Circuit Description Section 4
4.3 ANCILLARY STAGES
(a) Power Supply Circuit
The incoming a.c. supply is fed to the power transformer, T2 via
apair of r.f. suppression filters (L14/C24 and L15/C25) and the
on/off section of the function switch, S2. Three outputs are
derived from the power transformer which are utilised as
tollows:
—
IfD r-m.s. output is half-wave rectified via the diode,
MRl, and fed through asmoothing circuit (C23A/B and
R20) to produce anegative earthed 140V d.c. supply with
low ripple content.
(2) The 6V r.m.s. supply is used as the heater voltage for the
double-triode valve, VI. This supply is heavily decoupled to
r.t. via L9, L13, and CI2, to prevent extraneous signals
from being fed back to the power transformer.
(3) The third supply is derived by auto-transformer action
from the 105/125V primary winding of T2 to provide the
energisation voltage for the neon supply indicator when
S2 IS set to any position other than off.
(b) Attenuation Stages
The attenuation stages are provided in the outout circuit of the
insyument to enable the operator to control the r.f. output level
and, if necessary reset the output level accurately. The at-
tenuation controls consist of acoarse, step-attenuator, output
voLTAGE/dB, and acontinuously variable fine attenuator
OUTPUT VOLTAGE. Switch S3 and its associated resistors R8 to
KI5 form the coarse attenuator circuit whilst the fine attenuator
is formed by the variable resistor RVl.
4.4 FUNCTIONAL DESCRIPTION
{a) C.W. Mode
This ^ode of operation is selected by setting the function
switch, S.2, to the c.w. position. In this condition the MOV ht
supply is fed directly to the anode circuit of the r.f. oscillator”
Vib, which functions in the manner indicated in sub-section 4.2’
Although the h.t. supply is also applied to the anode circuit of
Via this stage will not oscillate since only part of the auto,
transformer T1 is in circuit; thus, the output at the r.f. socket
will be unmodulated r.f and no output will be available at the
A.F. socket.
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