Mechanism of a strong rise of Tc due to the calcium doping in Y1−xCaxBa2Cu2.8Zn0.2Oy

► Calcium is observed to restore superconductivity in Y1−xCaxBa2Cu2.8Zn0.2Oy. ► Parameters of the band spectrum and charge-carrier system are determined. ► The calcium doping is shown to give a noticeable contribution to the DOS function. ► The mechanism of calcium effect on Tc in doped YBa2Cu3Oy is discussed.