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Chapter 3. REPAIR INSTRUCTIONS
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TM-10-3930-620-34 Truck Lift Fork Solid Rubber Tired Wheels 6000 Capacity 180 Inch Lift Manual
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Ground Test
k. Drive Power Circuit. The drive motor operates
speed and therefore prevents excessive surge currents.
With the accelerator pedal at normal (up), the speed
when the power switch assembly transistors are turned
interlock switch contacts are held actuated as shown in
on by the pulses from the control unit. The direction of
the schematic diagram. This permits normal operation.
motor operation is determined by the direction of current
With the accelerator pedal depressed, the contacts
in the drive motor field winding. The schematic diagram
become normalled and the operate path to the control
shows one transistor circuit of a single power switch
relay is open.
module. Each module contains two circuits. There are
h. Direction Interlock.
The direction interlock
twelve such modules in the power switch assembly,
contacts on the forward and reverse contactors prevent
making a total of twenty-four circuits.
These are
both contactors from being energized at the same time.
connected in parallel in order to handle the high current
Assuming an initial forward direction and a switch to
required by the drive motor.
I. Capacitor and Diode Assembly.
reverse, the collapsing field of the F coil creates a
current through the coil and its parallel diode, keeping
(1) During the off time of the power switch
the contactor energized momentarily.  The F interlock
assembly, the collapsing magnetic field of the drive
contacts keep the circuit to the R contactor open at this
motor field winding tends to induce a current through the
time. When current has diminished sufficiently in the F
motor armature in the proper direction. Diode DI acts as
coil, the contactor releases, allowing the path to be
a free-wheeling diode to pass this current. This keeps
completed to the R contactor.
average drive motor current at a higher level than
i. Key Switch Bypass.
The key switch is
average battery current.
connected in parallel with two pairs of normally open
(2) When the power switch assembly cuts off
direction selector contacts.  As long as a direction is
after each drive pulse, there is a brief period before
selected and the accelerator pedal is depressed, this
diode D1 can sustain the motor conduction. During this
prevents the key switch from being used to turn off
period diode D3 conducts, charging the 1000 microfarad
power.  If the key switch is turned off, power is not
capacitors. The capacitors discharge through the 0.33
removed until the directional control lever is moved to the
ohm limiting resistor each time the power switch
neutral position or the accelerator pedal is released.
assembly is turned on.
This protects against a loss of bias voltage to the power
(3) Diode D2 connects across the drive motor
switch until the directional contactor has been de-
armature and functions only during rapid reversals of
energized.
direction.
During such a reversal, the armature
j. Control  Unit.
The  control  unit  provides
continues to operate in the previous direction for a brief
period. The armature acts as a generator at this time.
controlling voltages for the power switch assembly,
Diode D2 carries the generated current and therefore
monitors available battery voltage for an undervoltage
prevents an excessive voltage from appearing across the
condition, and monitors drive current consumption for an
armature.
overcurrent condition.
(4) The  two  5500  microfarad  capacitors
(1) With a direction selected, the setting of the
connect between B2- and B2+. These act as a filter for
speed potentiometer determines the voltage provided at
any voltage transients caused by switching in the system.
control unit terminal 5. This in turn determines the width
of the drive pulses supplied at terminal 3.
The
3-3. General Requirements for Test and Adjustment
transistors of the power switch assembly are on during
Precautions to be followed throughout all electrical
each drive pulse,(-5 volt input). The transistors are kept
testing and adjustments are as follows:
off in the absence of a drive pulse (+3 volt input) by the
a. Always check battery polarity.
bias voltage provided at terminal 1.  This voltage is
b. To shut down power, disconnect battery and
approximately +5 volts dc with respect to battery positive.
discharge capacitors according to paragraph 2-5.
(2) The control unit senses battery voltage
c. Always  disconnect  battery  and  discharge
between terminal 8 and B2+ .  If the battery voltage
capacitors before taking resistance readings.
between these points drops to about 24 volts, the drive
d. If battery is connected, never manually operate a
pulses are cut off until voltage is restored.
directional contactor unless key switch is on and control
(3) Drive motor current for the truck is through
system is operating.
the current sensing shunt.  The voltage developed
e. Do not ground or short any heat sinks (finned
across the shunt is monitored by the control unit between
assemblies) as they are integral parts of circuits. Never
B2+ and B1 +. If the current becomes excessive, the
eliminate or modify heat sinks.
control unit cuts off drive pulses. The current is sensed
f. Do not operate truck without the complete
with each pulse so that normal operation is restored
compliment of power switch transistors, except in
when the overcurrent condition is removed.
extreme emergencies.
Operation without the full
3-2

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