This week WTWF looks at the times in their solo careers when solo Beatles dug into the catalog of their former band.      In the early and mid 1970's, it was more a matter of completing material and releasing it to the general public.      As the years passed, and the spectre of the band receeded a bit, it was no longer blasphemy for Ringo or Paul to consider attempting songs actually released by the Fabs.      Some of these were charity or holiday discs, but the soundtrack to "Give My Regards to Broad Street" (produced by George Martin) is almost half an album loaded with eighties-vintage McCartney covering the Beatles.