Quote:
Originally Posted by johnyjf
It [this transformer] is installed in a light chassis box and when I power it up from the AC mains (no sec. connections) it makes a loud hum.
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Try powering it up
outside that box and with the transformer sitting on a block of wood. That box may be acting as a resonating chamber, substantially amplifying the normal residual hum, possibly coming from loose laminations.
Al.