[en] A 170 GHz, 2 MW, steady state gyrotron is being developed in collaboration between European research associations and industries to be used for the Electron Cyclotron Resonance Heating (ECRH) system of ITER, the gyrotron is presently in the prototype state. EFDA entrusted ENEA with the design of a new power supply for the gyrotron's body (Body Power Supply); in which, the traditional power vacuum tubes are replaced with solid state components (IGBT); furthermore ENEA had the task of developing the new BPS, following the initial conceptual design. The BPS is formed by fifty identical bidirectional (in current) modules, in series connected; the BPS's control system regulates the output voltage by changing the modulation index. OCEM SpA, under the ENEA's supervision, has completed the manufacture of the BPS. Tests done at the start of this year showed the perfect agreement between the initial technical specifications and the design parameters. Particularly the ramp-up/down (50-10 microsec.), the accuracy ( ± 0.5%), the modulation (45-20 kV-5 kHz) and the maximum phase-shift (< 10 ^o) have been tested. In the article, the new SWM (Stair-Way Modulation) scheme will be sketched and the test's results will be described in detail. (author)