Really need more information. What kind of power supply and what is it
supposed to be used for. If has a regulator it appears it may be bad. To
find the problem you have to go inside and measure the voltage before the
regulator if there is one.
You haven't given us much to go on. I presume though that this is
probably a linear power supply, maybe 3 or 5A output? This would
basically consist of a transformer, rectifier/filter capacitor and
some kind of regulator. The latter is probably a simple discrete
circuit consisting of a few components. One of them will likely be a
transistor in a metal can mounted on a heatsink.
If you haven't used this thing in a loooong time but it was working
the last time you put it away, the first thing I would suspect would
be any electrolytic capacitors, including the big one that filters the
output of the transformer/rectifier combination. The most likely
semiconductors to fail would be the rectifier diodes (2 or 4 discrete
diodes, or one square bridge rectifier perhaps) and the transistor.
But then... that might be all the semiconductors there are.
Do you measure any AC voltage on the output? A small AC voltage (aka
ripple) riding on the high DC would indicate a failed pass transistor,
but if the AC voltage is significant, the filter cap has probably
Of course if this is a switchmode supply you can forget most of this,
but most switcher failures result in loss of output rather than
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