  (abridged) Aims. To reconstruct the spectral shape of the quasar ionizing
radiation in the extreme-UV range (1Ryd <= E < 10Ryd) from the analysis of
narrow absorption lines (NAL) of the associated systems. Methods. Computational
technique for inverse spectroscopic problems - Monte Carlo Inversion augmented
by procedure of the spectral shape recovering and modified to account for the
incomplete coverage of the light source. Results. The ionizing spectra
responsible for the ionization structure of the NAL systems require an
intensity depression at E > 4Ryd which is attributed to the HeII Lyman
continuum opacity (tau^HeII_c ~ 1). A most likely source of this opacity is a
quasar accretion disk wind. The corresponding column density of HI in the wind
is estimated as a few times 10^16 cm^-2. This amount of neutral hydrogen should
cause a weak continuum depression at lamb <= 912A (rest-frame), and a broad and
shallow absorption in HI Ly-alpha. If metallicity of the wind is high enough,
other resonance lines of OVI, NeVI-NeVIII, etc. are expected. In the analyzed
QSO spectra we do observe broad (stretching over 1000s km/s) and shallow (tau
<< 1) absorption troughs of HI Ly-alpha and OVI 1031,1037A...
