A socket should, ideally, be fitted for T601 so that a range of transistors can be tried to obtain optimum noise levels. The white noise output should be adjusted to around 10V peak-to-peak using P601. If this sort of level cannot be obtained then experiment with different transistors.
The amplitude of the pink noise output should also lie in the same range as that of the white noise. If it is too small then R607 should be reduced and if it is too large then R607 should be increased.