.
I built this Electronics Australia sine wave / square wave generator in the late 1970s. It has given me excellent service for about 30 years - apart from when I inadvertently connected it to the HT supply of a radio instead of the grid. Fortunately the damage was only to a common enough transistor.
It generates from 15Hz to 150Khz in 4 ranges and since the advent of cheap frequency meters integrated into digital multimeters I have found out that is surprisingly accurate. Most useful for trouble shooting audio sections.