF one at the bottom is the replacement I settled on in the end, in part to avoid another blown cap, although it required cutting a bit of plastic off the insides of the cover.
At the top is also visible another nearby capacitor in the SMPS section (47uF 25V 105C); it doesn't even read on my effective series resistance meter, suggesting it's totally dried out, although there's no outward sign of failure. I should probably go through every cap on the low voltage side to make sure they haven't failed as well. The feedback opto-isolator didn't show any outward damage and the diodes checked out fine, but I replaced it just in case because I didn't want to set up a test-bed to see if it worked. For the actual switching IC there were no indications of how it should measure if it's intact, but Google searches suggested it would be almost always bad so I replaced it as well. One with short leads on the left is the original, one on right is one of the replacement parts.
The blown fuse was a matter of its own. Being enclosed, there was no indication of failure on the outside, but of course being it's a fuse, it's easy to measure even in-circuit. It wasn't until I cut it open that I realized it should probably normally be possible to screw the top open (though I wouldn't recommend this as it might influence its integrity). As can be seen, this one is quite clearly blown.
I already threw out the bad rectified bridge, but as usual, there was no outwards sign it was bad either, but again due to the nature of it's function, it should check out rather easily even in-circuit. The schematic I've added is from the TOPSwitch II switching IC datasheet, it's not exactly same as the batter charger, but close enough.
It's probably worth reminding, especially if there's a fault in the circuitry, those capacitors can retain charge far after the power has been disconnected, and need to be bled empty through a power resistor before touching anything. Incidentally, with a shorted rectifier bridge like this, it seems like you could get a shock from the power-plug if the cap and fuse are replaced!