Chapter 3 Theory of Operation
Models QT-740 & QT-750 - Service Manual
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contact closure at A1J4-1 and A1J4-2. This contact closure would be fed to the
table's PBL circuit to indicate the 40" SID position.
Receptor Cabinet Lock
The Receptor Cabinet magnetic lock has its own release switch (S15) for Receptor
Cabinet positioning. When S15 is pressed, the Receptor Cabinet locking magnet
is de-energized enabling cabinet movement.
Emergency Stop Switch
The Emergency Stop switch (S16) is a two-position push-button switch connected
across the 110 VAC secondary of transformer T1. Pressing the switch breaks the
110 VAC input connection applied to DC Motor Driver Board A2 (at one AC termi-
nal) thereby removing power from the table DC motor drive circuit. To restore
power, the switch must be manually released to the normally closed position.
In addition to providing an emergency shut-off function, the Emergency Stop
Switch serves as an indicator lamp that flashes at a 1-Hertz rate in the event one
of the following "faults" occur:
• Any of the obstruction sensors (S3 - S6) are activated (during tabletop down
movement)
• A drive belt fault condition occurs (i.e. snapped or disengaged belt), sensed
by normally-closed MOTOR BELT switch S22
• A sync belt fault condition occurs (i.e. snapped or disengaged belt), sensed
by normally-closed SYNC BELT switch S23
• DC Motor over temperature fault occurs (motor temperature exceeds 70°C),
sensed by DC Motor thermal switch S21
• The 20 VAC input from transformer T1 falls below 15% nominal
• Redundant down limit switch S2 or redundant up limit switch S9 is activated
When any of the above conditions occur, a low-to-high transition is applied to
oscillator A1U8-4, which causes a 1-Hz square wave output at A1U8-3. The out-
put from A1U8-3 turns transistor A1Q4 on and off, generating the FLASH signal
that drives the EMERGENCY SWITCH LAMP DS1 through connector A1J2-10.
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