I have an HP C1556 external tape drive with a power fault. The drive container's fan doesn't power up and the power light is off. Here's what I've tested so far :
mains to the board is OK (240V AC in the UK)
the fuse on the board is OK
there appears to be no DC out on any of the output pins
no apparant burnt out components
I'm don't really know much about power supplies, but I know how to identify the components and how to replace them since I did an electronics degree.
Can anyone advise whether there's a likely cause for this problem or a common fault in this kind of supply which I could look investigate. The board supplies a standard PC drive power connector which provides 5v and 12v connections. Neither DC rails show any output.
Is this a linear or switch mode power supply ? I'm guessing that its a switcher ie lots of electronics, and no mains transformer.
Assuming that it is a switcher, intact board fuse and no smoke, is a good sign. Most likely causes of this type of failure are open circuit or poor ESR small value electrolytics on the primary side - suspect anything with a value of 1uF to 47uF with a voltage rating of 25 to 50v, and particularly any positioned close to any component which runs hot. If you don't have a proper means of testing them, and I'm talking an ESR meter here, no other test will do, then just replace any suspects using low ESR 105 degree types.
The other very common cause of failure to start, is an open circuit strartup resistor. Locate the main smoothing cap - typically 47uF to 470uF rated at around 400v - then follow the print from the " + " pin, looking for a high value resistor, typically several tens to several hundreds of k ohms in value. If this is your problem, make sure that you replace it with a resistor rated to at least 400v, or two lower rated resistors of half the value each, connected in series.
Finally, a word of caution. Switch mode power supplies are potentially VERY dangerous. A very high voltage can remain across that main smoothing cap for several hours, and in some cases, several days, when a switcher is failing to start up. When the power supply is running, there will be about 385v across the cap, derived DIRECTLY from the mains, which is, of course, potentially lethal. Primary voltages can only be measured with respect to the main smoother's negative terminal. Keep one hand in your pocket at all times, is the best safety advice, if you are not used to working on switchers.
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