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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Bad transformer? posted by wamba on October 29, 2004 at 22:13:39:
Lethal voltages are present, so please do not attempt this if you are at all unsure of how to do these things safely.First, if it smoked then it is no longer safe to use and must be replaced.
If it only made a loud hum for a brief instant and you turned the power off quickly, or the fuse blew quickly, then it may be fine, but you should still test it.
To test it, unsolder the secondary leads from the rectifier. Measure the resistance of the secondary coil to see if it is open-circuit (bad). Also measure the resistance of the coil to the case to see if it is still insulated (open-circuit reading is good here).
Now measure the secondary voltage with no load, and with the transformer energized just long enough to get a reading. There should be little or no sound from the transformer under this condition. If it still hums loudly, it is bad.
Finally, construct a dummy load resistor to simulate the amp's demand. You can calculate the resistance value from the rated power and the open-circuit secondary voltage, assuming the amp is no more than 50% efficient. It is probably less efficient, but this will do to make sure the transformer is OK. Power is voltage squared over R. If the amp is rated at 50 watts total output, use 100 watts in this equation. The power rating of the dummy load should be larger than the actual power you are testing.
Measure the secondary voltage with the dummy load connected. It should not be much lower than the open-circuit voltage, and the transformer should only get moderately warm while driving it, and should not make an abnormal amount of hum.
If it passes all these tests, then I think it is OK to use. It might be easier to just replace it, but if it is a classic unit, finding a replacement might be difficult.
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Follow Ups
- Careful checking. - Al Sekela 21:21:04 10/30/04 (0)
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