It seems like DM 500s, 7000 and 7020 have the same tuner :

DBS-Tuner:
Frequency Range 950 …. 2150 MHz
Signal Level - 65 dBm …. - 25 dBm
Signal-to-Noise Level 12 dB max.
DBS-Tuner Input Connector F-Type female
Input Impedance 75 Ohm
AFC +/- 3 MHz
Demodulation Shaped QPSK
FEC Viterbi and Reed-Solomon
Viterbirate 1/2, 2/3, 3/4, 5/6, 7/8, automatic assimilation
Roll-off Factor 35 %
Demultiplexing according ISO 13818-1

Is there anyone who knows where to buy the tuner as sparepart ?

Same problem here - no power out from 'LNB IN'. What can be done - changing the tuner part ?

Have you checked out that 'LNB IN' conntact on the backpanel gives power to the LNB ? I have the same problem only my DM 500s shows around 50 % +/- signalstrenght with or without connection to antenna. I connected a sat finder to 'LNB IN' and the meter didn't give any light at all - but my four other receivers do give light when i use this method. This could mean that there is no power out to the LNB and that the tuner is broken.