2-14. SPOT FILM CHANGEOVER CIRCUIT (CIRCUIT FUNCTION DESCRIPTION 8.1)
Refer to figure 2-12. The changeover switch in the spot film device is a
mechanically actuated switch operated by the cassette tray itself. When a spot film is
commanded, the cassette tray moves forward to the in-beam position closing the
a. The spot-film-preheat adjustment is necessary to keep the large focal spot
filament in the under-table tube warm during fluoro so when a spot film is called for, the
boost time will not be excessively long. However, the filament cannot be hot enough for
emission during fluoroscopy.
b. During fluoroscopy, relay Re-6-SF located on the relay matrix PCB is
energized via lead FT from the fluoroscopic timer and contacts TE/TL are closed
supplying 115vac, via lead TL to the fluoroscopic exposure control and to the power
circuit to energize the fluoro contactor.
c. When the fluoroscopic timer reaches zero time, leads TL/FTE close and relay
Re-6-SF remains closed through the FTE diode, holding the contactor and a diode. The
fluoro exposure continues, but the fluoro time-expired buzzer sounds. When the fluoro
exposure is released, relay Re-6-SF will open.
d. If another exposure is attempted before the fluoro timer is reset, it will be
inhibited since Re-6-SF will not close. The fluoro time-expired buzzer will sound to alert
the operator to reset the fluoro time.
e. The amount of fluoro time is controlled by a five-minute, synchronous, motor-
driven timer, which normally runs whenever fluoro is active and is shut off when fluoro is
f. A mechanical cam, connected to the dial shaft, operates a single-pole
double-throw (SPDT) switch. The switch is held by the cam in its normally opened
position when the dial indicator is between five minutes and zero time, and transfers to
its normally closed position at zero time. The timer stops at this point. The cam,
consisting of two, 180 segments, is both positional and angle adjustable. Since the
timer uses a synchronous motor, the angular rate of motion will depend on the
frequency of the existing voltage. The rate of motion at 60Hz is 62 per minute. At
5OHz the rate of motion is 51 2/3 per minute.
g. The panel is provided with a reversible decal, indicating the time divisions for
each frequency. The zero point on both sides is at the same angular position relative to
the placement and the OFF position of the cam need not be changed. The decal facing
and start-cam (five minutes) position must be adjusted for the operating frequency of