  We present a description of the originally developed $\beta$-spectrometer
consisting of two Si(Li)-detectors with sensitive area thickness above 8~mm and
$4\pi$-geometry. The full absorption spectrometer allows for direct
measurements of $\beta$-spectra, disregarding the corrections to the response
function induced by the electron backscattering from the crystal surface. In
case of $\beta$-spectra of transitions to the excited state of the daughter
isotope additional 3" BGO-detector is used in order to detect the
$\gamma$-quanta in coincidence with the pair of Si(Li)-spectrometers.
