This is a square wave oscillation circuit which uses NE555, a very popular timer chip.
http://www.hobby-elec.org/e_ckt5.htm
Your first test instrument is propably a simple multivibrator as a signal generator.
http://my.integritynet.com.au/purdic/square-wave-generator.htm
two gates from a CMOS 4011 NAND chip form a simpel square wave oscillator where the mark space ratio can also be independantly controlled by varying the value of the resistors
http://www.mitedu.freeserve.co.uk/Circuits/Testgear/sqosc.html
http://home.att.net/~theremin1/Circuit_Library/twin_t_oscillator.html