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the extent to
which
carrier
energy can
be removed
from
the
demodulated
signal.
The
modulator output
is
approximately
4.0
V
(p-p) at
TP-106.
The
push-
pull
modulator
output
is
converted
to a single-ended
feedbyTlOl.
Record
Amplifiers-Q21
0,
Q211
The
fm
signals
from
the
modulator
is
applied to
record
amplifiers
Q210
and Q211.
Q210
drives
the
video
heads
through
driver
transformer
T203.
The
record
amplifiers operate
in
the
Record
mode
only,
as
B+
is
switched
off
in
all
other
modes.
R225
adjusts
the recording
current applied to the video
heads
for
optimum
recording
level.
Video Heads
A
and B
are
connected
in parallel
to the
secondary of
transformer
T203.
R217
equalizes the
recording
cur-
rent of
Video Head
B
to
that
of
Video
Head
A.
Video
Heads
A
slip-ring
and brush
assembly couples recording
current to
Video Heads
A
and
B.
The Video Heads
supply
signals
during
playback
operations.
The
angle
between
the heads
is
180°
±20".
to the following
playback
amplifiers
during
positive
excursions of the switching
waveform. While output
of
Video
Head
A
is
gated, the
output
of
Video
Head
B
is
blocked
from
the
playback
amplifiers.
This
is
done
as
follows:
When Q206
is
cut
off
(Q205 ON),
the cascode
amplifier
of
Channel
B
can not amplify
Head
B
output because of
the high
impedance (L202)
in
the
source of
Q202.
The
conditions described
above
are
reversed
with each 180°
rotation
of the
head
drum
by
the
signals
from
two
30
PG
coils
mounted
on the
scanner.
Thus,
the
output
of
each head
is
alternately
coupled
to
Q207
and combined
into a
continuous
rf signal
without
any
noise.
The
timing
relation
of
the switching pulses
is
shown
in Fig. 2-1.
Equalizing amplifiers-Q208,
Q209
The
rf
signal
from
Q207
is
amplified
by Q208 and
fed
to
the
limiters
through
Q209. The
collector
circuit
of
Q208
contains
a
resonant
circuit
consisting
of
R211, L204, and C211 which
provides
playback
equalization.
R211
and
L204
set
the resonant
fre-
quency
to
about
3.5
MHz
for
correct
playback
equalization.
Playback
Amplifiers~Q201, Q203, Q202,
Q204
Outputs from
the video
heads
are
coupled through
T201
and
T202
(load
ratio
1
:
1)
to
their respective
playback
amplifiers.
The
low-noise cascode amplifier
Q201
(Q202,
FETs)
and
Q203
(Q204)
amplify
the
weak
signals.
A
resonant
circuit in
the gate of
Q201
(Q203),
consisting
of
T201
(T202),
R201
(R202), and C201
(C202),
resonates
with
the
react-
ance
of the head and
increases
the
output
from
the
head
at
the resonant frequency
to
provide high
fre-
quency
compensation
for head-to-tape
characteristics.
Inductor
L203
in
the
collector
of
Q203
(Q204)
keeps
the
dc
collector
level
constant,
and
also
forms
a trap
with
R205
to
keep
the
audio
bias/erase signal
out
of
the
playback
amplifiers
during audio
dub
oper-
ations.
Switchers
and
Mixer
-Q205,
Q206,
Q207
The
Channel
A
switching
transistor,
Q205,
is
employed
in
the source of
Q201.
Similarly,
Channel
B
employs
switching
transistor
Q206
in
the source
of
Q202.
The
states
of
Q205
and
Q206
are
always
opposite.
For
example,
when Q205
is
saturated,
Q206
is
cut off
by
the opposite-phase rectangular
switching
pulses
from
FF401
(hybrid
IC).
When
Q205
is
ON,
the source of
Q201
is
shorted
to
ground,
so
that
Q201
can amplify
rf signals.
Output from
Head
A
is
amplified
by
Q201
and Q203, and
routed
Limiters-Q301
to
Q305,
D301
to
D310
The
limiter stages
eliminate
amplitude
fluctuations
caused
by
variations in
head-to-tape contact. Limiting
is
accomplished
by
the four diode
pairs
with
interstage
amplification supplied
by
transistors
Q301
to
Q305.
Each
diode conducts
when
the
signal
across
it
reaches
about
0.4
V
(p-p)
in
the
forward
direction.
Thus,
trie
signal
is
limited
in
both
directions to 0.8
V
(p-p).
In the
last
stage
D307/D308
(D309/D310)
conduct
when
the
signal
exceeds
0.7
V
(p-p)
signal,
so
that
2.8
V
(p-p)
is
developed
across the
combination
D307
and
D308
(D309
and D310).
R305
in
the base
circuit
of
Q302
is
adjusted for a correct operating
point
to
produce
a
symmetrical
limiter
output
waveform.
Demodutetor-D311, D312, Q306,
LPF
The
input
to
the
demodulator
is
differentiate!
t>y
R324
and
the
primary winding
of
T301.
The
jusJi-
pull
output
of
T301
is
applied to a
frequency
dopier
consisting
of
a pair
of
pulse detectors
D311ar»d
D312.
They
conduct
on
alternate half
cycles
to
produce
two
positive
output
pulses
per
input
qcl^e.
Thus,
carrier
frequency
is
effectively
doubled
ar»d
placed outside the video passband.
By
integratirJg
the pulse
output
in
the low-pass
filter
that
foiovws
Q306,
a
video output
is
obtained
that
is
proportional
to pulse
frequency.
R326
is
set
to balance
the
jul^e
2-2
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