Precision E-200-C Series User Manual

The
operator ran
avoid
this
temporary
detuning
procedure
by
taking
advantage of
the
fact
that voltage
is
developed
across
ONE
HALF
of
the
discriminator
load
network
even
when
the
discriminator
secondary is
properly
adjusted.
This
fact
permits
an
alternative
procedure
for
aligning the
PRIMARY
of the
discriminator
transformer,
as
follows.
Connect
the
V.T.V.M. or
20.000
ohms
per
volt
multimeter
to
the
JUNCTION
of
the two
discriminator
load
resistors
and
ground.
With the
generator set
to
the
I.F.
frequency,
a
reading
will be
obtained on
the
V.T.V.M.
(even
if the
secondary
happens
to be
at proper
midpoint
adjustment).
The
discriminator
primary
winding is
then
adjusted
for
maximum
meter
reading.
The
meter is
then
re-connected
across
the
cathodes
of the
discriminator
tube
and the
SECONDARY
winding
is
adjusted
for zero
voltage as
pre-
viously
described.
Care
must be
taken to
avoid setting
the
discriminator
sec-
ondary to
a
FALSE
zero
setting.
For
example,
the
secondary
trimmer
could
inadvertently
be
set to
zero
voltage at
the
right
or
left
of the
positive or
negative
peak
of the
discrim-
inator
"S"
response
curve.
To test for
the
proper zero
set-
ting,
the
operator
should
vary the
secondary
trimmer setting
very
slightly
to
the left
and right.
With
the
proper
Zero Set-
ting,
voltage
indications
of
opposite
polarity
should
be
obtained.
Without
disturbing
the
setting
of
the
Signal
Generator
tuning
dial,
transfer
the
coaxial
output
leads
to
the
grid
and
ground of
the
LAST
I.F.
amplifier
tube
(closest to
the
limiter
stage).
Disconnect
the
V.T.V.M.
or 20,000
ohms
per
volt
tester
from
the
discriminator.
Connect
one
end
of a
25,000
to
100,000
ohm
carbon
resistor
to the
high
side
of
the
limiter
grid resistor
(R.-10
on
the
diagram) .
Connect
the
V.T.V.M.
or
20,000
ohms
per
volt
tester
across the
free
end
of the
carbon
resistor
and
ground.
(
Disable
th
e_
receiver's
Jocal
oscillator
to
eliminate
false
peaks).
If
the
I.F.
system
is
known to
be a
simple
peaked
amplifier
you
need
merely
ad-
just
both
primary
and
secondary for
maximum
meter
deflec-
tion at
the
specified
I.F.
frequency.
If
the
I.F.
system is
the
overcoupled or
"double-humped"
type,
greater
care
must
be
taken to
obtain the
proper
I.F.
band
width. If
it
were
possible
to
physically
separate
the
two
windings
of a
properly
aligned
overcoupled
I.F.
trans-
former,
each
winding
would be
found
peaked at
the
same
specified
I.F.
frequency:
However,
when
the
windings
are
placed
in
proper
proximity to
each
other,
overcoupling
re-
sults,
producing
an
overall
"double-humped"
response
curve.
In
order to
perform
a
simple
peaking
operation
on
each
winding,
of such
I.F.
transformers,
the
overcoupling
effect
must be
temporarily
eliminated
or
minimized. This is
accom-
plished by
temporarily
shunting
the
primary,
for
example,
with a
1000
ohms carbon
resistor.
This
resistive
shunting or
"loading"
of the
primary
permits the
Secondary
winding
to
be
simply
peaked at
the specified
I.F.
frequency for a
maximum
reading on the
indicating
instrument.
After this is
accomplished
the
procedure is
reversed,
i.e.,
the
secondary
is loaded
with the
resistor and the
primary is
simply
peaked
to
the same
specified I.F.
frequency. The
resistor is
then re-
moved from
the
secondary, and the
inherent
overcoupled
design of
the
transformer
will then produce
the
typical
double-humped
response
curve,
centering about the
specified
I.F. frequency.
The output
leads
of
the
Signal Generator are
then
trans-
ferred to
grid and ground of
the
preceding I.F. stage and the
peaking
procedure (using the
loading resistor
technique in
the case of
overcoupled
I.F.'s) is performed on
the next
I.F.
stage.
The procedure is repeated for
each
I.F. stage until
all
I.F. transformers
have been
aligned. The last
I.F.
adjustment
is made
with the output of
the Signal
Generator injected
at
the R.F.
input grid of the
first detector
or
mixer, which in
this particular case is a
6SA7.
It is
important,
in
all
the
foregoing
adjustment
proce-
dures, to keep
the
Signal
Generator
output at as low a
level
as
will permit
reasonable
V.T.V.M. or
high-sensitivity multi-
meter
readings.
Excessive
Signal
Generator
output may
cause
overloading of
the I.F.
stages, which
could result
in
errors
arising from
overly broad
output meter
indications.
It is
always helpful to
securely
interground
all
test
instru-
ments and
the
chassis of
the set
being aligned. Simple and
positive
grounding
can be effective
through
use
of
an
alu-
minum
or copper
bench plate.
To
re-check the
overall response
of the
aligned I.F. system
the Signal
Generator
output
should be
injected
at the
mixer
input
grid.
The
V.T.V.M. or 20,000
ohms-per-volt
multi-
meter is
then
connected
across
the
output
of
the discrim-
inator.
With
the Signal
Generator set to
the same frequency
at
which the
I.F.'s
were
just
aligned, the voltage at the dis-
criminator
should be
zero.
However,
shifting the frequency
of the
Generator, 50 to
100
Kc (.05
to
.1
Mc) on either
side of
the
specified
I.F. should
give equal or nearly
equal
meter
readings,
but at
opposite
polarity.
Another way
to test
for
symmetrical response
is to
connect
the
meter
across the
limiter
grid resistor
and
note the
change
in
meter
reading
while
the
generator
tuning dial is
moved
to
100
Kc on
either side of
the
specified
I.F.
Approxi-
mately
the
same
change in
meter
reading
should
occur on
both
sides
of the
specified
I.F.
frequency.
If
reasonably
approximate
symmetry
does
not
occur, each
of
the
I.F. stages
should
be
again
adjusted,
because
good
F.M.
receiver
audio
performance
depends upon
the
best
possible
symmetrical
response
of the
I.F.
and
discriminator
circuits.
16

Precision E-200-C Series Specifications

General

Brand	Precision
Model	E-200-C Series
Category	Portable Generator
Language	English