I have a power supply for an lcd monitor that only works after being heated. It will work fine so long as the monitor is kept on. After cooling down, it's difficult to get started again. Occasionally it will start with being unplugged and replugged, but usually it requires heating. Eight minutes in the oven at 300F works consistently (wrapped in a thin towel).
In the past I've had reasonable luck with this kind of failure just by starting with the largest cap and pulling them until I find one that's obviously leaking or shorted (testing with an ohm meter). This time, however, the broken part isn't obvious to me.
I thought that perhaps I could isolate it by heating or cooling parts until the unit started or stopped working. I tried using a soldering iron for heating, and canned air for cooling, but neither had a clear effect. Is there any other technique I might use for heating or cooling individual parts?
Yes. Inverting the can gives a nice shot of cold liquid that rapidly evaporates. I quickly found that cooling one of the smaller electrolytics would switch off the power, so I'm fairly confident that it's the culprit.
Unfortunately it's a low esr cap, and I haven't found a way to get one to my door for less than 12 times the cost of the cap (due to minimum shipping costs). It nearly eliminates the advantage of repairing the supply.
I can get a non-esr replacement locally. Any guess how long that would last?
The local shop only carries NTE parts. At first glance, I can't find low esr caps in their catalog.
I know the feeling. Hard to understand why suppliers are unwilling to put $5 worth of components in an envelope and send it by normal post. Particularly if the customer expressly accepts the risk of its getting lost.
Yes. Inverting the can gives a nice shot of cold liquid that rapidly evaporates. I quickly found that cooling one of the smaller electrolytics would switch off the power, so I'm fairly confident that it's the culprit.
Unfortunately it's a low esr cap, and I haven't found a way to get one to my door for less than 12 times the cost of the cap (due to minimum shipping costs). It nearly eliminates the advantage of repairing the supply.
I can get a non-esr replacement locally. Any guess how long that would last?
The local shop only carries NTE parts. At first glance, I can't find low esr caps in their catalog.
Depending where in the circuit the cap is, a 'normal' ESR cap will probably work just fine for a considerable time. There are various things that you can do to help its life, not the least of which is to make sure that it is not pressed up close to a heatsink or power resistor. Also, try to get a 105 degree temperature rated cap, rather than an 85 degree. Your local supplier should carry those, if not low ESR ones. The ESR of 'standard' caps can be helped by connecting two in parallel, so that might be another thing to try if you have room to get two in, or one on the front and one on the back of the board.
A final thing that you could try, if you get desperate, is to try your local repair shop. They would definitely carry low ESR caps for their own repairs, and might be persuaded to sell you one. Failing that, they would almost certainly have a scrap power supply board around from something - TV, VCR, computer etc - that you could persuade them to part with. A 'replacement' cap could then be 'culled' from that board. Obvious word of caution. Any such second-hand cap might be as bad as the one you're replacing, and an ESR meter would be useful to check, but in my experience, as long as the scrap power supply that it's coming from is not too old, and the cap is not located right next to a heat source, chances are it will be ok.
Hope that helps. If you are seriously into repairing this sort of stuff, your very next investment should be an ESR meter (see Bob Parker's "Blue" at Anatek Corp's website). This is an excellent instrument and very reasonably priced. It will save you literally hours of wasted time and cans of freezer.
There's the overhead of picking and packing the products. Consequently, most companies have a $20 to $30 minimum purchase, or impose an handling charge for smaller orders. Some don't.
Hairdryers on low are good if you're careful not to overheat components. You can localise the area heated by cutting a hole of the desired size in a sheet of cardboard & using it as a mask to shield the rest of the PCB.
If you haven't already done so, look for dry joints in the PSU.
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. | ,. w , "Some people are alive only because
\\|/ \\|/ it is illegal to kill them." Perna condita delenda est
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I've no objection to being charged the true cost of picking etc, plus a fair profit margin. But the cheapest delivery option of the supplier I've been using is $10, being for sending by courier (often same day). I assume that's somewhat similar to what they're charged. If they'd put it in an envelope and stick a stamp on instead, I could save about $9.50.
Indeed I could presumably save more if they didn't insist on sending a copy of the invoice by separate cover.
I'm assuming that the cap is in the PSU. Have you got a spare PSU you could cannibalise for parts? (I usually have a bunch of spare PSUs in the junkbox.)
In the PSU? - Not long. It's also kind of begging it to let out the confetti. ;^)
I've never been desperate enough to pay the 10000% markup for NTE parts.
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W
. | ,. w , "Some people are alive only because
\\|/ \\|/ it is illegal to kill them." Perna condita delenda est
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Basic ground UPS in the US starts at around $8, way up from what it was just a few years ago. FedEx Ground is a bit cheaper -- and faster. The Post Office's Priority Mail starts around $8 for 2-3 day delivery.
Here in the UK, Farnell, whom I use a lot, have a completely unfathomable shipping policy. If I order a couple of transistors, one time they will come in a Jiffy bag, by standard surface mail. The next time, the very same parts will come packed in a box the size of your head, filled with plastic air bags, and shipped via UPS, who annoyingly vary their routes "for security puposes, innit mate?" such that the parcel might arrive anywhere between about 8am, when I'm not around, and 5pm. Another time, you might order a transformer that weighs a kilo or so, and they will ship that by Royal Mail Parcels. It's really quite bizarre.
Recently, they seem to have rationalised their inventory at the Leeds depot, and now supplement it from a warehouse in Belgium of all places. So I can place an order at 3pm, and most of it might arrive the next morning by Royal Mail, and a packet of resistors will come by UPS later on, shipped from bloody Belgium ! And of course, if the part that's urgent is the one that they are shipping from Belgium, you can guarantee that on that particular occasion, it will arrive a day after the parts that you put in there to 'make up' the order ...
I'm glad that they don't charge me for shipping. How on earth can they justify the cost (and in this case I'm almost inclined to include all the carbon footprint eco-bollox) of shipping a packet of resistors from Belgium to the UK, overnight ?
Heh. Well, a $0.63 part online is about $1.30 bubble-packaged from NTE at the local shop. With minimum shipping being about $8, that's $8.63 vs. $1.30 for NTE. The markup isn't so absurd in that context. Obviously for large orders it doesn't make sense.
It all depends on the circuit, and the quality of the cap. A good quality regular cap will out last these junky "low ESR" caps that are being used a lot. If nothing else, it will work until you need something else that makes the order worth while. Andy Cuffe
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