it's likely there is an open winding at the transformer (cause: power surge) or the rectifier is blown. if you aren't experienced in electronics, there is little you will be able to do. get the unit (s) to a technician.
Tip: if portable units, check that there is no Batt/mains selector switch which is set wrongly. Also if it is a language lab check the master console connection is set up correctly (and hence outputting the correct voltage to the student booths in question - many old tandberg labs had a fuse in the console for each booth).
romko
October 2006
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