Ampex Capitol 401 User manual

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MODEL 401
The Model
401
is
identical
to the
Made-I 400 in every respect
except the head a-s-sembly. The 401 incorporate-s --heads to-erase and
re-cord over-the full width of the tape. Thus tapes recorded on the
401 can be played back on the Model 300 and Model 201 without pre
erasing
the unused
portion of the tape and without
necessitating
level adjustment.
The Installation & Operation Manual of
the Model 400 which
follows
applies in all respects other than the half
track record
features.
Both the 400 and 401 will play back either full or half
track
recordings recorded on any AMPEX recorder.

ISSUE NUMBER 3
EFFECTIVE SERIAL #2722
AMPEX
MODEL
400
INSTALLATION & OPERATION MANUAL -
Index of
SECTIONS, CIRCUITS AND DRAWINGS
SECTION I
SPECIFICATIONS
SECTION
11 INSTALLATION
SECTION
UI OPERATION
SECTION
IV
HEAD
ASSEMBLY AND SPECIAL
PRECAUTIONS IN USE
SECTION V ELECTRONIC
ALIGNMENT
SECTION VI MAINTENANCE
SECTION
VII PARTS LISTS 4ND
STOCK NUMBERS
FIGURE 1 ELECTRONIC SCHEMATIC DIAGRAM
FIGURE 2 POWER
SUPPLY SCHEMATIC DIAGRAM
FIGURE 3 CONTROL
CIRCUITS
FIGURE
4 CONNECTION DIAGRAM FOR HIGH IMPEDANCE
MICRO
PHONE
FIGURE
5 CONNECTION DIAGRAM FOR 30 AND 50
OHM MICRO
PHONES
FIGURE 6 TOP PLATE
FIGURE
7 ELECTRONIC ASSEMBLY FRONT
PANEL
FIGURE 8 ELECTRONIC
ASSEMBLY TOP VIEW
FIGURE
9 MECHANICAL LINKAGE
3-12-51

SPECIFICATIONS - MODEL
400
Tape Speed. 15 inches per second and 7.5 inches per
second, with
motor speed change
and
equalization
switches conveniently located.
Frequency Re sponse
At 15 inches + 2db. 50 - 15,000
cycles
At 7 i/a inches
4-
2 db,
70
- 10,
000 cycles
+ 4 db.
30
- 15, 000 cycles
Signal-to-Noise Ratio. Over 55db. atboth7l/Zinchesper
second and 15 inches per second, as defined by the N. A. B. standards.
By
definition,
the
signal-to-noise ratio is the ratio of peak recording
level to- the total unweighted playback noise when erasing a signal of
peak recording level and in the absence of a new
signal.
Thus, bias
and
erase noise are included, as well
as
playback amplifier noise. All fre
quencies between 30 and 15, 000 cycles are measured. The peak record
ing level is defined as that level at which the overall input to output
total r. m. s. harmonic distortion
does
not exceed 3% when measured on
a 400 cycle tone.
Starting
T
i
me
. Instantaneous. When starting the tape accelerates
to
full speed
in less than i/io
second
because
capstan
motor
operates when
ever power is on.
Stopping Time. When playing atl5 inches per second, the tape moves
less than 2 inches after stop switch is operated.
Flutter and Wow, AtlSi.p.s. well under 2/10% r.m,s,
measuring
all
flutter components from 0 to
300
cycles using
a tone of
3,
000 cycles.
At 7 1/2 inches per second under 1/4%.
Playback Timing Accuracy. 2/10%.
Playing Time. 32 minutes at 15 inch speed with standard N. A. B. reel;
64
minutes at 7. 5
inch speed on each track. The standard R. M. A. reel
may also be used. The Model 400 Magnetic Tape Recorder
is designed to
record on
only
one-half of the standard 1/4" wide tape in accordance with
R.M.A, standards, By
turning
the reel
ovek’and recording on the other
half,
twice
the amount of program can be stored on a single
reel,
thereby
effecting a 50% saving
in tape.
Rewind Time. One and a
half
minutes
for the full N,A.. ree1
Model
4-jQ Section I Page 1

Ope ration Selector. A
single selector control lever is
used for
FAST FORWARD, PLAY,
STOP, and REWIND. A
separate record button
energizes record relay, which drops out when
machine is stopped. Shutt
ling back and forth of tape
made possible by changing from one
mode of
operation to the other.
Complete Plug-In Head
Housing, Erase, record, and play
back heads are contained in a
single plug-in head housing.
Simultaneous Monitoring, Separate
recordandplaybackampli
hers and heads allow the tape to be monitored
while recording.
Input. A switch allows
the recorder to accommodate either microphone
level low impedance input or to bridge 600 ohms plus 4 VU line balanced
or unbalanced.
Playback Amplifier. Plus 4 VU output into 600 ohms
balanced or
unbalanced.
Metering.
A
4 inch VU meter is mounted on the front panel and pro
vides for:
a.
Direct monitor of record
input
signal before or during recording.
b. Monitor of recorded output signal from playback head while re
cording or during playback.
c, Reading bias
current.
d. Reading erase current.
Phone Monitoring.
A
phone monitor jack is provided for direct
monitor of record input signal before or during recording, and monitor of
recorded output signal from playback head while recording or during play
back.
An A-B switch is incorporated in order that direct comparison can be
made between the original program and the recorded program while a re
cording is being made. The same switch transfers the VU meter for level
comparison and monitoring.
Mountings. Rack or
portable case,
Dime n s ions . Mechanical unit is mounted on 15 3/4" panel for stand
ard rack mounting. Electronic unit is mounted on 7" panel for standard
rack mounting. Power supply is on a 5 1/4’ panel for rack mounting, or
mounted in the portable case, Portable case holding both
the
mechanical
Model
400 Section I Page 2 4-11-51

unit and the electronic assembly has overall dimensions oh approximate
ly
13 3/4" high, 17 1/2" deep, and 20" wide.
Weight.
Approximately
75
pounds
for complete unit in portable
case.
Model
400 Section I Page 3 4-11-51

INSTALLATION
IMPORTANT:
Before
attempting
to
operate the recorder, first
read the
follow
ing
sections
on installation and operation.
The portable machine is shipped in a
ready-to-operate condition.
The rack mounted machine should be mounted on a
standard
19"
relay rack with the mechanical unit above the main
electronic panel. The
electronic power supply panel should be mounted directly above the mechan
ical assembly, or at least 5" below the electronic assembly.
Greater
prox
imity to the electronic assembly will result in increased
hum level. For
further
information on rack mounting, see Page 3 of
this section,
All connections are readily accessible from the front
panel.
Re
lease the front panel cover by unlatching and removing the top cover. The
numbers set off in parentheses in the following text refer to the
numbered
designations in Fig. 6
and Fig. 7.
I.
OUTPUT:
A. For Studio Line:
Plus 4
VU,
600
ohm
line output, balanced or unbalanced, is avail
able across terminals 2 and 3 of the LINE OUT connector, 12. Pin 1 is
the chassis ground. If unbalanced output is desired, tie either side of the
line
to
ground.
It is
necessary
to supply
600
ohm
termination to this output
a all times in order to maintain correct meter calibration while recording
or playing back. Therefore, if the output is not feeding a terminated line
or if the output is not
connected,
such as on remote pickups, the LINE OUT
PUT TERMINATION switch, 13, must be switched to IN.
B. For Connection to High Impedance Amplifier
Input:
Connect pin 3 of the LINE OUT connector, 12, to the high side of
the amplifier input. Strap pins 1 and
2
of the connector and connect to the
ground side of the amplifier input. The LINE OUTPUT TERMINATION
switch,
15,
must be left
in the
position designated
IN at
all times, as ex
plained in A above.
Model 400 Section II Page 1 3-26-51

II. INPUT:
The
following inputs are provided:
A. Microphones:
Any low impedance
microphone, the nominal impedance of which
is in the range
of 30
to
250
ohms
can be plugged in directly. Connect
microphone to pins 2 and 3 of the input connection,
14. Connect cable
shield to pin 1. Place the INPUT TRANSFER SWITCH,
15, in the MIC
position.
The microphone input transformer is strapped for the optimum
step up for a 150 to 250 ohm source. Therefore, in the case of micro
phones having 50 ohms or less impedance, 6 db.
additional gain can be
obtained
by
strapping
the input as
shown
in Fig.
5. This is not usually
necessary, however, and should not be done unless insufficient gain is
found to exist. If the input is re-strapped, serious frequency
discrim
ination
will exist should
the input be
fed from
a
source impedance great
er than 50 ohms.
High impedance microphones are not recommended for use on
this equipment. In general, the quality obtainable from
high impedance
microphones is not satisfactory for professional work.
In the event
that it
becomes necessary to connect a high impedance
microphone, the input
circuit
will have to be re-wired as shown in Fig. 4.
B. Bridging a Balanced Studio Line:
Connect balanced line to pins
2
and 3 of the input
connector,
14.
Pin 1 is
ground. Place
the INPUT TRANSFER SWITCH,
15, in the SAL
BRIDGE
Edsition.
Input levels of minus 10 to plus 10 VU can be accommo
dated. The load placed on the line is approximately 100, 000 ohms.
If the input transformer is
restrapped
for
50 ohms, the input
level accommodated on the balanced bridge position is minus 16 to plus
4VU.
For bridging higher or lower level lines, an external
bridge
can
be wired in the line to the machine.
This would consist of a resistance,
the value of which should be 50, 000 times ti.. RMS program voltage, in
series with each side of the line, A 50 ohm resistor should terminate
the output side of the two bridging rest . tors. The output of the bridge
must be
fed
to the microphone input
f the recorder.
C. Bridging an Unbalanced Source
Connect unbalanced line, radio tu:.r, etc. , to pins 3 and 1 of
Model 400 Section II Page 2

the input connector 14.
Pin us the ground side.
Place
the INPUT
TRANSFER
SWITCH, 15, in the UNBAL. BRIDGE position. This con
nection
,rovides
100,
000
ohms bridging
of
any RMS program voltage
grcater
than i/a volt.
III. PHONES:
Any sensitive high impedance
head phones can be plugged in the
jack 11, provided for monitoring of incoming line or
playback of the
re
cording.
IV, RECOMMENDED
RACK MOUNTING OF TWO MACHINES:
If two 400s or 4Ols are
to
be
mounted on a standard rack, the fol
lowing placement is recommended starting
at the
top of the
rack:
1. Install a
1
3/4
inch spacer panel at the top of the rack.
2.
Install below this panel the Mechanical
Assembly for machine
#1.
3. Install the
Electronic Assembly for machine
#1.
4.
Install a 1 3/4 inch or 3 i/a
inch spacer panel.
5.
Install the Mechanical Assembly
#2.
6, Install the Electronic
Assembly #2.
7.
Install a 5 1/4 inch
or
7 inch
spacer panel.
S. Install both
power supply panels at the bottom.
The cable on
the power supply for the upper
.recorder will have
to be :vtendcd
in length.
It is important that no power supply be closer
than 5 inches to
an Electronic Assembly!
Model 400 Section
II Page 3 3-26-Si

OPERATION - MODEL
400
I. REEL SIZE:
The turntable on the left side tape supply is
equipped to handle either the N. A. B. 10 u/a" reel of tape or the smaller
R. iv!. A. 5 and 7 inch sizes. Handling these different type
reels on this
turntable can be accomplished as follows:
1
N, A. B.
- 10 i/a" Reel - Place the Reel Size Switch 6 in
the Large position and place the reel on the turntable after positioning
the black plastic reel centering guide
that has been provided.
2 R.M.A. - 7 & 5" Reels - Place the Reel
Size
Switch 6
in
the
Small position
and
place the reel
on
the keyed turntable directly.
The turntable on the right side takeup reel will handle only the
N. A. B. 10
1/2"
reel and
the keys
have
not been provided on the turntable
to
drive the small reels. Attempting to use the small
R.
M. A. reels on
the right hand takeup turntable will seriously impair the performance
of the machine.
II.
TAPE THREADING: Thread tape as indicated in Fig. 6. The
takeup tension arm, 5, must be moved into position as shown, in order
to supply power to operate the machine.
III. POWER:
Power is supplied
to
the top plate through switch, 3,
which must be turned on to energize the motors, Main power is supplied
through switch,
9,
which must be turned on to operate the electronics
and top plate. The top
plate
and electronics are individually fused by
7 and 8, respectively.
IV. FUSES: Top
Plate Fuse
7
2 Amperes
Chassis Fuse 8 1 Ampere
V. SPEED SWITCHES: There are two switches associated with op
erating speed. Switch 4 governs
the motor,
and
19
corrects
equal
ization in the amplifiers.
VI. TAPE MOTION: The tape motion is controlled by means of a
mode selector switch 1 and the
PLAY buUei
2, directly to its left.
The selector switch should be moved firmly by means of the selector
knob attached, Hold the knob securely between the thumb and forefinger
while changing its position. Rapid, positive motion of the selector knob
is
recommended
in
preference
to slow action, This is to insure simul
taneous action of all necessary functions.
Model 400 Section
III Page 1 3-12-51

- IMPORTANT -
The trip
out switch will not disengage
the capstan idler at the
end of the reel unless the tape will
freely disengage the reel hub.
All
new factory
wound reels of tape
should be unwound and inspected,
as
they are usually looped to
the hub in a manner such that
they wiii not
come free. If
the tape does not disengage the
reel, the drive capstan
will wear
a
flat in the rubber capstan idler
wheel which will necessitate
the wheel’s replacement. Any adhesive material
accumulation
on the
hub must be removed with solvent
for the same reason,
1. Play and Record, The tape is set into play
motion
by
first depressing the PLAY button, 2, and then moving
the selector
switch, 1, to the FORWARD position. It is important
that the PLAY
button be depressed before moving the selector switch or the tape will
move at the fast forward spee;d instead of at the operating
speed. The
tape must be
completely
stopped
before starting in this mode. The se
lector switch should be moved firmly and rapidly to insure proper opera
tion.
2,
Stop. To stop
the tape motion, while it is moving in any
mode, merely return the selector switch, 1, to the OFF position, This
will occur automatically if the tape should break or run off of either reel,
CA U TI ON:
Never stop the machine
near
the end
of a
fast wind
when using the small reel, but allow the
tape
to run
off,
When the amount
of
tape remaining on the unwinding reel is less than 1/2"; the rate of ro
tation
of the reel becomes so great that it may not stop without throwing
a slack loop.
3, F a s t For w a r d, Move the selector switch,
1, toward
the
left without depressing the button, This
is a
fa?t forward
speed
which
is
used
in order to rapidly arrive at a point within the reel,
4, Rewind, Move the selector
switch, 1, toward the right.
This is a fast reverse and is used to rewind the tape completely
or
to
return
to an earlier sectior, using either the fast forward or rewind
mode, it is desirable to
remove the tape from direct contact with the
heads
by opening the gate of the head assembly, This is to prevent oxide
coating from depositing on the heads and
impairing the
performance,
and
reduces wear on
the heads,
In
putting
the recorder into play or fast forward immediately after
fast rewind, slack
in the tape may occLr causing the recorder to trip
off, In this case, return the mode .elector to "off", take
up
the slack
in the tape, and the machine
will
function
normally. The condition is
brought
about by the turntable motor spinning free for
a
short time in
Model 400 Section III Page 2 4-11-51

the
direntinn
it was
last rimning. The operator will find a number of
ways to avnisid.eiay.dne
to
this
condition. One of the simplest js to
move the mode selector a few
degrees in
the
direction the machine was
last
running
before, proceeding in the
desired mode.
VII. PLAYBACK: To play back a previously recorded tape, turn the
MST.ER and OUTPUT switc.h, 20, to the extreme left position designated
PL&Y.BAilIC. Then start the tape in motion as indicated under Play. If
the recording was made at the correct level, the VU Meter will peak on
o
zero.
No attempt should be made to alter
the
gain
of the playback
amplifier if the meter does not peak
on 0
zero. Otherwise, the cali
brationaf this meter as a record level indicator will be destroyed. The
playback gain will be close when playing any tape recorded to N.A. B.
specifications at l5?/sec.
VIII, RECORD: To record a new program on previously recorded tape,
or on blank tape, turn switch 20
to
the second position from the left which
is
designated RECORD-LEVEL. Turn the Record Gain control, 16,
clockwise until the level reads 0 zero on the VU meter on the most in
tense program peaks. The program can be audibly monitored through
either the ear phones jack, io, or the LINE OUT before the tape is in
motion, For correct meter calibration it is important that the LINE OUT
be properly terminated either external to the machine or by use of the LINE
OUTPUT TERMINATION switch, 13. See detailed instructions under
INSTALLATION. This direct monitor feature allows the program to
be
set
up through the machine without actually recording during the set up period.
When the program level is properly set, start the tape in motion
as indicated under
PLAY.
Then
push
the
RECORD button, 18. Be sure
there is a delay of
at
least
i/a
second before operating the
RECORD button,
as the surge from operating the
PLAY
control may magnetize the record
head, thereby increasing the noise level on the tape. The record indica
tor9 17, above the record button will now glow and the machine is recording.
It is desirable to
bheck the record bias and erase currents occas
ionally. In order to do this, reposition switch,, 20, to the positions desig
nated BIAS and ERASE, respectively, The erase is not cri’tical and should
read in the upper half of the meter scale, The bias on this machine, serial
#2701 and after, should read between - 1/2 and Fl/a on the VU scale, The
bias is somewhat critical and must be kept within the indicated
range
in
order to record the higher frequencies at 7 1/? inch tape speed.
The bias is adjusted by
means of
the
locked control, 11. The
meter calibration for bias measureme::it can be checked as indicated in
electronic alignment.
An
actual comparison of the recording with the direct program
Model 400 Section Ill Page 3 2-16-51

can be made both audibly and on the VU meter by
moving
the
selector
switch, 20, to the extreme left position for
playback
monitor,
and
then to the secondfrmn the left position for
monitor of
the
unrecorded
program. The levels as read on the meter and the quality as heard
will
be the same within the t,oieranceatof the machine. It should be
noted that
some
change in
level as
read on
the
VU
meter while switching from A to
B will be experienced if a tape of different manufacture than the one used
to set up the machine is used,
When the program being recorded is finished, return switch, 1,
to the central OFF position. The tape transport will stop and the
record
relay
will drop out, making the, record circuits
inactive.
The
record cir
cuits will
remain inactive until the record button is
again depressed while
the tape
is
moving in the
play
mode.
NOTE:
Because of the extremely wide frequency range of the VU meter,
some pickup of the erase and bias frequency will be experienced while
monitoring the record or playback level while recording. The pickup
of
this
oscillator frequency approximately 90 Ic. can be read on the VU
meter
approximately
15
Db. below the program level, but does
not inter
fere in
any
way with the performance
of the machine.
Model 400 Section III Page 4 2-16-51

HEAD HOUSING
The head housing, see Fig. 6, is a die cast assembly which
contains the three heads used in the recording process. The heads
are respectively erase, record, and playback as viewed from left
to right when facing the machine. The gate on the housing
holds
the
playback and record shield covers
and the tape-lifting
fingers.
The
function of
the
tape-lifting fingers is
to
remove
the tape
from the
heads when the gate is open during
rewind
or fast forward position.
This reduces head wear considerably. The tape may leave a de
posit on the heads if allowed to
contact
them at high speeds. Such
a deposit will seriously impair
the
performance of the machine and
should be guarded against
by
always
opening
the
gate on
fast forward
and rewind. If a deposit is left, it
may be
easily removed with
carbon tetrachloride on a soft rag. Never use metal of any kind to
touch the head surfaces.
Model 400 Section IV Page 1

HEAD
ALIGNMENT
Remove the top cover from the head housing by
removing the
two screws from the top, and pulling cover gently back
and up.
Looking at the head housing from the
front, the three heads
from left
to right are: erase, record, and
playback.
The azimuth angle of the erase head requires no adjustment,
and should not be touched.
The record and playback heads
should
be
aligned only after
reading and fully understanding the procedure under ELECTRONIC
.A.LIGNMENT.
The actual physical alignnuit of the record and playback heads
consists of placing a 1/4" spintite socket wrench on the left hand elastic
stop nut on each head and adjusting back and forth until the
proper azi
muth angle is arr.ived at. This is accomplished by first
playing the
standard tape and adjusting
the
playback head until maximum response
is obtained on the high frequency adjustment tone provided at the be
ginning of the standard tape. The record head is then aligned with
the
playback head by recording 15, 000 cycles on a blank tape, and
adjusting
the record head for maximum playback output.
Model
400 Section IV Page 2

HEAD DEMAGNETIZATION
Occasionally the heads become magnetized through
some electri
cal fault which may occur in the amplifiers or by coming into
physical
contact with a magnetized object. In
order to demagnetize these heads
completely in such events, a demagnetizer should be
used.
A
demagnetizer
suitable for this purpose is manufactured by AMPEX as an accessory item.
The demagnetizer is used by opening the gate of the head housing
and allow
ing the demagnetizer to come into contact with each head, The tip is split
at the end. This split section should be placed across the gap in the center
of each head. Move the tip up and down across the head surface slowly.
Remove the demagnetizer very slowly, allowing the A. C. field to die
off
gradually. Repeat
this operation on all
three heads,
In the event
demag
netization is not effected, repeat the process.
The capstan or tape guides may become magnetized by contact
with
a
magnetized
tool. Should this occur, the magnetized part may be
demagnetized with an A.
C.
solenoid placed over the part and slowly pulled
away.
Model 400 Section
IV
Page 3

ELECTRONIC ALIGNMENT
The amplifiers are set on standardized
voltage curves at the
factory
and
thoroughly
pre-checked.
The following
alignment
procedure is recommended for check
ing electronics alignment
in the
field,
No
further adjustments should be
necessary, however, unless alterations occur in shipment, or as re
quired
in routine maintenance.
A standard tape is
available for alignment purposes
and contains
the following frequency run recorded
at
15 inches per second, 10 db.
below 15 inch operating level.
1 Kc. for level adjustment
15 Xc.
for playback azimuth alignment
50 Cycles
100
200
400
800
1600
3200
6400
8000
9000 ii
10000
11000
12000
14000
tt
15000
I.
ALIGNMENT
OF PLAYBACK CIRCUIT:
A. Slide out
the electronic assembly from the
cabinet
far enough
so that
adjustments can be made on the
controls located on the chassis.
B. Set the TAPE
SPEED
and
EQUALIZATION SPEED switch to
15 inches
per second.
C. Put the METER
& OUTPUT switch, 20, to the PLAYBACK-
LEVEL position.
D.
Terminate the line output
either by
means
of the LINE OUT
TERMINATION switch,
11, or by external termination of
600
ohms.
Model 400 Section V Pge 1

E..
Connect amplifier
and loudspeaker to the output or
plug in
a
pair
of high impedance head phones so that voice
announcements can be
heard
on the standard tape.
F. Thread the standard tape
on the machine and set the tape
in motion in the PLAY position.
0. Adjust the 15 inch playback level control,
R-139,
Fig.
8,
for a.rea.ding of
- 10 on the VU meter on the 1000 cycle tone.
H. Adjust the playback
head azimuth as discussed in Section
V.
if necessary.
I. .Adjus.t the high frequency
playback equalizer, R-131, Fig. 8,
for
flat response.,
+
2 dli.
to
8000 cycles
on
the standard
tape. The re
sponse beyond
800Ocycles should slowly rise to approximately plus
3 db.
at 15,
0.00
cycles.
J.
Turn the front
panel EQUALIZATION SPEED switch, 19,
to the 7. 5 position.
K. Re-tun the beginning of the standard tape and by means of
the 1000
cycle test tone, adjust the 7 1/2 inch playback level control,
R-140, Fig.
8, for - 10 on the VU meter, The tape should be running
at 15 inches per
second.
The above will satisfactorily align the playback circuit for
opera
tion at both speeds.
II. ALIGNMENT OF THE RECORD
CIRCUITS:
The 7 i/a" and 15"
per second record alignment should not be attempted until the playback
is properly aligned.
Perform
the following in
the order
indicated:
A. Bias Adjustment:
1. Output of playback amplifier must be properly terminated
as in playback
alignment.
2. Set the speed switches, 4 & 19, to 7 i/a inches per
second.
3. Thread blank tape on the machine and set the tape into
motion
in the PLAY
position.
4. Connect an audio oscillator to the input connector, 14,
set
15 for
unbalanced
input,
and adjust the frequency to 2000 cycles.
Model 400 Section V Page 2 2-16-51

5. Push the record button, 18.
tr.
fl
6. Adjust bias control on front
panelkfor
maximum playback
level of 2000 cycle tone while recording and playing back. If the output
reads within
2
dli.
of the maximum, the
distortion characteristic will not
be altered, It is desirable to be within i/a db. of this maximum in order
to achieve wide frequency range recording at 7 i/a" per second.
B.
Noise
Balance:
A.
noise balance control is provided
to
eliminate excessive low
frequency noise and null second harmonic distortion. The noise balance
should
not be touched unless all heads have been thoroughly demagnetized
with an AMPEX Head Demagnetizer
or equivalent. See Section IV.
If noise of a crackling nature is still found to exist in the output of the
machine, connect a 1 rnfd. condenser across the output of the machine
and adjust
the noise balance control, R-123, for minimum record noise
as read on
a
sensitive meter or
heard in a loudspeaker.
C. 7 i/a inch Record Equalization Adj us tment:
1. Reset oscillator to 400
cycles.
2. Set record gain, 16, so that VU meter connected on
playback reads
- 10.
3. Adjust record head azimuth as discussed in
Section IV.
if necessary.
4.
Reset oscillator to 8000 cycles.
5, Adjust 7 1/2 inch record equalizer,
C-b?, Fig. 8, for
-10
on playback VU meter.
6, Frequency response should be + 2 db. from 70 to 8000
cycles. Due to tape saturation,
frequency ctecks cannot be made at
7 i/a inch tape speed, at this level,
beyond 8000 cycles. In order to run
respdnse
checks beyond 8000 cycles,
a
more sensitive
meter such as the
Hewlett Packard Model 400 Vacuum
Tube Voltmeter, must be connected
to the machine output.
The response run should then be run at least 20 db.
below
operating level or - 15 dbm, as read at the output
terminals. A flat
amplifier preceding
a
standard
VU meter n-nght be used to supply the nec
essary sensitivity in absence of a
more sensitive meter,
D. 15
inch Record Equalizer:
1. Set speed switches 4 and 19 to
the
15"
positions,
2. Reset oscillator to 400 cycles.
Model 400
Section V Page 3 4-11-51

3. Set record gain so that VU meter on playback reads -10.
4.
Reset
oscillator
to 8000 cycles.
5. Adjust 15 inch record equalizer, C-l08, for -i tn
playback VU meter.
6. Frequency response should be
1-
2 db. from 50 to
15,000 cycles.
E. Erase Adjustment:
Erase is factory adjusted.
No
readjustment
is
necessary.
F. Record Level Meter Calibration:
The control see
Fig. 8
is
adjusted such that the
program
level as read on the
VU
meter is the same as monitored from the play
back head or the incoming line. This is accomplished
by
recording a
400 cycle tone or program
in
the
absence of an audio oscillator at 0
zero
on the VU meter with the METER
&
OUTPUT switch in the PLAY
BACK-LEVEL position. Then switch the METER & OUTPUT switch to
the
RECORD-LEVEL position and adjust the RECORD LEVEL METER
CALIBRATION control Fig. 8 for 0 zero on the
VU
meter.
0. Bias Meter Calibration:
The
bias meter
calibration is adjusted as follows:
After
the bias is adjusted in A above, meter the bias in
the
normal fashion and
adjust the meter shunt, R-l50, for 0 zero reading on the
VU
scale of
the meter. R-150 is located in back of the METER & OUTPUT switch.
Model 400 Section V Page 4 2-16-51