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H
52005-301D
The
potential
should
be
+55
Vat
tag 12
,
relative to
tag
9, on
the component
side
of the
right-hand
printed board.
20.
It
is
probable
that
readjustment
of
R V1 will be
necessary
if
any of the transistors VTl
to
VT4
are
changed.
Resistor RV3
21.
This
control is
used to set
the
gain of the amplifier.
The
gain
should
be 400
between test
points TPl and
TP2, on
the right-hand
printed
board,
measured
as
follows
:-
(1)
Disconnect
the
positive
feedback at tag 11
on
the component side of the board.
(2)
Apply
25 mV
at
1
kHz from
a 600
Q
source to
TPl on the wiring
side
of the
board,
using tag
21 on
the
component
side as
earth.
(3)
Read the output
on a
high impedance voltmeter connected
to TP2 on
the
wiring side
of
the board, using tag
21
as
earth.
Adjust
RV3, if
necessary,
to obtain 10 V r
.
m.s.
22.
R
V3
may
require
readjustment if
a
ny
of the
transistors
VT5 to
VTll
are changed,
par-
ticularly VTG.
Resistor
R
V4
23.
Distortion in the output
can be minimized by means
of
this
control.
It
also
varies the
positive
d.c.
voltage
at
test
point TP2
on
the wiring
side
of the right-hand printed board,
rela-
tive
to
tag
9
on the
component side.
This
voltage
must
lie in the range 26
V
to
29 V
when RV4
is
set.
Cha
nging
any of the
transistors VT5
to VTll
may m
ake
readjustment of R
V4
necessary.
R
esistor
R
V5
24.
Varia
ble resistor RV5,
forming
pa
rt of
the level-sensitive potentiometer network,
is a
means
of
setting the
negati
ve
feedback providing
level stabilization, as
follows :-
(1)
Set
the
instrument so that
it
is oscillating
normally at 1
kHz.
(2)
Check
the
voltage at
test
point TP2
on
the wiring side of the right-hand
printed
board,
relative
to tag 21 on the component side, using a high impedance
voltmeter.
It
should
be
11
v
r.
m. s
.
Adjust RV5 if necessary.
25.
R
V5
may need readjustment if thermistor THl or
TH2
is
changed.
Resistors R
V8
to RV19
26.
These
set the
limits of the frequency bands.
Normally,
they
should not be moved.
PRESETS- VOLTME
TER
27.
In
the
monito
red
attenuator
, only
the
voltmeter
circuit
has preset
controls.
If it
is neces-
sary
to
change
components
in the input voltmeter, check
afterwards
that
the
presets are
s
et
correctly
if the best
accura
cy
is
required, although
it
is unlikely that
much
readjustment
will be
needed.
2H.
To
check
and
,
if
necessary,
adjust the presets proceed as follows
:-
(1)
Remo
ve
the plastic
plate
beneath the trimming-hole mask, on
the
top of
the
\'oltmeter
Chap. 5
Nov.
79
Page
5
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