HP 7970B Service manual

~--------------------HEWLETTiPPACKARD--------------------~
07970-90383
OPERATING AND SERVICE MANUAL
PART 2
7970B/7970C
DIGITAL MAGNETIC TAPE UNIT
TAPE TRANSPORT
Printed-Circuit Assemblies:
07970-60481, Series 1101
07970-61010, Series 1323
07970-61020, Series 1229
07970-61080, Series 1042
07970-61150, Series 1049
07970-62172,Series1322
07970-62173, Series 1329
07970-62086, Series 1013
07970-62089, Series 1206
Copyright HEWLETT-PACKARD COMPANY 1972
11000 Wolfe Road, Cupertino, California 95014, U.S.A. PRINTED: AUG 1973

7970B/7970C Description
SECTION
DESCRIPTION
1-1.
INTRODUCTION.
1-2. This section describes the transport for the Hewlett-
Packard 7970B/7970C Digital Magnetic Tape Units and pro-
vides a functional description of the transport circuits. Speed
critical assemblies are defined and optional assemblies are
described.
1-3. PHYSICAL DESCRIPTION.
1-4. The tape unit consists of a transport casting as-
sembly and a sheet metal housing assembly. The housing
assembly contains the power supply, the power distribution
assembly, the control and status assembly, and interfacing
provisions. The read and write modules are also contained
in the housing assembly.
1-5. The transport casting assembly consists
of all tape
moving assemblies, reel motors, capstan servo motor, ten-
sion arm assemblies and the circuits associated with these
assemblies. The magnetic tape head assembly, photosense
assembly, write interconnect assembly, and read preampli-
fier assembly are also mounted to the transport casting as-
sembly. All standard and optional switch assemblies are
mounted
to the transport casting assembly. All assemblies
are accessible and
all circuits will operate with the transport
assembly open.
1-6. SPEED CRITICAL ASSEMBLIES.
1-7. The tape unit will operate at tape speeds between
10 ips and 45 ips. The actual tape speed of the unit is speci-
fied when the unit is ordered. The following are limited or
critical
to tape speed:
a. Reel Motor Assembly.
b. Capstan Motor Assembly.
c. Capstan Servo
PC Assembly.
d.
Magnetic Tape Head Assembly.
e. Read Modules.
f. Write Modules.
1-8. REEL MOTORS.
1-9. Reel Motors B1/B2, part number07970-60170, are
used on tape units with tape speed ranges between
10 ips
and 37.5 ips. Reel Motors B1/B2, part number07970-60171,
are used on tape units with tape speed ranges between 37.6
and 45 ips.
Part 2
1-10. CAPSTAN MOTOR ASSEMBLY.
1-11. Capstan Motor Assembly MG1,
part number07970-
60140, is used on tape units with tape speed ranges between
10 ips and 37.5 ips. Capstan Motor Assembly MG1, part
number 07970-60141,
is used on tape units with tape speed
ranges between 37.6 ips and 45 ips.
Note
Capstan Motor Assembly MG1, part number
07970-60140, may be used on tape units oper-
ating at 45 ips. However, program restrictions
must be maintained.
1-12. CAPSTAN SERVO PRINTED-CIRCUIT
ASSEMBLY.
1-13. The capstan servo PCA
is equipped with a dummy
connector
that functions as a strapping network. The
dummy connector
is installed in one of six positions, de-
pending upon the speed range of the tape unit and capstan
motor assembly used. See figure 7-10, Capstan Servo PCA
A9 Schematic Diagram, for details of dummy connector.
1-14.
CAUTION
Ensure that dummy connector is installed in the
proper position prior
to applying power to the
unit. Attempting to operate the unit without the
connector installed
or with the connector in the
wrong position may result in damage
to the unit.
MAGNETIC TAPE HEAD ASSEMBLY.
1-15. The magnetic tape head assembly consists
of tape
guides, a tape cleaner, and, depending upon configuration,
read, write, and erase heads. The tape head assembly
is speed
critical; refer
to section VI for part numbers and speed ranges
of the magnetic tape head assembly.
1-16. READ MODULES.
1-17. The read modules are described in detail in part 3
or part 5 of this manual.
1-18. WRITE MODULES.
1-19. The write modules are described in detail in
part 4
of this manual.
1-1

Description
1-20. BASIC AND OPTIONAL ASSEMBLIES.
1-21. The tape unit is available in a variety of configura-
tions. Regardless
of tape speed, the following assemblies are
basic and applicable
to all tape units.
a. Transformer Assembly.
b. Photosense Assembly.
c. Tension Assemblies (two).
d. Capstan Assembly.
e. Reel Motor Assemblies (two).
f. Reel Servo Printed-Circuit Assembly.
g. Capstan Servo Printed-Circuit Assembly.
h. Control and Status Printed-Circuit Assembly.
1-2
7970Bj7970C
i. Power Distribution Assembly.
j. Power Regulator Printed-Circuit Assembly.
1-22. The control switch assembly
is a basic assembly;
however, if the tape unit is equipped with write data cir-
cuits, the control switch assembly wHi contain the write en-
able indicator.
1-23. The following assemblies are optional and, depend-
ing upon the use
of the tape unit, mayor may not be a part
of the tape unit.
a. Write Enable Assembly.
b. Density Select Assembly.
c. Unit Select Assembly.
Part 2

7970B/7970C Theory
of Operation
SECTION II
THEORY OF OPERATION
2-1. INTRODUCTION.
2·2. This section provides an overall functional descrip·
tion of the tape transport and description of the transport
circuits. Block diagrams are included
to aid in understand-
ing the operation
of the transport circuits.
2-3. OVERALL FUNCTIONAL DESCRIPTION.
2-4. The tape transport controls the movement of mag-
netic tape, provides power for read and write data circuits,
and supplies status signals
to the interface. Off-line com-
mands (High-Speed, Reverse, Forward, Rewind, and Off-
Line) are generated by the interface. The off-line and on-
line commands are processed by the control and status cir-
cuits and result in controlling signals for the capstan servo,
reel servo,
and data circuits. Status signals for the interface
and front panel indicators are also provided by the control
and status circuits. Figure 2-1
is a functional block diagram
of the tape transport circuits.
2-5. At initial power-on, the capstan
motor circuit is
open, and the reel servo motors are shorted.When the LOAD
switch is pressed, the capstan motor and reel motor circuits
are completed. When tape tension
is established, the tension
arms swing away from the tension limit switches. With tape
tensioned, the capstanand reel
motor returns are maintained.
When the LOAD switch
is released, the control and status
circuits initiate a load
point search. During load point search,
the reel servo circuit operates with voltage feedback and the
capstan servo pulls tape forward
at 20 ips.
2-6. When the load
point tab is detected by the photo-
sense assembly, the control and status circuits terminate the
load point search, and tape motion stops. The control and
status circuits provide a load
point status to the interface,
and the
front panel LOAD indicator illuminates.
2-7. Pressing the ON-LINE switch establishes interface
control
of the tape unit. Except for RESET, the front panel
controls have
no control of the tape unit. Pressing RESET
releases the interface control and allows front panel control
of the tape unit.
2-8. When the tape unit
is under interface control and
data
is being processed, the tape unit pulls tape at synchro-
nous speed
as required. The reel servo circuits operate with
voltage feedback and tension
is maintained. When the end-
of-tape tab is detected, the control and status circuits pro-
vide the interface with an end-of-tape (EOT) status.
Part 2
2-9. A Rewind command generated by
the interface
will initiate the rewind sequence. The control and status
circuits place the capstan servo circuits
in a high-speed re-
verse mode and switch the reel servo feedback circuits. The
capstan rewinds tape
at 160 ips and the reel servo operates
with current feedback. When the trailing edge
of the
beginning-of-tape (BOT) tab
is detected, the control and
status circuits terminate
the rewind mode, and after a one-
second delay, establish a load point search mode.
2-10. Pressing RESET releases the tape unit from inter-
face control. Pressing REWIND in this condition rewinds
the tape
on the supply reel. When the tape is run-off of the
take-up reel, the tension arms
contact the tension arm limit
switches, the capstan
motor circuit is broken and the reel
motors are shorted. This provides dynamic braking
of the
reel motors.
2-11. CIRCUIT DESCRIPTIONS.
2-12. The following paragraphs describe the power sup-
ply circuits, controlandstatus circuits, capstan servo circuits
and reel servo circuits
of the tape transport. Block Diagrams
of the circuits are provided as an aid in understanding the
operation
of the circuits. Refer to section VII for detailed
schematic diagrams
of the tape transport.
2-13. POWER SUPPLY AND POWER DISTRIBUTION
CIRCUITS.
2-14. The power supply
of the tape transport consists of
a power transformer, three center-tapped bridge rectifiers,
filter capacitors, and bleeder resistors. (See figure 7-2.) The
primary circuit
of the power supply transformer includes a
switch
that allows selection of 115 or 230 Vac operation, a
power on-off switch, and a line filter/power connector_
When 115 Vac power
is selected, the two primary windings
of the power transformer are in parallel and fuse F1 pro-
vides overload protection. When 230 Vac power is selected,
the two primary windings
of the power transformer are in
series, and fuse F2 is placed in series with the primary wind-
ing to provide overload protection.
2-15. The power distribution circuits are
on the power
distribution PCA. The assembly contains secondary fuses
and connectors for power distribution. The assembly also
contains a printed-circuit connector for the power regulator
printed-circuit assembly. Silicon diodes in the ±20 volt cir-
cuits and the ±12 volt distribution circuits provide circuit
protection.
2-1

Theory of Operation 7970B/7970C
SUPPLY REEL MOTOR
M
SUPPLY REEL
TENSION ARM
REEL SERVO
bl
CIRCUITS
TAKEUP REEL MOTOR
M
TAKEUP REEL
TENSION ARM
TENSION I
SWITCHES t I
I
FEEDBACK I
CONTROL I
OFF-LINE
I
CAPSTAN MOTOR
COMMANDS I
(FRONT PANEL!
CAPSTAN M
·1
STATUS
MOTION CONTROL I
SERVO
(FRONT PANEL!
CONTROL
CIRCUITS
'1
1
AND STATUS I I
ON-LINE
CIRCUITS
COMMANDS TACHOMETER I I
(INTERFACEI
1 I
I I
STATUS
'-.J
(INTERFACEI
TENSION/TENSION LATCH
PHOTOSENSE BOT/EOT
ASSEMBLY
7970 508
t I
CONTROL AND STATUS
r-~,
I DATA CIRCUITS I
I
(SEE PARTS I
3,4 OR 51
L __ -.J
Kl
(LOCATED ON
REEL SERVO PC
AI
Figure 2-1. Tape Transport Functional Block Diagram
2-16. Unregulated power from the power supply
is dis-
tributed to the power regulator printed-circuit assembly and
to heatsink-mounted power transistors. Unregulated ±40
volts (57.5 volts nominal) from the power supply is routed
through heatsink-mounted resistors to the power regulator
printed-circuit assembly. Regulated power (+5
V, +12V, and
-12V) from the regulator-controlled transistors (heatsink-
mounted) are distributed to the transport circuits and data
circuits by the power distribution printed-circuit assembly.
2-17. Unregulated ±20 volts (22.5 volts nominal) from
the power supply
is provided for the reel servo circuit. Steer-
ing diodes CR1 and CR2 allows the ±20/40 volt lines to
switch from 22.5 volts dc to 57.5 volts dc.
2-2
2-18. POWER REGULATOR CIRCUITS.
2-19. The power regulator printed-circuit assembly con-
tains a + 12 volt regulator circuit, a -12 volt regulator circuit,
and a +5 volt regulator circuit. (See figure 2-2.) The power
regulator printed-circuit assembly also contains a reel servo
voltage switching circuit and a delay circuit. These circuits
are described with related circuits.
2-20. +12 VOLT REGULATOR. The +12 volt regulator
uses an integrated circuit voltage regulator with an internal
temperature compensated voltage reference. Regulation
is
obtained by dividing the output voltage (R3, R4, and R5)
Part 2

7970B/7970C Theory
of Operation
+40V (NOM) +40V UNREG
-40V (NOM) -40V UNREG
+20V UNREG
-20V UNREG
TRANSFORMER +20V (NOM)
ASSEMBLY
~&
+12V REG
-20V (NOM)
~:9
-12V REG
+12V (NOM)
~&
+5V REG
J /1-
~
+
'"'"
REF
+5 VOLT -12 VOLT +12
VOLT
REGULATOR REGULATOR REGULATOR
~
~
7970·509
Figure 2-2. Power Supply Voltage Regulators, Simplified Diagram
and comparing the divided voltage with the internal refer·
ence. The
output voltage of the +12 volt regulator is adjust·
able by variable resistor R4. Series pass transistor
Q2 (lo-
cated on a heatsink external
of the regulator) is protected
by current limiting. The current foldback knee
is set to ap-
proximately 2.8 amperes by R1 and R2. Short circuit cur-
rent is set to approximately 1.3 amperes by R7 and R8
(located on heatsink external
to the regulator).
2-21.
-12 VOLT REGULATOR. The -12 volt regulator
uses an integrated circuit voltage regulator. The reference
for the
-12 volt regulator is derived from the regulated +12
volt source. Current limiting
of the -12 volt regulator is set
at 1.2 amperes as determined by R9 and R20. Diode CR14
protects the -12 volt regulator against excessive common
mode voltage. Diode CR17 limits the
output voltage to -14.7
volts in the event that the +12 volt reference
is lost.
2-22.
+5 VOLT REGULATOR. The +5 volt regulator
uses an integrated circuit voltage regulator. The reference
for the
+5 volt regulator is generated from the +12 volt regu-
lator
output using R23, R25, and R26. Current limit of the
+5 volt regulator is approximately 4.0 amperes controlled
Part 2
by current sense resistor R6, located external
of the regu-
lator on a heatsink. In the event
of an over-voltage, silicon
controlled rectifier CR1 conducts and shorts the +5 volt
supply. The 4 ampere short circuit current will blow fuse
F5 which
is in series with the +10 volt unregulated supply.
2-23. CONTROL AND STATUS CIRCUITS.
2-24. The control and status circuits process commands
from the front panel controls and interface and generate
controlling signals for the tape transport and data circuits.
The control and status circuits also generate status signals
for the interface and front panel indicators.
2-25. After power
is applied, the tape is not ten-
sioned, limit switch
W2S1 is open and the capstan and
reel servo circuits are disabled (figure 2-3). The capstan
and reel servo circuits are disabled because relay
K1 is
deenergized and because ground voltage is supplied to
the Tape Tension signal line through the B contacts of
deenergized relay Kl. Ground voltage supplied to Q4,
on the control and status
PCA, results in an inactive
output from
the LDFD FF to the capstan servo circuit.
2-3

Theory of Operation
Also, ground voltage supplied
to Q22 on the capstan
servo
PCA holds switch Q20 on the capstan servo PCA
closed, grounding the input to motor drive amplifiers
Q2, Q4, and Q6. Ground voltage applied
to Q19 on the
reel servo amplifier PCA, by the Tape Tension signal
and through the normally closed contacts
of the LOAD
switch, cause it to close switches Q16, Q17, and Q20
on the reel servo amplifier PCA. With the inputs to the
reel servo amplifiers grounded,
the reel motors are dis-
abled. Relay
K1 is also kept deenergized through the
normally closed contacts
of the LOAD switch and Q13
on the reel servo amplifier PCA. When the LOAD
switch is pressed, +5 volts is supplied through R9 and
R103 to activate the Tension signal to Q13 and Q19 on
the reel servo amplifier
PCA. The high input to Q13
energizes relay K1. (Once energized,
K1 remains ener-
gized, through R109, CR11, and Q13, until tension is
lost.) Pressing the LOAD switch also applies a high
input to Q19
on the reel servo amplifier PCA. Nand
gate Q19 holds Q16, Q17, and Q20 open if either
of its
inputs are high. With
K1 energized, both inputs to Q19
are high causing switches Q16, Q17, and Q20
on the
reel servo amplifier
PCA to open. Thus, both require-
ments for enabling
the reel servos (K1 energized and
switches Q16 and Q17 open) are met and the reel
servos tension the tape. Thus, with K1 energized and
Q20 on the reel servo amplifier PCA open, Q22 on the
capstan servo
PCA (through Q21) opens Q20 on the
capstan servo
PCA and enables the amplifier. With tape
tension established, tension arm switch
W2S1 closes,
supplying a high input
to terminal D of the Load For-
ward FF and to Q19 to the reel servo amplifier PCA.
2-26. When the LOAD switch is released, the load point
search sequence
is initiated. Load Latch (U1A and U2A) is
set. The output of the load latch is gated with REW (false)
to clock the Load Forward flip-flop (U12A). The LFWD
signal
is gated with DELAY to provide the capstan servo
with the load point search command.
2-27. The DELAY signal
is generated by a delay circuit
located onthe power regulator printed-circuit assembly. The
delay circuit provides a one-second delay when changing
from a high speed operation
to a synchronous or load point
search mode.
2-28. When the load point tab
is detected (BOT), the
Load Forward flip-flop
is cleared by LP through U16D and
U13B. Pressing RESET will also clear the Load Forward
flip-flop.
When Load Forward is cleared, tape load point
search motion
will stop. The edge transition of LDFD clocks
Load Complete flip-flop U12B. Except for a rewind condi-
tion, Load Complete remains set during normal operation
of the tape transport.
2-29. Rewind flip-flop U6B
is cleared by pressing the
REWIND switch when the unit
is off-line, or by inter-
face command REW when the unit is on-line. When the
Rewind flip-flop
is cleared, the Rewind Status flip-flop
(U4B and U3A) is set. This provides rewind status SRW
2-4
7970B/7970C
to the interface and rewind status to the control switch
assembly
to illuminate the REWIND indicator.
2-30. When the trailing edge
of the load point tab is de-
tected during rewind the Load Point flip-flop is set and the
Rewind flip-flop
is set through the rewind clock input (U6B-
11). Load point also gates U3B to prevent rewind commands
from clearing the Rewind flip-flop. With the Rewind flip-
flop set, the Load Forward flip-flop is set through gate U13A.
This starts the load point search mode.
2-31. The control and status printed-circuitassembly con-
tains a unit select network that allows up to four units to be
controlled by one interface. The network consists
of a
jumper
(W1) and 5 connecting pins (OFF, 0, 1, 2, 3)_ The
position
of the jumper determines the unit to be selected by
the interface.
If the units connected to the interface are
equipped with the unit select switch option, the jumper
must be in the OFF position. With the jumper in the
OFF
position the select match circuitry of the control and status
printed-circuit assembly
is disabled. If the tape units are not
equipped with the unit select switch option, the jumper
must be connected
to positions 0 through 3 depending upon
unit designation.
2-32. The unit select command from
the interface
(CSO, CS1, CS2 or CS3) is gated with ON-LINE to
generate SOLA and SOLB (selected and on-line). The
SOLA condition allows status signals to be supplied to
the interface. The SOLB condition is gated with COMP
(load sequence completed) to generate EXT (external
control). The EXT condition allows interface commands
to be processed. The OFF-LINE command from the
interface does
not require the EXT condition. The OFF-
LINE command is gated with the unit select command
to clear the On-Line latch (U1D and U2B).
2-33. The Rewind command (REW) from the interface is
gated with EXT to clear (assert) the Rewind flip-flop (U6B),
provided that tape is not at load point. The forward, reverse,
and high-speed (FWD, REV and HIGH SPEED) are gated
to provide motion commands to the tape transport servos
and data circuits.
2-34.
All status signals to the interface are gated with
SOLA. The load point status (SLP) indicates that the tape
unit
is at load point (load point tab under photosense head).
The end-of-tape status (SET) indicates
that the tape is at or
beyond the end-of-tape tab. The rewind status (SRW) indi-
cates that the tape is rewinding or in an automatic load
point search operation. The ready status (SR) indicates
that
the tape unit is selected, on-line, and that the load sequence
is complete (not rewinding and not in a load point search
mode). The on-line status (SL) and file protect status (SFP)
indicate that the.unit is on-line, and that the tape reel in-
stalled on the supply reel hub is not equipped with a write
enable ring.
Part 2

7970B/7970C
2,35. The one-second delay (generated on the power
regulator printed-circuit assembly) following a high-speed
reverse command prevents additional high speed commands
from being processed by the control and status circuits. The
delay also prevents the load point search from occurring for
one-second following a rewind operation.
2-36. CAPSTAN SERVO CIRCUITS.
2-37. The capstan servo circuits control the speed and
direction
of tape motion across the magnetichead assembly.
The capstan servo consists of a capstan motor/tachometer
and a capstan servo printed-circuit assembly. The servo cir-
cuit employs current and velocity feedback. The velocity
feedback
is provided by the magnetic moving coil tach-
ometer attached to the capstan motor shaft. Current feed-
back is provided by a pair of sensing resistors in the capstan
motor return circuit. Figure 2-4 is a block diagram of the
capstan servo circuit.
HSFWD------~----------------~_:~::~
HSFWD
LPSEARCH-----------------+~----~
Theory of Operation
2-38. Motion commands from the control and status
printed-circuit assembly control switching circuits and a bi-
polar ramp generator. The output of the ramp generator
and high-speed control signals control the capstan closed-
loop servo amplifier. The capstan servo amplifier consists
of
an integrated circuit servo preamplifier (U4) and a capstan
power amplifier
(Ql through Q6).
2-39. The forward and reverse switching circuits contain
temperature compensated zener diodes
that provide a basic
reference voltage for forward and reverse speeds. When a
FWD command from the control and status circuits of FWD
service switch S2 is placed in the on position (up), Q7 is re-
verse biased. Bias current for CR6 is then provided through
R33
to establish a -6.2 volt reference for the ramp generator
(U2 and U3). A REV command
or placing the REV service
switch
in the on position (up) will provide bias current for
CR2 through R27. CR2 will provide a +6.2 volt reference
for the ramp generator. The forward and reverse reference
voltages
at the output of U3 are independently adjustable
to control the forward or reverse speeds.
.12V
L---o-~ OFFSET
12V
TENSION
TENSION ----------------4~
'- .... --------------------------------------------------------------_ TO REE L SE RVO
cCA
7970511A
Figure 2-4. Capstan Servo, Block Diagram
Part 2
2-7

Theory of Operation
2-40. The bi-polar ramp generator consists
of two inte-
grated circuit operational amplifiers (U2 and U3), a symetri-
cal 8-volt clipping network (CR7 through CR11), and a
ramp control network (R42/C28).
2-41. Operational amplifier U2 initially operates as a
saturating
comparator when a forward or reverse command
is given or removed. Near the end of a ramp-up or ramp-
down
U2 changes from a saturating comparator to a linear
amplifier with the non-inverting input
at O-volts.
2-42. The output of U2 is held to 8 volts by a symetrical
clipping network (CR7 through CR11) which establishes
current for the ramp control network. The slope
of the
ramp
is determined by the current through R42 (RAMP
control) into integrating capacitor C28. Feedback through
R66 nulls the reference input voltage
to U2 and the output
voltage of the ramp generator (U3-6) is held steady by the
ratio determined by R66 (R35 + R34)
or R66 (R29 + R28).
2-43. When the Forward or Reverse command
is removed,
the current through R66 drives operational amplifier
U2 in-
to saturation and the ramp integrates the 0 volts. As the
output of the ramp generator (U3-6) approaches 0 volts, U2
reverts to a linear amplifier and the output of the ramp
generator
is maintained at 0 volts.
2-44. A High-Speed Forward command (HSFWD) from
the control and status circuits or placing the +
160 service
switch in the on position (up) will allow CR12 and CR13
to
conduct and reverse bias Q8. With Q8 reverse biased, C25
is charged through R49. The exponential voltage at the base
of Q9 rises to +12 volts but is clipped at +6 volts when Q9
saturates. When the HSFWD command
is removed or the
+160 service switch is placed in the off position (down),
U1B conducts placing U1B-4, CR12, and CR13 at 0 volts.
This allows Q8
to conduct and C25 discharges through R48.
The threshold caused by the base emitter turn-on voltage
of
Q9 and the diode drop across CR14 results in a delay of
approximately 100 ms before motion starts or stops.
2-45. A High-Speed Reverse command (HSREV)
or plac-
ing the
-160 service switch in the on position, (up) will cause
the high-speed reverse ramp circuit
to function the same as
the high-speed forward ramp circuit, except that voltage
polarities are reversed. Capacitor C26
is charged through
R56 and discharged through R54 and R55.
2-46. The LOAD command from the control and status
circuits does
not control a ramp circuit. The load switch of
the capstan servo is a single step input to the capstan servo
amplifier resulting in a nominal 20 ips tape motion.
2-47. The
outputs of the bi-polar ramp generator, high-
speed forward ramp generator, high-speed reverse ramp gen-
erator and load switch form a summing junction at the input
of the capstan servo preamplifier (U4). Diodes CR17 and
CR18 provide clipping to protect the amplifier from over-
load. The preamplifier drives the capstan
motor power am-
plifier (Q1 through Q6). The dc gain
of the power amplifier
2-8
7970B/7970C
is 10 volts per volt determined by RIO and R11. The power
amplifier
is operated in class B with Q6 providing negative
current for forward motion and
Q5 providing positive cur-
rent for reverse motion.
2-48. A
notch filter in the velocity feedback circuit from
the tachometer
is selected to attenuate the mechanical re-
sponse
of the motor-tachometer combination. A compensat-
ing network in the current feedback circuit
is also selected
depending upon synchronous speed
of the tape unit.
2-49. Transistor switch Q22 senses the presence
of
tape tension. While the tape is tensioned, Q22 is on,
keeping switch Q20 off. However, when tension is lost
Q22 turns off, allowing Q20
to turn on and switch the
input of motor drive amplifiers Q2, Q4, and Q6 to
ground. This disables the capstan servo. The capstan
motor circuit is completed through the B contacts of
relay K1 on the reel servo amplifier PCA when K1 is
energized. Relay
K1 is energized when the LOAD push-
button is pressed. Once energized, it remains energized
until tension is removed.
2-50. REEL SERVO CIRCUITS.
2-51. The reel servo circuits consist of a tension circuit,
a voltage switching circuit, a delay circuit, voltage/current
feedback switches, tension arm photosense circuits, pream-
plifiers,
motor power amplifiers, and reel motors. Figure 2-5
is a block diagram of the reel servo circuit.
2-52. At initial power-on, the tension circuit
is disabled.
The normally closed contacts
of LOAD pushbutton switch
prevent Q13
of the tension circuit from conducting. Pressing
the LOAD control allows Q13
to conduct, energizing relay
K1. With
K1 energized, the capstan and reel servo motor
circuits are completed. As tape is tensioned and the tension
arms swing away from the limit switches, power through the
limit switches maintain a forward bias
of Q13. When power
is removed, or tape tension is lost, the relay contacts short
across the reel
motor windings to provide dynamic breaking.
2-53. The voltage switching circuit
is located on the
power regulator printed-circuit assembly. During a high-
speed operation forward or reverse, power
to the motor
power amplifiers is switched from 22.5 volts to 57.5 volts
(nominal).
2-54. During a high-speed reverse operation (rewind), the
HSREV command from the control and status circuits
is
gated with TENSION. When both reel motors approach full
r/min, the
motor voltage exceeds the break-down voltage of
CR4. The condition established by the gating of HSREV
and Tension allows current through CR4
to forward bias
Q5. Voltage switch
Q6/Q7 conducts placing +57.5 volts on
the
+20/40 line. Diode CR2 on the power distribution
printed-circuit assembly
is back-biased preventing +57.5
volts from entering the +20 volts line.
Part 2

7970B/7970C Theory
of Operation
I
I
I
I
I
I
I
I
+SV
KIA (ENERGIZED)
~~~~-4~~~~~~~~~~~AI
-20/40V
R27
R26
+20V ------------iM----- ... ---+-.. +20/40V
-20V
-----------t4----i>---+---,--II .. -20/40V
HSFWD
MOTOR VOLTAGE
DELAY TO
">cHI-_ CONTROL AND
STATUSPCA
FEEDBACK CONTROL
R47
VOLTAGE
FEEDBACK
).
SUPPLY
REEL
MOTOR
L __ _
-SV
7970-512
Part 2
RIOS
VOLTAGE
FEEDBACK
TAKEUP
REEL
MOTOR
>- .... - __ -<11- ____ ----------- ... I KIC (ENERGIZED)
.J..
-20/40V
RB
R7
Figure 2-5. Reel Servo, Block Diagram
2·9

Theory of Operation
2-55. During a high-speed forward operation, the voltage
switching circuits function the same
as in the fast reverse
operation. HSFWD
is gated with tension to allow motor
voltage to switch the -40 volt switch (QI2/QI3).
2-56. The delay circuit is located on the power regulator
printed-circuit assembly. The delay circuit provides an addi-
tional one-second delay following the end of a high-speed
command. The one-second delay prevents further high-speed
commands from the interface from being processed during
the one-second period. The one-second delay
is also used to
switch the reel servo operating mode from voltage feedback
to current feedback. The additional one-second following a
high speed operation allows the reel servo
to operate in a
current feedback mode until reels are slowed.
2-57. During normal operation, U5A-3 and U5B-4 rest
at
ovolts. A HSFWD or HSREV command from the control
and status circuits will cut-off U5A
or U5B and base current
for Q15
is supplied through R28 or R29. When Q15 con-
ducts, capacitor C8 immediately discharges through Q15
causing Q16 and Q18
to be cut-off. Feedback control to the
reel servo changes from approximately +8 volts
to approxi-
mately -8 volts. The negative potential also reverse biases
Q17 and the DELAY line switches
to +5 volts.
2-58. When the high-speed command (HSFWD
or
HSREV) is removed and Q15 loses base current, capacitor
2-10
7970B/7970C
C8 is then charged through R33 (one-second time constant).
When C8
is charged to +5V, Q16 and Q18 conduct and the
feedback control changes from
-8V to +8V. The DELAY
switches
to 0 volts.
2-59. The reel servo tension arm assemblies contain dual
element photo-conductors
that are illuminated by a lamp
shining through a slotted disc. The slot
is in the form of a
spiral attached
to the tension arm. As the arm moves, the
slot exposes different areas
of the photo-conductor. As a
result, the
output of the photo-conductor is proportional
to the position of the tension arm.
2-60. The reel servo preamplifier is an integrated opera-
tional amplifier that amplifies the position error of the ten-
sion arm. The tension arm photo-conductor output is single-
ended, therefore, an off-set is provided by R39 or R41.
The preamplifier drives a class B
motor drive amplifier.
The
motor power amplifier has a gain of 10 volts per volt.
2-61. When operating in a normal mode (synchronous
or load speed), the servo operates with voltage feedback.
The +8 volts from the feedback switching network (feed-
back control) back-biases the feedback FET switch (Q14
and
QI5). Feedback is then provided through R47 and
RI05. During a high-speed operation, the feedback control
changes
to -8 volts. The feedback FET switch is forward
biased and feedback
is current through the switch.
Part 2

7970B/7970C Performance Checkout
SECTION III
PERFORMANCE CHECKOUT
3-1. INTRODUCTION.
3-2. This section provides checkout instructions to ver-
ify that the tape unit conforms to published performance
specifications. The test procedure assumes
that the follow-
ing general conditions apply
at all times unless specific in-
structions to the contrary are stated as part of a test routine.
3-3. TEST EQUIPMENT REQUI RED.
3-4. For the purpose of this procedure, it is presumed
that the person conducting the test will be using either a
computer
or an off-line test set that is capable of meeting
the following requirements. It is also presumed that operat-
ing instructions for the equipment
is provided by documen-
tation applicable
to the equipment.
a. Provides all standard functional commands.
b. Responds
to all status outputs.
3-5.
An HP 13191A Control and Status Test Board is
available as a service accessory and will meet the needs for
all adjustments. Computer operation
is also suitable. The
following electronic test instruments (or equivalent) are
also required:
a.
HP 140A Oscilloscope with HP 1421A Time Base
Generator.
b.
HP 1421A Dual-Trace Preamplifier (for HP 140A).
c. HP 5245L Counter with HP 5265A Digital Volt-
meter Plug-In.
3·6. In addition
to the listed test equipment, a trans-
port test tape HP part number 5080-4525 that generates
various signals for accurate speed measurements is required.
3-7. CHECKOUT PROCEDURES.
3-8. Performance checkoutprocedures for the tape unit
consist of:
a. Preliminary Power-Off Checks.
b. Operator Control Checks.
Part 2
c. Service Switches and Accessory Checks.
d. Tape Path Evaluation.
e. Power Supply Voltage Checks.
f. Tape Speed and Capstan Servo Checks.
g. Transport Function, Motion, and Status Checks.
3-9. PRELIMINARY POWER-OFF CHECKS.
3-10. Preliminary power-off checks are performed as
follows:
a. Tape Rollers: The tape rollers must operate freely
and have no end play.
b. Head Crosstalk Shield: The head crosstalk shield
should operate freely and should have clearance between
face
of head and shield (room to slip a punched card thick-
ness without binding).
c. Tension Arm Limit Switches: The three limit
switches associated with the tension arms must operate
when the arm
is approximately 1/8 inch from the rubber
stop. The roller on the switch arm should be approximately
on the center
of the arm when the arm is fully against the
stop. There should be positive travel of the switch lever be-
yond the point at which it actuates the microswitch. The
check can
be made audibly if ambient noise level permits,
or with an ohmmeter if room noise is too high.
d. Write Enable Sensing: The write enable sensing
finger nominal location dimensions should be as follows:
(1) The distance between the outside surface
of
the write enable sensing finger and the out-
side flange diameter of the reel turntable
should be 3/32-inch (nominal).
(2) In the file protected position (inoperative),
the dimension between the
outer tip of the
sensing finger and the
outer face of the reel
turntable should be 3/16-inch (nominal)
above the turntable surface.
(3) In the write enable position (with solenoid
energized) the
outer tip of the sensing finger
should be 1/16-inch below the turntable sur-
face.
(4) The sensing finger must
not touch the turn-
table under any condition.
3-1

Performance Checkout
e. Reel Retaining Knob: With the locking lever
re-
leased, the reel should slip over the rubber grip ring easily,
and it should be possible
to easily rotate the reel. When the
lever
is closed, positive resistance should be felt as the rub-
ber is compressed. In the locked position, it should not be
possible
to move the reel by hand. If slippage is suspected,
place a piece
of masking tape on the reel, and another on
the
hUb. A mark placed in alignment on both pieces of tape
should
not become misaligned by more than 1/8 inch in 16
hours of operation. To correct tape reel slippage, release
locking lever and loosen the pozidrive screw,
rotate the reel
retainer
knob clockwise, and tighten the screw. Repeat until
tape reel mounts firmly and does
not slip.
3-11. OPERATOR CONTROL CHECKS.
3-12. Verify position
of 115/230 volt selector switch;
connect unit
to appropriate power source and check the
following operating modes.
3-13. TAPE LOADING AND WRITE ENABLE. Place a
reel
of tape (with write enable ring) on unit and thread with
the power switch on. After threading tape, press LOAD and
verify
that the following takes place.
a. Tape tension is established. RESET indicator il-
luminates.
b. WRITE ENABLE indicator illuminates.
c. Tape moves forward at 20 ips
to load point tab
(BOT tab).
d. LOAD indicator illuminates.
3-14. ON-LINE TRANSFER AND RESET. Place the
control and status
PC assembly unit select jumper in the
OFF pin (this is not the same as OFF on unit select option)
and load tape. Following completion
of a load sequence
with tape positioned
at load point (LOAD indicator on),
press ON-LINE and verify
that unit will respond to external
commands. Pressing ON-LINE while load point search
is in
process will also result in ON-LINE operation upon com-
pleting the search (LOAD indicator is ON). While ON-LINE,
the unit will not respond to local controls with the excep-
tion of RESET.
3-15. DYNAMIC BRAKING AND RECOVERY FROM
POWER FAILURE. Place unit in rewind mode after tape
is
well into take-up reel. When full rewind speed is reached,
turn power switch off to simulate power failure. Tape should
stop without any damage to tape. Proceed with recovery to
on-line status by following normal load sequence. Except
that when tape unit goes into the load point search mode,
load
point tab may be simulated by pressing the RESET
button. This can then be followed by an on-line command.
3-16. REWIND OPERATION. The REWIND control
is
operational only while the unit is in the reset state which
permits local control. The rewind command will override
3-2
7970B/7970C
the load command and will return the tape to the load point
position (LOAD indicator on with tape stopped). Rewind
may be terminated prior
to load point by pressing RESET.
Transfer
to on-line is also possible immediately following
this sequence.
3-17. SERVICE SWITCHES AND ACCESSORY
CHECKS.
3-18. There are three service switches incorporated
in the
capstan servo card. These together with the REWIND and
LOAD controls provide a means of operating all five drive
modes for service andlor adjustment purposes. There
is no
capability for cyclic drive operation.
3-19. The HP
13191A Control and Status Test Board is
available for use with the tape unit. With this test board it
is possible to completely adjust and verify the performance
of the control and status function of a tape unit under off-
line conditions. Complete operatingdetails are included with
the test board. The HP 13191A Control and Status Test
Board provides normal drive modes
as well as cyclic pro-
gramming which is suitable for adjustment of the capstan
start/stop ramp. The board also includes status indicator
lamps
to verify all normal status functions. The test board
is installed in the connector of the tape unit control and
status PCA.
3-20. TAPE PATH EVALUATION.
3-21. With undamaged tape threaded
on the tape unit
and with the unit in the appropriate operating mode, evalu-
ate the following characteristics of the tape path.
a. Capstan Height: Capstan height should be such that
the tape is guided equal distance in from the outer and inner
edge
of the capstan. The air escape grooves provide a means
for judging.
b. Tension Position: Arms should be aligned with the
centering marks on the rear
of the casting. Position is ac-
ceptable provided the arm is not out of location by more
than the diameter
of the arm.
c. Tape Tracking Over the Tape Guides: Tracking
over the guides should be smooth with no evidence
of edge
forces anywhere in the tape path. Transverse reflected light
may be used
to assist in judging. Light reflections across
the 1/2-inch width
of the tape should not be bent due to
points of stress.
d. Photosense Position: Face of photosense head as-
sembly should be parallel to the path of the tape and posi-
tioned 1/8 inch from the tape.
3-22. POWER SUPPLY VOLTAGE CHECKS.
3-23. Both regulated and unregulated supplies should be
checked for the following specifications.
Part 2

7970B/7970C
3-24. REGULATED SUPPLY VOLTAGES. Regulated
voltages will remain within tolerances over a primary voltage
range
of ±10 percent. DC voltages and tolerances are to be
as specified below. The peak-to-peak ripple values are nom-
inal and minor variances may be expected. Ripple is stated
under tension-only conditions.
a. The
+5 volt supply must be +5.000 ± 0.050 Vdc.
b. The ±12 volt supplies must be 12.000 ± 0.360 Vdc.
3-25. UNREGULATED SUPPLY VOLTAGES. Unregu-
lated supply voltages are a direct function of line voltage.
The following table
of nominal values should be judged
only at a line voltage
of 115 volts. RMS ripple values are
nominal and apply at 60
Hz.
a. ±40 Vdc: Nominal value is ±57.5 volts; ripple of
150 mv POp (sawtooth).
b. +20 Vdc: Nominal value
is +22.5 volts; ripple of
600 mv Pop (sawtooth).
c. -20 Vdc: Nominal values is -22.5 volts; ripple of
300 mv Pop (sawtooth).
d. +10 Vdc: Nominal value
is +12 volts; ripple of
900 mv POp (sawtooth).
3-26. TAPE SPEED
AND CAPSTAN SERVO CHECKS.
3-27. The tape speed and capstan servo checks consist of
measuring capstan motor offset current, measuring long term
speed variation (tape units capable
of reading data), measur-
ing startand stop time and distance, measuring instantaneous
speed variations, and measuring fast forward, reverse, start
and stop. Included
is a dynamic tape skew check for tape
units equipped with read and write electronic circuits.
3-2S. CAPSTAN MOTOR OFFSET CURRENT. Connect
a suitable dc voltmeter
or oscilloscope across the 3 ohm 1
percent resistor (R21
or R22). The return side of the resis-
tors is connected to pin 2 of CJ-1. With the tape under ten-
sion but no tape motion, the voltage should not exceed the
following referenced
to 0 Vdc.
a. Maximum acceptable operating limit: ±100 mv dc
at 25°C.
b. Adjustment recommended if greater than:
±SO mv
dc.
3-29. LONG TERM SPEED VARIATION. Measure the
tape speed accuracy over any interval greater than one sec-
ond and under each of the five drive speeds. The principle
of measurement requires that a tape having highly accurate
Part 2
Performance Checkout
bit-to-bit distances be read, and
that the output of the read
preamplifier be connected
to a suitable counter. HP part
number50S0-4525 TransportTest Tape provides frequencies
accurately spaced
to better than 0.1 percent. For the follow-
ing tests reference frequencies are based on using the signal
that will be availabel from the channel 3 preamplifier of nine-
track units and channel 6 preamplifier of seven-track units.
This has a bit-to-bit spacing
of 0.00150 inch and will pro-
duce a frequency of 10,000 Hz when reproduced at a tape
speed
of 30.000 ips. Frequencies for other speeds are in
direct ratio
to the change in speed relation to 30 ips. Ex-
ample: 60 ips would produce 20,000 Hz. Use this principle
and check the following:
a. High-Speed Forward:
160 ips, basic frequency is
53,333 Hz.
(1) Maximum acceptable operating limit: ±1050
Hz (±2 percent).
(2) Adjustment recommended if greater than:
±SOO Hz (±1.5 percent).
b. Rewind (High-Speed Reverse): Checked identically
to the limits applicable to high-speed forward, except that
the rewind mode is used.
c. Forward and Reverse Drive: Frequency depends
on tape speed and must be calculated for speeds
other than
25 ips and 37.5 ips which are tabulated below. Maximum
acceptable operating limit
is based on ±1 percent with ad-
justment recommended if speed error is +O.S percent or
greater.
(1) The 25 ips drive nominal frequency
is S,333
Hz. Acceptable limits are ±S3 Hz; adjustment
is required if greater than ±65 Hz.
(2) The 37.5 ips drive nominal frequency
is
12,500 Hz. Acceptable limits are ±125 Hz;
adjustment
is recommended if greater than
±100 Hz.
d. Load Point Search: 20 ips; basic frequency
is
6,667 Hz.
3-30.
(1) Maximum acceptable limit is ±1330 Hz (±20
percent).
(2) No adjustment
is provided.
START MEASUREMENT. Start measurements
(both forward and reverse) are made
as follows. Measure-
ment of the characteristics defined requires that the tape
unit be driven with commands
that are adjustable to periods
greater than the specified start/stop times and
that there be
a provision to synchronize an oscilloscope sweep directly
from these commands (both start and stop). The tape
to be
read may be any previously recorded tape, preferably at
SOO cpi, as this provides the best resolution where measure-
ments from preamplifier outputs are required. Wavelength
3-3

Performance Checkout
accuracy
is not a factor in this test. Do not make measure-
ments during read-after write operation.
Note
The definitions apply for both forward and re-
verse directions. The capstan tachometer voltage
will reverse polarity when direction
is changed;
however, there will be no observable difference
in the preamplifier
output waveforms.
a. Start-Ramp Delay Time: The time (following a start
command) required for the capstan tachometer voltage
to
reach a value that is greater than aVdc but less than 3 per-
cent of the value reached at normal drive speed. Specifica-
tion is 0.5 ± 0.5 ms.
b. Start-Ramp 100 Percent Time: The time (follow-
ing a start command) required for the analog
output of any
preamplifier track
to first reach 100 percent of the peak-to-
peak (or O-to-peak) value established while reading an all
"1 's" tape under steady state drive conditions. This is a
function
of tape speed; the following limits apply with the
capstan ramp adjustment being made during the forward
start mode only.
(1) The 25 ips start-ramp 100 percent time
should be 14.5 ± 0.2 ms for forward ramp,
±0.3 ms for reverse ramp.
(2) The 37.5 ips start-ramp 100 percent time
should be 9.5 ± 0.2
ms for forward ramp,
±0.3 ms for reverse ramp.
3-31. START DISTANCE. Start distance
is calculated
as follows. Measurement of the characteristics defined re-
quires that the tape unit be driven with commands that are
adjustable
to periods greater than the specified start/stop
times and
that there be a provision to synchronize an oscil-
loscope directly from these commands (both start and stop).
The tape
to be read may be any previously recorded tape,
preferably
at 800 cpi as this provides the best resolution
where measurements fram preamplifier
outputs are required.
Wavelength accuracy
is not a factor in this test. Do not
make measurements during the read-after-write operation.
Note
The definitions apply for both forward and re-
verse directions. The capstan tachometer voltage
will reverse polarity when direction
is changed;
however, there will be no observable difference
in the preamplifier
output waveforms.
a. Start-Ramp Time: This is the time required to ac-
celerate the tape from zero ips to the drive speed. It is de-
termined by subtracting the start delay time (measured)
from the start-ramp 100 percent time (also measured).
3-4
7970B/7970C
b. Start Balance Time: This is the time difference be-
tween the start-ramp 100 percent time and the specified
start time for the tape speed involved. This period
of time
is allowed to provide a balance in tape distance as it offsets
the time delay
at full drive speed represented by the stop-
ramp delay time.
c. One-half
of the start-ramp time plus the start bal-
ance time (both in milliseconds) multiplied by the tape speed
in inches per second equals the start distance (in inches x
10-3
). Specifications 0.187 ± 0.020 inch.
3-32. STOP MEASUREMENT. Stop measurements
(both
forward and reverse) are made as follows:
a. Stop-Ramp Delay Time: The time (following a stop
command) required for the analog output of any preampli-
fier
to fall to a peak-to-peak (or O-to-peak) value that is less
than
100 percent but greater than 97 percent of the value
established
under steady-state drive conditions while read-
ing in all "l's" tape. Specification is 0.5 ± 0.5 ms.
b. Stop-Ramp 100 Percent Time: The time (follow-
ing a stop command) required for the capstan tachometer
voltage
to reach a level between 3 percent of normal drive
speed value and aVdc. Specification
is a function of tape
speed and must
not exceed a time that is at least 0.2 ms
less than the specified stop time for the tape speed involved.
3-33. STOP DISTANCE. Stop distance
is calculated as
follows:
a. Determine stop-ramp delay time.
b. Determine Stop-Ramp Time: This
is the time re-
quired to decelerate the tape to zero ips from the drive
speed.
It is determined by subtracting the stop-ramp delay
time (measured) from the stop-ramp 100 percent time (also
measured).
c. The stop-ramp delay time plus one-half the
stop
ramp time (both in milliseconds) multiplied by the tape
speed in inches
per second equals the stop distance (in
inches x 10-3
). Specification is 0.187 ±0.020 inch.
3-34. INSTANTANEOUS SPEED VARIATION. Instan-
taneous speed variation
is a measurement of the short term
departure of the tape velocity from its long term average
speed. Measurement
is made as follows:
a. Write a length of all "l's" tape at 800 cpi and re-
wind as reqUired to be in position to reproduce this section
of tape.
b. Connect oscilloscope
to any convenient preampli-
fier
output, position so that waveform is centered on screen,
and adjust main sweep sync for positive slope and
to trigger
at the zero crossover.
Part 2

7970B/7970C
c. Use the delayed sweep feature to position the de-
layed sweep at the nextzero axis crossover (this corresponds
to one bit-to-bit distance) with the delayed sweep auto trig-
gered from a main (delaying) sweep.
d. Adjust delayed sweep rate
to permit good resolu-
tion of the time shift band at the zero axis crossover. During
this evaluation the vertical gain and the delaying sweep time
may be adjusted
to optimize the resolution. If the correct
point is being observed, it will be the waveform that is com-
ing from top left of screen to bottom right with the slope
depending on the gain and delayed sweep speed.
e. Observe the jitter band under conditions in step
"d" in microseconds peak-to-peak time displacement and
multiply by tape speed
to determine this distance in micro-
inches. This value must not exceed the following limits
which are based
on ±3 percent instantaneous speed variation:
(1) 800 cpi must
not exceed ±37.5 microinches
(75 microinches peak-to-peak).
(2) 556 cpi must
not exceed ±54 microinches
(108 microinches peak-to-peak).
(3) 200 cpi must
not exceed ±150 microinches
(300 microinches peak-to-peak).
Note
The above values apply
to both the forward and
reverse modes.
3-35. DYNAMIC TAPE
SKEW. Dynamic tape skew is
that variation in tape velocity which generates a differential
time position between the two outermost tracks on the tape.
Measurement
is made as follows:
a. Write a length of all "l's" tape. Rewind as required
to reproduce this section of tape.
b.
Use dual-trace oscilloscope and connect to pream-
plifier output of the two outside tracks (channels 4 and 5).
Use chopped mode (triggered by channel A) with the main
sweep synchronized for positive slope triggered
at zero axis
crossover. (Use negative slope in reverse direction.)
c. Adjust
both channel gains and positions to super-
impose the two waveforms (amplitude only; there will be
varying degrees
of time difference due to static skew char-
acteristics ).
d. Use the delayed sweep feature to present the next
(one bit-to-bit distance later) zero axis crossover
on the de-
layed sweep. The delayed sweep must be on internal sync,
triggered on the negative slope. (Use positive slope in
reo
verse direction.) The earliest of the two tracks will sync the
delayed sweep and the
other will arrive later and will have
jitter
that represents the time differential in microseconds
Part 2
Performance Checkout
peak-to-peak. Again gains must be adjusted as high as pos-
sible (both the same) and the delayed sweep trigger set for
maximum stability
of the stable waveform. There will be a
considerable dead zone in the delaying sweep adjustment.
Use the first operating position as the sweep is moved out
from minimum delay position. There is no significance to
the time differential between the stable and unstable wave-
forms. This does not represent static skew. The difference
is a function of oscilloscope gain and position settings.
e. Measure the peak-to-peak time band
of the unstable
waveform
as it crosses the zero axis. Convert this to micro-
inches peak-to-peak for the tape speed involved. Worst case
must not exceed ±50 microinches (100 microinches peak-
to-peak) and applies to forward or reverse mode.
3-36. FAST
FORWARD/REVERSE START/STOP
CHARACTERISTIC. Fast forward and fast reverse charac-
teristics are measured by synchronizing an oscilloscope with
the appropriate drive command, and observing the dc out-
put of the tachometer using the TACH test point on the
capstan servo printed-circuit assembly.
CAUTION
Do not issue sequential fast drive commands
without allowing time to reach full speed
or to
return to zero speed. Failure to observe this pre-
caution may cause excessive power dissipation
in the reel servo amplifier circuitry. Minimum
time between commands should be 1 second
or
the sum of the start and stop times (whichever
is greater).
3-37. Fast forward/reverse start
or stop times are meas-
ured by observing the time (following a start or stop com-
mand) required for the tachometer output to either reach
its maximum value (for start time)
or to fall to zero (for
stop time). Nominal times are from 400 to 700 milliseconds.
Typically,
the value will be 600 milliseconds.
3-38. Fast forward/reverse start
or stop distances are de-
termined by the time required for the ramp to move from
one state
to the other. This time is nominally 500 milli-
seconds. One-half of this time multiplied by 160 ips equals
the nominal start/stop distance
of 40 inches.
3-39. TRANSPORT FUNCTION, MOTION, AND
STATUS CHECKS.
3-40. The following checks cover the
1/0 lines in the
status and motion command connector. Use appropriate
off-line test equipment
to verify proper performance as in-
dicated.
3-41. FUNCTION COMMANDS. Select,
CS: The con-
trol and status PC assembly includes a jumper that can serve
to establish unit identification where the front panel select
3-5

Performance Checkout
option
is not present. If no select function is desired, the
jumper can be placed in the
off position which will permit
response
to controller commands, when unit is in on-line
with load sequence completed,
and is not rewinding. Verify
the following conditions
to check the select (CS) line:
a. Unit responds to commandswith jumper connected
to OFF.
b. Unit responds to 0 through 3 positions when cor-
responding CSO through CS3 is selected by the controlling
device.
c. When front panel select option
is included, place
internal jumper in the
off position and verify that response
to pushbuttons 0 through 3 is same as in step "b" above.
When the front panel OFF pushbutton
is pressed, unit must
not respond to any commands from the controlling device.
3-42. Verify
that the off-line (CL) line clears the write
condition and returns the selected tape unit
to the reset
condition.
3-43. MOTION COMMANDS. Verify
that the following
external commands will place the selected and on-line tape
unit in the corresponding drive mode:
a. Forward (CF): Tape drives forward.
b. Reverse (CR): Tape drives reverse
then stops at
load point tab.
c. Rewind (CRW): Tape unit enters rewind mode, re-
mains on-line.
d. High Speed (CH): Tape unit will respond only when
this
is combined with either a forward or reverse command.
When so combined, tape will drive
at 160 ips speed. CH with
CR will drive reverse past load point.
3-6
7970Bj7970C
CAUTION
Do not issue sequential fast drive commands
without allowing time to reach full speed or to
return to zero speed. Failure to observe this pre-
caution may cause excessive power dissipation
in the reel servo amplifier circuitry. Minimum
time between commands should be 1 second or
the sum
of the start and stop times (whichever
is greater).
3-44. STATUS OUTPUTS. Verify
that the following
status
outputs are true (low assertion) when a selected and
ready tape unit
is in the condition indicated.
a. On-Line (SL): True when selected tape unit has
been manually placed on-line.
b. Ready (SR): True when selected
unit is on-line,
tape loading cycle
is completed, and tape unit is not in re-
wind mode.
c. Load Point (SLP): True when selected tape unit
has tape positioned
at the load point reflective strip.
d. Density (SD2, SD5, or SDS): Verify that selected
tape unit will display the density selected by the tape unit
density switch if this option
is present. Without this option,
the tape unit will be set
at SOO cpi but there will be no SDS
output.
e. Rewind (SRW): Verify that this status remains
true
as long as the selected unit is in the rewind mode.
Must remain true until tape
is repositioned at load point
tab.
f. File Protect (SFP): True when selected unit is not
write-enabled.
g. End-of-Tape (EOT): True when selected unit has
moved end-of-tape tab
beyond photosense head. Will re-
main true until tab again passes photosense head in the re-
verse direction.
Part 2

7970B/7970C Repair and Replacement
SECTION IV
REPAIR AND REPLACEMENT
4-1. INTRODUCTION.
4-2. This section provides repair and replacement pro-
cedures for the tape transport of the HP 7970B/7970C Dig-
ital Magnetic Tape Units.
a. Avoid unnecessary component substitution; it can
result in damage to the PCA circuit board and/or adjacent
components.
b. Do not use a high·power soldering iron. Excessive
heat may lift a conductor
or damage the board.
4-3. REPAIR.
CAUTION
Do not use a sharp metal object such as an awl
or twist drill to remove solder. Sharp objects
may damage the plated-through conductor.
4-4. The etched printed-circuit assemblies (PCA's) used
are of the plated-through type consisting of metal bonded
to both sides of an insulating material. The metallic con-
ductors are extended through the component holes by a
plating process. Soldering can be performed on either side
of the PCA with equally good results. Table 4-1 lists recom-
mendedtools and materials for use inrepairing etched PCA's.
The following are recommendations and precautions pertin-
ent
to PCA repair work.
c. Use a suction device (table 4-1) or wooden tooth-
pick to remove solder from component mounting holes.
d. After soldering, remove excess flux from the solder
areas and apply a protective coating
to prevent contamina-
tion and corrosion.
Table 4-1. Printed-Circuit Assembly Repair Equipment
ITEM USE
DESCRIPTION RECOMMENDED MODEL
Soldering Tool Soldering and unsoldering Wattage rating: 47-1/2 to 56-1/2W Ungar
#776 Handle with
Tip Temp: 8500 to 900°F Ungar
#4037 Heating Unit*
Soldering Tip* Soldering and unsoldering Shape: pointed Ungar
#PL111*
Suction Device Removes molten solder Soldapullt by Edsyn Co.,
from connection Arleta, California
Resin (Flux) Removes
excess flux from Must not dissolve etched circuit
Freon
Solvent soldered area before appli- base board material or conductor Aceton
cation
of protectivecoating bonding agent Lacquer
Thinner
Isopropyl Alcohol (100% dry)
Solder Component replacement, Resin (flux) core, high tin content
printed-circuitboard repair, (60/40 tin/lead), 18
gauge (SWG)
and wiring connections preferred
Protective Contamination and corro-
Good electrical insulation,
Krylon R** #1302
Coating sion protection corrosion-prevention properties Humiseal Protective Coating,
Type 1B12 by Columbia
Technical Corp., Woodside
77,
New York
*For working on etched boards; for general purpose work, use Ungar #1237 Heating Unit (37.5W, tip temp of 750°
to 800° F) and Ungar #PL113 1/8-inch ch isel ti p.
** Kryton, Inc., Norristown, Pennsylvania
Part 2 4-1

Repair and Replacement
4-5. The following procedures are recommended when
component replacement is necessary.
a. Remove defective component from board.
b.
If component was unsoldered, remove solder from
mounting holeswith a suction device (table 4-1)
or a wooden
toothpick.
c. Shape the leads
of replacement component to
match mounting hole spacing.
d. Insert component leads into mounting holes and
position
component as original was positioned. Do not force
leads into mounting holes; sharp lead ends may damage
plated-through conductor.
Note
Although
not recommended when both sides of
the PCA are accessible, axial lead components
such
as resistors and tubular capacitors can be
replaced without soldering. Clip ends
of defec-
tive component near body. Straighten leads left
in board. Wrap leads
of replacement component
one
tum around original leads. Solder wrapped
connection and clip
off excess leads.
4-6. REPLACEMENT.
4-7. The 7970B/7970C is of modular design. All major
transport assemblies are easily accessible for repair
or re-
placement. The following paragraphs describe replacement
of critical transport assemblies. Refer to section VI for de-
tailed views of mechanical assemblies.
4-8. TAPE ROLLER AND BEARING ASSEMBLY.
4-9. The tape rollers are precision referenced to a step
on the mounting shaft. The bearings are very critical to pre-
loading. When replacing the tape roller and bearing assem-
blies refer to section V for adjustment procedures to prop-
erly preload the bearings.
4-10. CAPSTAN (10 -37.5 IPS UNITS ONLY).
4-11. The capstan replacement requires that the capstan
motor assembly be removed from the transport. The posi-
tion of the capstan is referenced to the mounting surface of
the capstan motor assembly, The following procedures de-
scribe capstan replacement.
a. Disconnect capstan motor and tachometer connec-
tors from capstan servo printed-circuit assembly.
b. Remove four socket-head screws securing the cap-
stan motor assembly to the transport casting.
4-2
7970B/7970C
c. Loosen two socket-head set-screws holding the cap-
stan to the motor shaft and remove capstan.
d. Place the capstan
on the motor shaft and using a
scale with 1/64-inch graduations, position the front edge
(top) of the capstan 1-1/32 (±1/64) inch from the motor
mounting surface. (See figure 4-1.)
e. Tighten capstan set-screws and reinstall capstan
motor assembly.
f. Reconnect capstan motor and tachometer connec-
tors to capstan servo printed-circuit assembly.
Figure 4-1. Capstan
PositiEll1
4-12. CAPSTAN (37.6 -45 IPS UNITS ONLY).
4-13. The capstan replacement for 37.6 -45 ips units is
the same as the 10 -37.5 ips units except that a single screw
secures the capstan
to the motor shaft. The capstan position
on the shaft is not adjustable.
4-14. REEL HOLDDOWN-ASSEMBLY.
4-15. The replacement
of the reel holddown assembly is
not critical; however, an adjustment of the reel retaining
knob must be made
to ensure that the tape reel mounts
firmly and does
not slip. Refer to section V for a descrip-
tion of the reel retaining knob adjustment.
4-16. TENSION ARM ASSEMBLY.
4-17. The replacement
of the tension arm assembly is not
critical; however, the tape rollers are critical to preloading.
The following procedures describe replacing the tension arm
assembly.
a. Disconnect the tension arm photosense connector
from the reel servo assembly.
Part 2