Chapter 3 Theory of Operation
Models QT-740 & QT-750 - Service Manual
29
A1J1-9 and A1J1-1 to DC Motor Driver Board A2 H1 and H2 terminals. A1K5 and
A1K6 are also energized in a similar fashion, which releases the Motor Brake and
redundant Table Brake. The delay of 150 milliseconds allows A1K3 to energize
before A1K4 energizes. A similar sequence occurs in reverse when the UP foot
pedal is released; A1K4, A1K5, and A1K6 are de-energized in approximately 1
millisecond and then A1K3 is de-energized approximately 150 milliseconds later.
At the same time, the /UP signal applied through connector A1J2-1 is inactive
high, and UP relay A1K1 is de-energized (through components A1U2A, A1U4B,
A1U1A, and A1U4A). As a result, the CONTROLLER(+) signal from DC Motor
Driver Board A2 is routed to the DC motor (through A1J1-10) as the MOTOR sig-
nal. This causes the motor to drive the Scissors Assembly downwards.
To control downward motor speed, resistors R1 through R4 are placed in parallel
with the DC motor. The resistors are connected to 6-Way Table Control Board
connector A1J1-2 and A1J1-12. When the /DN signal is active, DOWN relay A1K2
is energized connecting brake resistors R1 - R4 in parallel with the DC motor at
A1J1-12 and A1J1-2.
Table down motion is stopped in any of the following ways:
• When the DOWN foot pedal is released, the /DN signal returns to an inac-
tive high level, de-energizing DOWN relay A1K3 through A1U2D, A1U4D,
A1U1B, and A1U4C and INHIBIT relay A1K4 through A1U2A, A1U4B, and
A1U1C. When de-energized, the CONTROLLER(-) input is open, disabling
the DC motor.
• After 24 seconds from the time the active low /DN signal was first
applied, the output at one-shot A1U6A-6 returns to a low level. (A1U6A
was triggered by the DN signal produced by A1U2B-4.) The low output
from A1U6A-6 de-energizes DOWN relay A1K3 through components
A1D17, A1U4D, A1U1B, and A1U4C and INHIBIT relay A1K4 through
A1U2D, A1U4D, and A1U1C. The motor drive circuit is interrupted
thereby stopping tabletop vertical motion. Releasing the DOWN foot
pedal before 24 seconds will cause a low level to be applied to A1U6A-3,
thereby resetting A1U6A and stopping tabletop vertical motion.
• An Obstruction Sensor (S3-S6) is located at each end of the Transverse
Assemblies below the tabletop frame. The function of the Obstruction
Sensors is to disable table motion in the event that an object, such as a
chair, table, etc., becomes wedged below the tabletop as it is traveling
downward. This feature is designed to avoid damage to the table only
and is not intended as a safeguard against personal injury. Activation of
any obstruction switch (S3-S6) applies a motion inhibit signal that de-
energizes relays A1K3 and A1K4, thereby disabling DC motor operation.
• Limitation of downward travel of the Scissors Assembly is accomplished
by Down Limit switch S7 and redundant Down Limit switch S10. The
switches are mounted on the table base plate close to the Ball Nut
Assemblies. When the tabletop is lowered, the lead screws drive the Ball
Nut Assemblies toward the Down Limit Switches. At the point where
switches S7 and/or S10 are activated, a low DN LIMIT signal is applied
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