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In Reply to: What makes a transformer HUM? posted by Soundripples on April 2, 2007 at 08:21:19:
If it only hums from time to time, the problem may be DC on the AC line. This can be caused by an appliance that rectifies the line, such as a hair-dryer on half-power, or an electric hot water heater. It might even be in your neighbor's house.If it hums all the time, it is likely to be loose laminations as rick_m suggests.
Follow Ups:
high voltage also may contribute. Try using a voltmeter to check incoming AC. this may be high. using a variac, you can check to see what voltage will cause excess hum.
Jun
Any way to correct loose laminations?
Depends on the construction and how easy it is to take apart.If there is no obvious way to tighten up the clamp, varnish impregnation might help. This is best done in a vacuum so that the varnish gets into all the nooks and crannies.
Years ago when I was young(er) and stupid(er) I varnished a noisy but otherwise excellent 300VA power transformer by immersing it in a pot of marine varnish which was heated up to near boiling point to expel the air and make it thinner. I had already made a jig so that I could suspend the XFO by its wires above the pot to drain. It took AGES to harden, didn't look very attractive and I could not get it back into its half screening can.
But it stopped humming!
.
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