2-10. TIME-SELECTOR SWITCH
The timer is an analog-type switch using a resistance-capacitance (RC)
combination. On the impulse-timer deck of the time-selector switch are 24 resistors
connected in series which form the "R" part of the timing circuit. As the selected time
increases, the total resistance is increased because a greater number of series-
connected resistors are selected. These resistors are inserted into the charging path of
C13 timer capacitor shown in figure 2-8 in lead D2T.
a. The voltage across the impulse-timer deck is some portion of positive (+)
12vdc as adjusted by the short-time-adjust pot (P2) also shown in figure 2-8. This
positive voltage is on leads TR1 and D2T. The back-up timer deck of the time-selector
switch utilizes resistors in the same manner as the impulse-timer deck. As the selected
back-up time increases, so the total resistance increases.
b. The back-up timer allows an approximately 10-percent increase in exposure
time 1 at 50Hz, in the event of failure of the impulse timer. The time increase will be
greater at 60Hz. The appropriate back-up time is automatically selected as the regular
exposure time is selected. These resistors are inserted into the charging path of C1
capacitor in the timer back-up circuit in lead DBR (figure 2-9). The voltage across the
back-up timer deck is some portion of positive (+) 24vdc as adjusted by pot R2 in the
back-up-timer circuit. This positive voltage is on leads DBR and 3D.
c. The remaining deck of the time-selector switch acts as an interlock to the ma
overload. From lead EER, 115vac is applied across the timer-switch contacts, and exits
to lead ERX. When the time selector is on the open contacts of the deck, the
safety-reset lamp illuminates to show an improper technique has been entered into the
control. It also prevents an exposure.
2-11. BACK-UP TIMER (CIRCUIT FUNCTION DESCRIPTION 6.2)
Refer to figure 2-9. The back-up timer operates in two modes. The first is
normal exposure, when the back-up timer causes the back-up contactor to pull in prior
to the beginning of an exposure, and releases the back-up contactor after the SCR
contactor turns off. During phototiming, if the phototimer terminates the exposure prior
to time out, the back-up contactor does not release. The second mode provides for
exposure interruption by releasing the back-up contactor immediately when the
overload condition is detected.
a. Upon the initiation of the PREP function (or cassette advance during spot
filming), signal H3 from the exposure circuit goes to 115vac, energizing relay K4. K4
opens the filament-standby shunt through contacts XBR/XD, bringing the tube filament
to full emission.