Inovonics 375 User manual
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SECTION
1.0
GENERA
L
IN
F
ORMATION
T
he
Inovon
ics 375
is
a compact, up-
to-date
magnetic recording
elec-
~
tr.onjcs_
p~~ka_ge,
completely
self
-contained with
integral
power
supp
ly.
Ava
il
able in several versions,
the
375
is equal ly well
suited
to
new
insta
ll
ations,
or
to
improve
the
performance
and
rel
iabil
it
y
of
older
professional tape recorders.
Th
e 375 is
de
li
vered with prop
er
interconnect cabling
for
whatever
tape
transport
is
specif
i
ed
for
us
e with i
t.
When
no
specification
is
made,
the
unit
is
supp
lied with
cab
ling
for
Ampex
350/351
series
machines.
Among
t
he
features
incorporated
are:
Ca
l
ibrated
positions
on
Record
and
Reproduce
gain
controls.
Provision
for
remote switching
of
all
monitor
and
equalization
functions,
and
sol
id-state
switching to eliminate contact noise
problems.
3-speed equalization to
accommodate
any
combination
of
NAB
and
IEC
charact
eristics.
Harmonic
and
phase
distortion
nulling
circuits.
Able
to
acco1TJTiodate
a
wide
variety
of
original
-equipment
and
replacement heads .
Optional
SYNC
Reproduce
Ampl
i
fier
wi
th
independent equalization
and
automatic
SYNC
-
INPU
T monitor
transfer
switching (-03 version
onl
y).
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I.I
SPECIFICATIONS
NOTE:
Performance
of
a magnetic recording system
is
limited
in
part
by
the heads
and
tape employed.
The
Inovonics
375
is
intended both
for
upgrading
older
recorders in broadcast
and
similar
applications,
as well as
for
'
new
studio
installations.
Hence
two
sets
of
specifi-
cations are given, each with respect
to
the heads, tape
and
standards
generally in use
in
each
installation
situation.
A
third
group con-
tains
those
specifications
which
are
not
application
oriented.
1.1.1
Specifications
derived using
3M
206
tape
and
a Taber
full
track
replacement
head
assembly
on
an
Ampex
351
transport.
Frequency
Response
(in
Hz)
15
ips
7. 5 i
ps
3.
75
ips
+2dB
+2d8
+3dB
20
20k
20
-
15k
20
-
8k
Signal-to-Noise Ratio
(in
dB,
referred
to a "peak" record level
6dB
above
200nW/m,
or
approx.
3dB
below
3%
THO;
20Hz
-
20kHz)
15
i
ps
7.
5 i
ps
3.75 ips
Erase/Bias Frequency
lOOkHz
Erasure
OVERALL
u'wtd.
NAB
wtd.
-67 -76
-69
-77
-66 -
74
STANDBY
u'wtd.
NAB
wtd.
-76 -87
-
76
-
86
-72
-83
70d8
erasure
of
500Hz
signal recorded
3dB
below
tape
saturation.
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Recorded
Distortion
(at
15-mi
1
wavelength bias
peak
-
typical)
LIN
out
LIN
in
Operating
Level
.45%
.02%
6dB
above
Op
Level
2% .015%
8dB
above
Op
Level
3% .02%
lOdB
above
Op
Level
5.5% 2.2%
1.1.2
Speci_
fications
d_
~i:-ived
u_sing,
_)
M
250
ta
· e
··
an
0Re
.
ratingl.evel
of
320nW/m
and
Nortro~fcs
9227
(erase),
9203
(record)
and
9213
(repro-
duce) heads -70-mil track width format, typical
of
multi-track
appli-
cations.
Frequency
Response
(in
Hz)
30
ips
+2dB
60
22k
(SYNC
reproduce
15
i
ps
+2dB
30
-
22k
·response essen-
tially
identical
7.5 i
ps
+2dB
20
-
16k
to
normal
overa
ll
)
Signal
-t
o-Noise Ratio
(in
dB,
referred
to approx.
640nW/m
or
3%
THO;
20Hz
-
20kHz)
30
ips
15
i
ps
7.5 i
ps
Erase/Bias Frequency
250kHz
Erasure
OVERALL
u'wtd. wtd.
-71
-78
-67 -
77
-67 -76
SYNC
u'wtd.
wtd.
-71 -
80
-67
-77
-67
-
77
STANDBY
u'wtd. wtd.
-80 -87
-
75
-86
-74
-85
70dB
erasure
of
500H
z signal recorded
12dB
above
Operati
ng
level,
or
approximately
3dB
below
tape
saturation.
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Recorded
Distort
i
on
(at
15-mil wavelenth
bias
peak
-typical )
Operating
Level
~
3_
dB
above
Op
Leve1
__
6dB
above
Op
Leve
1
LIN
out
.5
%
1%
2.5%
LIN
in
.15%
.3%
.8
%
1.1.3
Specifications
Independent
of
Application
Equaliz
ation
3 speed with pushbutton
selection,
automatic switching be-
tween
INTER
and
LOW
when
transport
provides
closure.
Accom
-
modates
NAB,
IEC
(or
mixed)
characteristics
for
3.75 through
30
i
ps
operation.
Amplifier Oistor~ion
Record:
Input
Outputs
<
.1%
THO
at
Op
Level
<.25%
THO
25d8
above
Op
Level
Reproduce:
< .
1%
THO
at
Op
Level
<.5%
THO
at
+24dBm
Line Output (clipping level
+25dBm)
Sensitivity:
-
20
to
+8d8m
(-8
to
+8d8m
with -
01
option
input transformer)
Impedance:
lOK
bridging
Line Output: feeds
600
l
ine,
balanced
or
unbalanced,
terminated
or
not,
at
+4
or
+8d8m
for
Zero-VU
.
Headphones:
Front panel jack
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------
Panel
Controls
RECORD
GAIN
(with pre-
set
CAL
position)
.
REPRO
LEVEL
(with pre-
set
CAL
pos
ition)
MONITOR:
selects
between
INPUT,
REPRO
and
SYNC
(-
03
versions)
SAFE/READY
RECORD
EQUA
L
IZATION
:
manual
selection
of
HIGH,
INTER
or
L
OW
.
POWER
Power
Requirement
105
-130
VAC
(_2
_39~_
ay~l
_?:_~~~)50/60Hz,
.3A
(plus transport)
Size
and
Shipping
Weight
3½"
X
19
11
X
10"
15 1
bs
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SECTION
2.0
INSTALLATION
ANO
USE
2.1
Upon
receipt
of
the equipment, inspect
for
shipping
damage.
Should
any
be
observed,
notify
the
carrier
irrmediately;
if
not,
proceed
as
outlined
below.
It
is
suggested
that
the original ship-
ping carton
and
materials
be
saved for possible future reshipment.
2.2
The
375
is
packaged
to
mount
in a standard
19
-inch equipment
rack, requiring 3½-inches
of
panel space.
When
replacing original
electronics
in
an
overbridge
above
the
transport,
it
may
be
necess-
ary to
drill
and
tap
an
additional
two
holes
to
accommodate
the
375
panel .
2.3
The
signal,
head
and
transport
connectors are d.irectly compatf-
bl~
with
Ampex
350,
351
and
354
equipment. Units ordered
for
use
with
other
transports.
or
for
use
in stereo pair~ are s·upplied-
wi_th
a_ppropr:__iate
_·
interconnect cabling.
2.4 Unless supplied otherwise
to
special
order,
units intended
for
mono
or
stereo
installations
are delivered properly aligned
for
use
with
3M
206
tape
at
an
operating level
of
200nW/m,
with
Ampex
351-
type
heads
(1.5mHy
erase,
5-lOmHy
record,
and
lHy
reproduce)
and
lOOkHz
bias. Units
for
multi-track
use
normally operate with lower
inductance heads
(0.5mHy
erase,
4-5mHy
record,
and
400mHy
reproduce),
utilize
250kHz
bias,
and
incorporate the optional
SYNC
reproduce
amp-
lifier.
Heads
with inductances
other
than those suggested
above
can
be
accommodated.
Section
4.1.4.2
outlines
the necessary changes.
2.5
As
delivered, the
375
is
calibrated
to operate
at
a
+4dBm
signal
level.
Should
operation
at
+8d8m
be
necessary,
clip
the
jumper
shun-
tingthe
4.7K
resistor
near the
line
output connector
and
recalibrate
as described in paragraph
4.1.2.8
.
2.6
The
low
source
impedance
of
the
375
line
output
results
in only
about
½dB
level
change
from
an
unloaded to
600
ohm
loaded condition.
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The
375
should, nevertheless,
be
connected
to
its
intended load
prior
to final
ca
librati
on.
2.7
Even
with the optional input
isolation
transformer option sup-
plied (-01 version
only),
the
375
in~ut
impedance
is
lOK
ohms
or
greater.
Should the equipment feeding the
375
require a terminating
load, a
600-ohm
resistor
should
be
placed in
parallel
with the
375
input.
2.8
No
provision for meter indication
of
bias
or
erase current
is
made,
as the inherent
stability
of
the
circuitry
is
greater
than
that
of
the
heads
and
meter
at
bias frequencies.
2.9
When
two
or
more
375
1
s are
employed
in dual
or
multi-track ap-
plications,
the interconnecting cabling supplied delivers
AC
power
to
all
units
and
to the
transport,
if
it
derives
power
from
the
elect-
ronics.
The
cabling also "slaves"
all
erase/bias
amplifiers
to
a
single
oscillator,
thereby avoiding bias
11
beats
11
or
the necessity
of
synchronizing
two
or
more
oscillators.
-8-
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SECTION
3.0
OPERATION
ANO
FUNCTIONAL
DESCRIPTION
, 3.1
OPERATION
3.1.1
The
RECORD
GAIN
and
REPRO
LEVEL
controls are provided with a
detented
CAL
position
at
full
CCW
rotation.
They
are normally
left
in these positions except
for
temp0rary correction
for
an
improperly
recorded
tape or
abnormal
line level .
Range
is
a
dequate
to
record
from
a
-20dBm
line
level
(-8dBm
with optional input transformer)
and
to produce
normal
line
output
from
a tape under-recorded
by
lOdB
.
3.
1.2
The
MONITOR
buttons determine whether the
line
amplifier will
derive
its
signal
from
the
incoming
program
(INPUT),
the reproduce
head
(REPRO),
or
in versions with -03 option, the record
head
(SYNC)
.
When
any
button
is
only
slightly
depressed to release
all
three to
their
11
out
11
position, the monitor selection
is
transferred to the
rear
panel
REMOTE
connector.
On
versions with the -03 option,
mon-
itor
is
automatically switched
from
SYNC
to
INPUT
when
entering the
record
mode.
3.1
.3
The
alternate-action
SAFE/READY
switch,
when
depressed, per-
mits the
375
to
enter
the record
mode
when
the transport
is
in "record
11
•
When
this
button
is
"out", the
375
will not
enter
the record
mode,
thereby preventing accidental erasure
of
a tape
or
that
track
of
a
dual
or
mult-track recording.
3.
1.4
The
RECORD
button places the transport in the record
mode.
The
button will not illuminate, though, nor \'Jill the
375
go
into
11
record
11
unless the
SAFE/READY
button
is
depressed.
In
dual
or
mult1-track
in-
stallations, depressing
any
RECORD
button
will
place
the transport in
the record
mode;
only those 375's in
11
ready
11
will record,
however.
3.1.5
The
EQUALIZATION
buttons
select
record
and
reproduce equal-
-lizationappropriate
to
the transport speed
employe
_
d.
Three independ-
ent
sets
of
equalizers are provided, although
most
transports are only
-9-
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dual
speed
units.
In
these cases the
third
position
may
be
used
for
a
European
or
other non-standard curve.
In
the event
of
use
with a
transport
providing
an
equalizatio
-switching pole
on
the speed
switch, the equalization
selection
an
be
transferred
to
change
auto-
matically
between
INTER
and
LOW
(if
he
transport
is
appropriately
wired)
by
depressing
an
EQUALIZATION
utton
slightly
so
that
all
three buttons are
11
out
11
•
3.1.6
The
alternate
action
POWER
switch controls
AC
power
to
the
375
and
to the
transport,
if
it
is
powered
by
the
electronics.
3.1.7
The
PHONES
jack
is
connected ahead
of
the output transformer
through a
470
ohm
resistor.
It
is
suitable
for
headphone
monitoring
or
as
an
unbalanced, uncalibrated
line
output
for
servicing.
3.1.8
The
recessed
slide
switch behind the adjustment cover
panel
enables
or
defeats the
linearization
circuit,
(see paragraph 4.2
.1).
-10-
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3.2
CIRCUIT
DESCRIPTIONS
3.2.1 General
Electronic
circuitry
for
the Recording, Reproducing
and
Erasing •
Amplifiers
is
contained
on
a
single
11
piggy-backed
11
plug-in assembly.
The
lower board (Reproduce)
carries
the Reproduce, optional
SYNC
and
Line Amplifiers; the upper board (Record), the Erase/Bias
and
Record
Amplifiers. Another
single
PC
assembly within the chassis contains
the
power
supply
and
bias
oscillator
circuits.
3.2.2
Reproduce
Amplifier (schematic 129000).
Signals
from
the reproduce
head
enter
on
pins
13
and
14
.
An
option-
al
head
input transformer (-04 version only)
can
be
strapped
in
to
permit use
of
low
(4-5mHy)
inductance heads. Resistor
Rl
is
selected
to
dampen
head
resonance
and
yield
smoothest playback response (see
paragraph 4.1.
2.4).
Transistors
Ql
and
Q2
form
a complementary feedback-pair input
stage,
Q3
serving primarily as
an
emitter-follower buffer.
DC
feedback
is
maintained through
R19
and
Rll,
bypassed
at
audio frequencies
by
C2.
AC
feedback
is
routed through
an
appropriate equalization network
by
FET
switches
Q4,
Q5
or
Q6.
A secondary function
of
Q3
is
to provide
a phase-inverted reproduce signal
(at
the
collector)
for
the repro-
duce
phase compensation
circuit.
This
signal,
coupled through
C7,
interacts
with the in-phase
emitter
signal fed through
R25
to provide
an
adjustment
of
reproduce phase
shift
to
complement
and
cancel the
phase
shift
which
normally occurs during the recording process.
FET
switch
Q7,
normally on, defeats
this
compensation, except
at
the
one
speed
for
which
the adjustment
is
made
(see paragraph
4.2.2.2).
IC2
imparts voltage gain
to
the reproduce
signal,
with
R28,
the
REPRO
CAL
adjustment, providing a control over the gain
and
a
means
of
obtain-
ing a given signal level
for
a
variety
of
head
outputs. This amplified
signal
is
fed through
R29,
bridged
by
the
front
panel
REPRO
LEVEL
con-
'
-11-
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trol,
to
the Line Amplifier.
3.2.3
SYNC
Amplifier
The
optional
SYNC
reproduce
amplifier
(-03 version only)
is
essen-
tially
identical
to the
Reproduce
Amp
l
ifier
just
discussed, except
that
the
head
input transformer
is
always used,
and
the
v·alue
of
R53
is
increased over
that
of
its
counterpart
R3
to reduce overall
gain. This
is
required as
the
record
head
used as a reproduce
head
has
greater
output than a
true
reproduce
head
of
similar
inductance.
3.2.4 Line
Ampl
i
fier
FET
1s
Q8,
Q9
and
Q28
perform
an
input switching function
for
the Line
Amplifier, passing
signals
from
the
Reproduce
Amplifier, input source,
and
SYNC
Amplifier,
respectively.
The
gates
of
these switching
tran
-
sistors
are
normally held
at
-
20
vo
l
ts,
introducing
an
effective
open
circuit
between the source
and
drain.
When
the
gate
is
brought to
ground
potential
either
by
the
front
panel
MONITOR
switch
or
through
the
REMO
TE connector, the source-drain resistance drops, passing the
selec~ed signal .
Transistor
Q29
and
associated
circuitry
is
part
of
the
SYNC
reproduce
option (-
03
version
only),
switching
the
line
amplifier
from
SYNC
to
INPU
T monitor
when
the
375
enters
the record
mode.
R34,
36,
ClO
and
12
are
part
of
an
active
low-pass
filter
with
cutoff
beginning
at
about
30kHz.
This ai
ds
in reducing whatever bias signal
might appear
at
the
line
output. primarily in
the
SYNC
reproduce
mode.
ICl performs
the
voltage gain function
of
the Line
Amp
l
ifier,
with
gain
established
by
feedback
resistors
R35
and
37.
Transistors
QlO,
Qll
and
associated
components
provide the output
current
required
for
driving
low
impedance
loads
and
l
ong
cab
l
es.
Protection
from
output
short
circuits
is
afforded
by
diodes
CR3
and
4.
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3.2.5
Reproduce Line Amplifier
Although not
available
as
a
factory-installed
option,
prov1s1on
is
included in the
PC
artwork
to
incorporate a second Line Amplifier
dedicated
to
the reproduce function.
Use
of
this
user-installed
option
is
relegated
to
special
applications
or
effects
generation.
3.2.6
Record Amplifier (Schematic
129100)
I~put
signals
from
the
front
panel
RECORD-GAIN
control
are
further
attenuated
by
the
RECORD
LEVEL
calibration
control,
Rl. Resistors
R2
and
R3,
capacitors
Cl
and
2
and
first
gain
stage
IC!
form
an
active
low-pass
filter
with
cutoff
beginning about
30kHz.
The
filter
removes
RF
and
other
spurious
signals
outside
the audible range
which
might otherwise cause high frequency overloading during recording. A
portion
of
the mplified input signal
is
directed
to
the Line Amplif-
ier
through
RECORD
CAL
control
R7
for__
_source monitoring.
C4
imparts a 6dB/octave
rising
characteristic
to
the imput signal as
required
for
recording pre-emphasis.
Depending
upon
the
tape
speed
and
equalization
characterisitc
used, however, only a
rise
at
very
high frequencies
or
no
increase
at
all
may
be required.
C7,
connec-
ted
in
the feedback path
of
IC2,
therefore
provides a complementary
falling
characteristic
which
cancels
the
effect
of
C4.
Record equali-
zation
trimmers
R34,
35
and
36
are
introduced
into
the
IC2
feedback
loop
when
the
appropriate
FET
switch
is
activated,
and
11
shelf
11
the
falling
characterisitc.
This
affords
control over the
inflection
point
of
the
effective
pre-emphasis curve provided
by
the
combined
network
characteristics.
Rll
and
CS
modify the pre-emphasis curve
slightly,
causing a droop
in
the
SkHz-and-above region. These
component
values can
be
changed
for
smoothest
overall
response, but
are
supplied as the
best
com-
promise
for
heads
typical
of
the
application
for
the version
order-
ed.
-13-
www.SteamPoweredRadio.Com
A
3dB
boost
at
50Hz
(required
for
the
NAB
record
characteristic)
is
effected
by
shunting
C4
by
Rl0.
FET
Q5
is
normally
off,
but
can
be
turned
on
for
one
or
_
more
equalization positions
if
IEC
or
other
11
flat-low-end
11
record curves are desired. This
is
accomplished
on
the
Power
Supply
PCB
(see paragraph 3.2.
9).
·
CR3
and
4,
IC3,
and
associated
circ~itry
forms
the
11
linearizer
11
net-
work.
When
Q9
is
turned
on
by
activating
the
LINEARITY
switch under
the adjustment cover panel, a variable non-linear
characteristic
can
be
imparted to the record signal
that
when
properly adjusted cancel~
a major portion
of
the tape-generated
third
harmonic
distortion
(see
section
4.2.1).
IC4
and
associated
components
form
the constant-current
head
driver
stage.
Relay
Kl
connects the record
head
to the output
of
IC4
in
the record
mode,
and
to the input
of
the
SYNC
amplifier for
SYNC
re-
produce (-03 version only). A bias
trap
composed
of
L2/C22,
C24
and
L3/C23
keeps
bias
from
disturbing the
head
driver
stage
or
overloading
the
SYNC
amplifier.
FET
switch
Ql0
opens the input
to
IC4,
except
when
the
375
is
in.the record
mode.
3.2.7 Erase/Bias Amplifier
The
bias signal generated
on
the
Power
Supply
PC
assembly
is
raised
to the required
power
level
for
erasure
and
bias
by
Ql, 2, 3
and
4.
Ql
and
2
raise
the
lVrms
signal to a proper level
to
just
saturate
the
class
11
C"
output stages,
Q3
and
4.
R23
and
25
control drive
and
symmetry,
respectively.
and
are factory-adjusted.
Transformer
Tl
raises the amplifier output voltage to approximately
-the_
150
~
v_o1_
ts
·p~.:p-req·
ui
re·d
for
erase
head
.dri
vfng
requirements.
CaJ?
_
acito
·r
Cl6
resorjates the erase
head
at
the operating frequency.
C25
and
BIAS
ADJUST
control
R50
couple the required bias signal
to
the
record head.
In
the case
of
erase heads
of
lower than
nominal
induct-
ance,
an
inductor,
L4,
is
inserted
in
series
to bring erase
head
-14-
www.SteamPoweredRadio.Com
inductance
up
to the
nominal
value (see paragraph
4.1.4.2)
.
3.2.8
Record
Logic
Timing
In
order
to
insure inaudible
11
punch-in 1s
11
and
11
punch-out1s
11
, the
375
must
enter
the record
mode
with the proper sequence
of
events.
IC5,
C26
and
R58
form
a Miller
Integrator
which
generates a
linear
ramp
when
the
unit
goes
into
"record."
At
the
start
of
the
ramp,
the Line Amplifier
is
first
switched
from
SYNC
to
INPUT
monitor
(-03 version only).
Next
Kl
is
energized,
transferring
the record
head
to the output
of
the record amplifier. A
short
delay
intro-
duced
by
R45
and
C19
applies the input signal to the
head
driver
stage concurrent with the relay closure.
At
this
point the bias
and
erase
field
begins to build.
Upon
leaving the record
mode,
the
opposite chain
of
events occurs.
11
Punch
-i
n
II
and
II Punch-outII
ti
me
is
symmetr
ical
at
about
100
milliseconds.
3.2.9
Power
Supply (Schematic
129200)
Triac
Yl
operates
in
conjunction with the
front
-panel
POWER
switch
to control primary
AC
power
to the
375
and
associated tape
transport,
if
deriving
power
from
the
unit.
This saves the switch contacts
from
heavy
surge currents
and
inductive loads.
The
chassis-
mounted
power
transformer applies
AC
to the bridge
rect-
ifier,
diodes
CRl
-4. A bipolar
11
raw
11
supply
of
about
+30
volts
appears across
filter
capacitors
Cl
and
C2.
ICl
is
the positive supply
regulator,
with
Ql
connected to provide
up
to
250mA,
at
+20
volts
(nominal) . Resistor
R3
and
circuitry
in-
ternal to ICl limit the available current to
this
value
and
establish
short
-
circuit
protection.
The
negative
power
source
is
referenced to
the regulated
positive
supp
l
y,
and
utilizes
Q3
and
Q4
as a
different-
ial
input
pair
driving the
compound
output stage
Q5/Q6.
CR6,
7,
and
-15-
www.SteamPoweredRadio.Com
8
and
Rl
provide current
limiting
and
short-circuit
protection.
Transistors
QlO
and
11
are connected in
an
emitter-coupled multi-
vibrator
configuration,
oscillating
at
the
erase/bias
frequency
established by the value
of
C7
and
trimmed
by
FREQ
adjustment
R33.
Q12
amplifies the
oscillator
signal
and
applies
it
to the
parallel-
resonant network Ll/C8, transforming the square
wave
into
sine
wave
form.
Transistor
Q9,
an
emitter-follower buffer
stage,
provides a
low
output
impedance
for
the
IV
rms
bias
oscillator
signal.
Trans'
istor
Q7
and
its
associated
components
form
part
of
the equal-
ization
switching system, placing the
375
in
INTER
EQ
when
no
other
command
is
received,
either
from
the
front
panel
EQUALIZATION
switch,
or
a
transport
speed switch closure.
Transistor
QB,
normally
off,
disables the reproduce (and
SYNC,
on
-03
versions) phase compensation network, except
at
the
one
speed
for
which
the network
is
optimized
and
for
which
a
strap
is
connected (see sec-
tion
4.2.2).
Similarly, the record
amplifier
pre-emphasis
characteristic
includes
a
low
frequency boost, as required
for
NAB
equalization,
which
can
be
defeated
at
one
or
more
speeds by the
installation
of
diodes as
shown
on
the schematic.
SCRl,
a
silicon
controlled
rectifier,
replaces the electro-mechanical
"record" relay usually required
by
Ampex
300-, 350-,
and
440-
series
transports.
The
SCR
is
triggered
"on"
in the
transport
PLAY
mode
when
the
front
panel
RECORD
button
is
depressed,
and
latches
until
the
transport
leaves the
PLAY
mode.
An
optical coupler
Al
and
tran-
sistor
Q2
isolate
the ground-referenced
Record
and
Erase/Bias Amplifier
logic
signal
from
the
transport
control function voltages.
-16-
www.SteamPoweredRadio.Com
SECTION
4.0
ALIGNMENT
AND
MAINTENANCE
4.1
4. 1.1
4.1.
2
Routine Calibration
Equipment
Required:
Head
Demagneti~er
Appropriate Reproducer Alignment
Tapes
Audio
Oscillator
AC
Voltmeter
REPRODUCE
4.1.2.1
Depress
INPUT
monitor button. Clean
and
demagnetize
all
heads,
moving
very slowly while the demagnetizer
is
near the
heads,
and
withdrawing
it
about a yard
from
the
head
assembly
before unplugging
it.
4.1.2.2
Depress
REPRO
monitor button
and
thread
an
alignment tape
appropriate
for
the equalization .to
which
the
electronics
is
switched.
4.1.2
.3
While
reproducing the highest frequency
on
the tape,
adjust
reproduce
head
azimuth
for
maximum
output.
4.1.2.4
If
the resonance
of
the reproduce
head
with
its
cable
is
near
the top
of
the passband (as
is
usually the case for best
signal-to-noise
performance), a
peak
in response will
be
ob-
served
at
the highest frequencies.
RI
on
the
Reproduce
Amp-
lifier
card
is
provided to
damp
this
resonance.
To
determine
whether
its
value
is
correct
for
the
head
used,
set
High
Speed
H.F.
control
so
that
5kHz
playback
is
flat
with respect to the
reference frequency tone
on
the alignment tape.
Note
response
at
the highest frequencies,
and
raise
the value
of
RI
to
in-
crease,
or
lower to decrease,
this
level.
4.1.2.5
Set
H.F.
control for smoothest response
from
reference
fre-
quency
to highest frequency.
-17-
www.SteamPoweredRadio.Com
4.1.2.6
If
alignment tape track width
is
the
same
as reproducer
track width,
set
L.F. control
for
smoothest response
from
reference frequency to lowest frequency.
If
not, as with
full-track
tapes
and
half-track
reproduce heads, wait
until
step 4.1.5.4 to trim
the
control.
4.
1.
2.7
Repeat
the preceding -
two
steps
for
the
other
speeds.
It
is
advisable to
make
final
azimuth
setting
at
the lowest speed
to
be
used.
4.1.2.8
Turn
front
panel
REPRO
LEVEL
control
fully
ccw
to
CAL
posi-
tion
and
adjust
R34
REPRO
CAL
on
Reproduce
card
for
an
indi-
cation
of
Zero-VU
while reproducing a reference tone recorded
to the desired operating
level.
If
it
is
wished to operate
into a
+8dBm
line,
remove
the jumper across the
4.7K
resistor
near the output connector before
making
this
adjustment.
4
.1.
3
4
.1.
4
SYNC
REPRODUCE
(-03 version only)
The
SYNC
reproduce function,
on
375's
so
equipped,
is
aligned
in the
same
manner
as the
normal
reproduce section described
in 4.1.2.1 through 4.1.2.7. Playback
is
from
the record
head
in
this
case,
and
there
is
no
front panel level adjustment.
ERASE/BIAS
4.1.4.1
The
Model
375
is
available with the option
of
lOOkHz
(-00
version)
or
250kHz
(-02 version) erase/bias frequencies.
The
lower frequency
is
intended
for
operation with older
metal
heads, as are
found
on
the
earlier
Ampex
vacuum-tube
recorders.
Heads
for operation
at
250kHz
must
be
of
lower inductance
and
greater
efficiency.
4.1.4.2
For
lOOkHz
operation the
375
is
designed to drive
an
erase
head
in the 1.2 to
1.8mHy
range.
At
250kHz,
the requirement
-18-
Other manuals for 375
1
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