Marconi Instruments Catalog:
"A wide frequency range together with high output. purity of waveform. and frequency stability, are primary requirements in any high-quality general-purpose oscillator. All of these features are combined in this compact Marconi R-C Oscillator. The frequency coverage of 20 Hz to 200 kHz with i -watt output at low distortion can be put to good use in a variety of applications ranging from audio work to carrier telephony testing. For distortion measurements on high-quality audio amplifiers. a built-in band-pass filter can be connected in series w ith the output to provide a 1 kHz test tone with harmonic content less than 0·1 %. Frequency response measurements are facilitated by the semilogarithmic frequency scale w hich gives the same order of setting accuracy at all frequencies. the overall frequency characteristic of ± 0·5 dB. and the virtual absence of switching transients between ranges. The large tuning dial has an effective scale length of over 48 inches and ensures excellent discrimination. The panel meter and 60 dB step attenuator are direct reading both in source e.m.f. and. for a 600 ohm load. in decibels relative to 1 mV. Fundamentally, the instrument comprises a two-stage thermistorstabilized Wien-bridge oscillator in which the capacitive elements are varied by the frequency control and the resistive elements by the range switch. Even-order harmonics and hum are minimized by the use of a push-pull transformer-coupled output stage; at high frequencies the quality of output is maintained by the provision of a ferrite-cored output transformer. A high order of frequency stability is assured by a carefully designed oscillatory circuit and a seriesstabilized power supply."

Pictures of the inner guts: