12.20.3
Circuit Description
The oscillator is of the Wien bridge type, C1 and C2 being the capacitors, R4
(2.4k
Ω
) being the resistor in series with C2 and R3 plus the parallel combination of
R1 and R2 (= 200 + 2.7k//12k = 200 + 2.2k = 2.4k
Ω
) being the effective shunt
resistance across C1. C1 and C2 are mounted on separate boards and are
selected in pairs by the frequency switch which is also external. The values of C1
and C2 and the corresponding frequencies are shown in the table.
A d.c. coupled pair of transistors VT1 and VT2 is used to provide the
necessary gain. The voltage gain is determined by R8 and the thermistor Th1. The
amplitude of oscillation builds up until the resistance of the thermistor is precisely
twice the value of R8 thereby giving the minimum voltage gain of 3 required to
maintain oscillation. The potential of the base of VT1 is determined by R1 and R2.
C3 and R6 serve to prevent spurious r.f. oscillation. C4 and C5 in parallel present a
low impedance between one end of R8 and the 0V line.
The oscillator is required to give a constant output level independent of
ambient temperature. With the circuit so far described the output would depend
upon the temperature. The oscillator signal from the collector of VT2 is therefore
applied to an emitter follower VT3, and peak rectified by D1 to charge C7 and C8 in
parallel. This circuit has long charge and discharge time constants due to the high
values of R10 and R11 respectively. The potential on C7 and C8 is compared with
the potential at the junction of R16 and AOT1 by emitter followers VT4 and VT7,
which are d.c. coupled to the bases of a long tail pair VT5 and VT6. If the former
potential exceeds the latter, VT6 is turned on and the current from its collector
passes through R8 and TH1. This provides extra heating in Th1, which in turn
tends to decrease the oscillator signal, thereby producing the required stabilisation
of the signal amplitude. On test of the board AOT1 is selected so that the collector
current of VT6, measured by inserting a suitable meter in place of the test link, is:
0.5mA
if ambient temperature
20C to 25C
0.55mA
if ambient temperature
15C to 20C
0.6mA
if ambient temperature
10C to 15C
The output circuit comprises a unity gain d.c. coupled pair VT8 and VT9. The
input to this pair is successively adjusted by AOT2 and AOT3 so that the output is
1.414V r.m.s.
±
0.05dB.
Because of the appreciable time taken for the output to become stabilised
after switching on, the oscillator is kept on even when it is not actually being used.
In order to minimise the risk of cross-talk into other circuits, decoupling is provided
in the positive and negative supply rails by R23, C11 and R24, C12 respectively.
12.21 Cassette Power Unit
12.21.1
Purpose
One of these is mounted on the underside of each cassette to supply the
+20V and –20V supply lines thereof.
