(2) Lead XA1 to pin 12 of IC3 is a high which "closes" pins 10 to 11
connecting P1 and P11 to lead XDA ma-set. The boost potentiometer P21 is into pins 8
to 9 of IC3 which are open because pin 6 is low. At the start of exposure OC1 turns on,
pins 5 and 6 of IC3 are high, closing pins 9 to 8 and 11 to 10, boost potentiometer P21
is shorted out, and ICs 5 and 6 are held inhibited. At the end of exposure, all return
back to a standby state.
MA-METER AND SHUNT CIRCUIT, CIRCUIT FUNCTION DESCRIPTION 5.0
Refer to figure 2-7. The ma meter is a one-ma full-scale device, scaled for three
dc-ma ranges of current indication, 0-10ma, 0-100ma and 0-400ma. With this
arrangement, all radiographic ma selections indicate within the central half of the meter
from 1/4 scale to 3/4 scale. The indicating range is selected by providing three
adjustable meter shunts, which when selected, parallel the meter and pass the majority
of the X-ray tube current around the meter.
a. The system is configured so the meter shunt of interest is actually in parallel
to the combination of the focal-spot-overload sensor and the ma meter connected in
series. The focal-spot-overload sensor is adjusted to 6,000Ω, and the ma meter has an
internal resistance of approximately 80Ω. This combination produces a series
resistance of 6,080Ω. At 1ma meter current, the voltage across the combination is 6.08
volts for a full-scale meter indication.
b. The meter shunts are sized to pass the remainder of the X-ray tube current.
On the 10-ma scale, the shunt resistance is approximately 675Ω, on the 100-ma scale
61.4Ω and on the 400-ma scale, 15.2Ω. The approximate shunt for the expected ma
range is selected through a relay-pole series on the ma relay selector circuit board.
When the fluoro spot-film mode is selected, two different shunts must be selected
dependent on the mode of operation. When fluoro is active, the 0-10-ma scale is
selected via a relay on the spot-film changeover to pass the shunt current through lead
MMF to the low-current ma-meter shunt.
c. When the radiographic mode is selected by the advance of the spot-film
device cassette carrier, the shunt current passes through lead MS1. Lead MS1
connects to a jumper which provides selection of either the middle-shunt range for spot
radiographic ma of 25 to 75ma, or the high-current shunt for ma of 100 to 300ma. This
provides field selection of the spot radiographic ma. The ma-meter shunt is calibrated
using a dc-current source set to the appropriate ma-selection indicator. The variable
potentiometer is adjusted so the meter indicates the correct ma.