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CIRCUll'
WBXIFTION
MIDELL 150A
D.
Zero Suppression
A low-current
*lo
volt
supply
is derived
from the main dc supply
using
lo-volt
sener diodes.
Potentiometer
Rl7h may be set at any
voltage
from -10 to +lO volts,
this
voltage
being
applied
through
ap-
propriate
dropping
resistors
to the feedback
point
to achieve
sero
suppression.
The potentiometer
is the front
panel
control
marked
SET, while
switch
S3,
which
detemAnes
the portion
fed back,
is
Labeled
RANGE.
E.
Other
Controls
Three controls
are set at the factory
and should
require
only
infre-
quent attention
by the user.
R118 is an internal
control
marked DC AMP BAL.
It
is used to zero the
IX amplifier,
i.e.,
to set the output
voltage
to sero when the de-
modulator
output
is zero.
This
is not very
critical
since
any unbal-
ance will
simply
be fed back to the inout
to produce
a small
error
signal
to correct
itself.
RlS7is
markedME!WR:CAt.
This
is the variable
portion
of the meter multiplier
resistance
to allow
for
meter-to-meter
sensitivity
differneces.
FCL77,
marked CURRENT BALAE!CE, may be set at some voltage
which will
cause a current
to flow
through
Rl75 to the chopper
am.
This
current
is used to compensate
for
a
small
generated
"chopper
current"
which
would
otherwise
flow
in the input
circuit.
This
"chopper
current"
differs
from chopper
to chopper,but
is fairly
stable
for long
periods
of time.
Its
effect
on any current
range
could
be removed with
the
ZERO SUPPRESS controls,
but the Current
Balance
method used here gives
an effective
aero input
current
for
all
ranges.
F.
Power Supply
A standard
half-wave
rectifier
followed
by an R-C filter
is used to
supply
unregulated
B+ and B- to the output
cathode
follower.
The unregulated
B- is regulated
to -150 volts
in V7, OA2, and is used
for
the negative
returns
for
the dc amplifier.
Unregulated
B+ is fed to the plate
of V8, 12BU,
the series
tube in a
225-volt
electronic
regulator.
The output
voltage
from this
regulator
is divided
by R510 and P.511 and compared to reference
tube V9, a
5651.
The difference
signal
is amplified
by
cascade
amplifier
VlO, a 12AX7,
and applied
to the grid-cathode
circuit
of the series
tube.
This
regulated
225 volts
supplies
Bc directly
to the dc amplifier,
through
a decoupling
filter
(R176,
CllO)
to the
second and third
ac amplifier
stages,
and through
another
decoupling
filter
(R103, ClOh) to the
first
ac amplifier
stage.
Regulated
B+ and B- also
supply
currents
to the 10 volt
sener diodes
which
are used for
xero suppression.
This
gives
two-stage
regula-
tion
for
these
very
critical
voltages.
The first
two ac amplifier
filaments
and the first
dc amplifier
fila-
ments are driven
by a bridge-rectified
6-volt
d.c.
supply.
The R-C
filter,
R512, C507, and ~5~8,
insure
lowripple.
4-2
0763
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