The Macintosh Performa does not use a ferro-resonant transformer. This type of transformer (large) was used in conjunction with a larger capacitor to produce a constant output voltage [also known as "constant voltage transformer (CVT)"], resonating between 50 and 60 Hz. It is a very old design and the power supply was very heavy, and it is seldom used in the industry.
We recommend that the customer use a true sinewave uninterruptable power supply with all Apple's platforms for best results.
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