CPI CMP 200 Series Service Manual page 295

7
Theory of Operation
7.2.12 AEC (Cont)
The start signal also exits from the generator control board. This is inverted by U6, and then inverted
again by Q4 and Q5. The output of Q5, when active, opens the analog switch that is part of the sample
and hold circuit at the input of U8A during an AEC exposure. This analog switch is closed at all other
times. The sample and hold circuit will sample any electrical noise at the output of U7A during standby
operation, and subtract this noise from the AEC signal during an AEC exposure. This ensures that the
output of U8A is proportional to the AEC chamber output voltage only, and is not influenced by noise.
The output of U8A connects to the common inputs of the analog switches that are in series with gain pots
R11 to R14. These analog switches are controlled by the chamber-select signal, and the switch
corresponding to the active channel will be closed. This connects the output of U8A to the AEC gain
adjustment potentiometer R11 to R14 corresponding to the active channel. The AEC signal, which will be
a ramp, is now fed to the input of U8B. The analog switch in the feedback loop of U8B is opened during
an exposure, allowing the AEC ramp to appear at the outputs of U4A and U4B.
The AEC ramp output from U4B is taken to the generator control board where it is processed as
described earlier in this section, and fed to comparator U9 on the AEC board where it is compared to the
AEC reference voltage. The output of U9 is normally high, switching low when the magnitude of the AEC
ramp equals the reference voltage. This PT stop signal is further processed on the generator control
board.
The START signal from Q4, and the LEFT, MIDDLE, and RIGHT field select signals from Q1, Q2, Q3
respectively are brought to the AEC chambers via J1 to J4.
Jumpers JW1 to JW8 swap the left and right fields on J1 to J4. Jumpering pins 2-3 of the field selector
jumpers (JW7 / JW8 for channel 1, JW5 / JW6 for channel 2, JW3 / JW4 for channel 3, JW1 / JW2 for
channel 4) connects the left field select signal to pin 6 and the right field select signal to pin 2. Jumpering
pins 1-2 of the field selector jumpers connects the left field select signal to pin 2 and the right field select
signal to pin 6.
Refer to MD-0936, sheet 6:
This is the functional schematic of AEC assembly 739389, used with various 3-of-5-field ion chambers.
The AEC chambers are connected to J1 and J2. The AEC signal from the AEC chamber(s) is routed to
the input of analog switches U1C and U1D on the AEC board. These analog switches are controlled by
the chamber-select signals.
The chamber-select signals originate on the generator control board as per sheet 1. Only one chamber
may be active at one time, and the active channel is indicated by DS1 and DS2 on the AEC board. The
chamber-select signals are inverted by U6. The signals CH 1 and CH 2 are connected to the control
inputs of the analog switches described in the previous paragraph. The analog switch corresponding to
the selected AEC channel will be closed when that channel is selected, thus connecting the AEC signal to
the input of U7A.
The channel 3 and channel 4 select lines from U6 are inverted by Q6 and Q7. The outputs of Q6 and Q7
are redesignated as the PORTRAIT and INVERTED signals, respectively. These signals select four
different combinations of the L-M-R fields from the five available fields on the AEC chamber.
The start signal also exits from the generator control board. This is inverted by U6, and then inverted
again by Q4 and Q5. The output of Q5, when active, opens the analog switch that is part of the sample
and hold circuit at the input of U8A during an AEC exposure. This analog switch is closed at all other
times. The sample and hold circuit will sample any electrical noise at the output of U7A during standby
operation, and subtract this noise from the AEC signal during an AEC exposure. This ensures that the
output of U8A is proportional to the AEC chamber output voltage only, and is not influenced by noise.
Use and disclosure is subject to the restrictions on the title page of this CPI document.
Page 7-14
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CMP 200 X-Ray Generator Service Manual
CPI Canada Inc.
Ch # 901476-09 Rev. C