DC power supply goes wonky

I have a 12 volt power supply that converts 110AC to 12V DC (actually 13.5.) I haven't used it for some time. I was using it to check some wiring and find it is now putting out 21 VDC. ??? ds
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Is there a question there? Sounds like a rectifier blew.

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Whatever you are using to check it's output voltage is upside down. Flip it over and it will read 12 instead of 21.
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It may be intended to have a load on it. Some cheap supplies will have a big number when unloaded.
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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.
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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 dried out.
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 overvoltage.
-=s
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