Taken from the HP-8640B Service Manual:
The Modulation Oscillator is a Wein-bridge type. Transistors Q7 to Q12 form a differential amplifier. The gate of FET Q11 is a high impedance non-inverting input of the amplifier. Transistor Q12 is an emitter-follower buffer amplifier. Trimmer capacitor C9 compensates for the high frequency phase shift of the amplifier. Transistors Q9 and Q10 provide voltage gain and drive the complementary symmetry output transistors Q7 and Q8. The inverting input to the amplifier is the emitter of Q9. Diodes CR2 to CR4 bias and thermally compensate Q7 and Q8. Components R19, C11, and C12 frequency compensate the amplifier. Resistor R26 provides negative dc feedback.
A frequency-selective Wein ladder forms a positive feedback path. This network is a band pass filter with zero phase shift at the maximum of the pass band. The frequency of oscillation is determined by the resistors and capacitors of the ladder. In the FIXED FREQ range, C6 and C7 are the ladder capacitors and either R2 and R6 or R3 and R5 in parallel with R6 are the resistors. In the variable frequency ranges, C1A and C1B are the variable ladder capacitors and R1 and R4 (each in parallel with one or none of the resistors on the A11A1 Frequency Select Switch) are the resistors. Capacitors C2, C3, C4, and C5 set the frequency end points and maximize flatness for a given frequency range. The negative feedback path is a voltage divider in which the amount of feedback is determined by the output signal level. The amount of feedback adjusts to maintain oscillation at a constant amplitude. The voltage divider consists of R28 and RT1, a thermistor assembly. Diodes VR1, VR2, CR5, and CR6 add a small amount of odd-harmonic distortion to stabilize the amplitude characteristic of the oscillator.