Incubate 65oC for 30 min. Re- extract with chloroform isoamyl alcohol. Transfer final supernatant to a tube containing 170 ul 6M ammonium acetate. Add 0.7 ml isopropanol. Yields are sufficient that a labelling reaction using 4 ul of phage DNA makes good probes.
Problems are usually due to too high or too low a level of inoculation.