Eddystone 990R - AGC Circuits and Manual RF;IF Gain Control

Video
Amplifier
.
(TR30 &
TRJl)
Audio/video
output
from
the
"AM”,
11
CW" and "PM”
positions
of
S7B
is fed to the
base
of
TR30 (GM378)
which
serves
the dual purpose
of
audio
amplifier
and emitter
follower
to
feed the
main
Video
Amplifier,
Audio output
is
fed
via
low-pass
filter
CI76/CH27/
C177
to the
Audio
Section
which
is
described later
(page
10).
Video response
is
maintained
at
the emitter
of
TR30
which
is then
direct-coupled
to
the
base of
TR31
(GM378).
The low frequency response in
this
stage
is effectively
boosted
by
applying
considerable
high-frequency
attenuation
in
the
form
of
C178.
Video
output is
developed
across
R149
and
connected to
SKI
2
(
BNC
socket)
via
a
short
coaxial lead.
The
complete
video
circuit is
on
a
separate printed board mounted
at the
rear
of
the
set and is
operative
in
all
positions of
the
MODE
SWITCH,
drive being
derived
from the
selected
detector
via
S7B.
AGO Circuits and
Manual
RP/lF Gain
Control
,
(TR21
&
TR22)
Separate
manual
gain
controls
and AGO
circuits are provided for
the RF
Stage
(together
with
the
IP Pre-amp)
and the
IP
Stages.
The
desired
mode
of
operation
is
selected by
the
MANQAL/AGC
SWITCH
(S6) which routes
the
base returns
of the
various
stages to
the
appropriate
parts
of
the circuit. ’’Forward”
AGC
is employed.
In the
case
of
manual
operation,
the
transistors
are
returned
to the sliders
of
two
potentiometers
(RV1 and
RV3)
wired
directly across the
10V
supply.
Both
potentiometers
are
combined in
one
assembly
with
concentric controls. Gain
is
reduced
when
the
sliders move
towards
the negative
end
of
the tracks.
Both
manual
gain
controls
are
switched out
of
circuit
and are
totally
inoperative when using
AGC.
The
AGC system
comprises
two separate
AGC Rectifiers
(D7
and
D8)
with
associated DC
Amplifiers
TR21
and
TR22
(2
x
ASY29)*
The
diodes,
which are
fed
from
the final IP
Amp-
lifier
(TR17) via
C144 and
C145? drive
the bases of
the
DC
Amplifiers
more positive
on
receipt of a
signal,
thus
causing
their collector currents to increase (n-p-n
transis-
tors).
Initial
no-signal
collector
currents are
set by RV5 and
RV6
to
produce
identical
control
voltages
on
the
AGC
line
to those
obtained
with the
manual
controls
at their
maximum
settings.
Gain of
the
RP AGC
Amplifier is controlled by RV2.
Some
measure
of
delay
is
introduced
on
the
RP
AGC
line
by
virtue of the
zener
diode
D6
which
holds
the voltage
constant
at low
signal
levels.
As
the
signal
increases
in
strength,
voltage
developed
across
R97
becomes
greater, the
zener diode
loses control
and the
line
voltage
then
varies
in
sympathy
with
the
signal. This
refinement
helps
maintain
optimum
signal/noise
performance
by shutting down the
IP
Section
in
advance
of
the
earlier
stages.
Meter Circuit
.
A
sensitive
centre-zero
micro-ammeter is
fitted for relative
carrier
level
measure-
ment
and
is
also
usable as
a
tuning
indicator. Scaling
is
in
arbitrary
divisions
0-10.
The
meter
operates
from
the
IP AGC line
at ”CW” or "AM"
and
from
the
output
of
the
Discriminator
at
"FM"
,
switching
being
achieved
automatically with
change
of
mode
(S7C
and
S7D).
Series
resistors
(R108 and
R125) prevent loading of
the
associated
circuits
and arrange
the
meter
sensitivity
to suit
the available voltage.
At
"CW" and "AM”, the
meter is
returned
via
S7D
to
a
pre-set
METER
ZERO
CONTROL
(RV4) which
allows
the
meter
needle
to
be
biased
electrically
to
a
normal
left-hand zero
against
the
standing "no-
signal”
potential across
R106. In
these two modes,
the
meter
presentation
is
logarith-
mic
with
the
MANUAL/AGC SWITCH
at
"AGC” and
linear
(up
to
the
threshold
of
overload)
with
the
switch
in
the
"MIAL"
position.
Normal
centre-zero
operation
is employed
in
the
"PM” mode,
the
meter
being
returned
directly
to earth
through
S7D.
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9
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