Fluke Biomedical 06-526 - Manual (Page 22)

Nuclear Associates 06-526 & 06-526-2200
Operators Manual
3-2
The high voltage utilized by the ion chamber is generated by the blocking oscillator circuit comprised of
Q4 and associated components. Zener diode CR3 determines the output voltage thus enabling the circuit
to be less sensitive to changes in input voltage.
The electrometer and the blocking oscillator operate from an internal +5 V supply generated by U5.
Transistor Q3 detects low battery voltage and turns on the LOW BAT display through R36.
U3 is a buffer for C13 that retains the last output of U4 through R32. The output of U3 is displayed on the
VRM through selection of the DOSE switch position (SW2B). R33 is the zero (offset) adjustment for U4.
CR1 provides a 1.2 V reference for the digital display VRM (at pin 7).
3.5 VRM Display
The voltage ratio module consists of an auto-zeroing, dual-slope analog to digital converter and a 3 1/2
digit liquid crystal display (LCD). The VRM converts two independent voltages into a display reading
representing the ratio of the two voltages:
Display = (IN HI - IN LOW)
(RFH-RFL)
Because of the compact nature of the VRM board, it should be completely replaced if defective, rather
than attempting repairs.
3.6 Calibration and Adjustments
This instrument has been calibrated to ± 5% at 75 kVp with 4 mm Al filtration at 72°F (22°C) and 760 mm
of Hg (one atmosphere).
The user should be aware that changes in altitude
and temperature will affect the reading (refer to Air
Density Corrections in Section 2).
The voltages listed below should be checked before continuing with adjustments or calibrations.
1. TP6 - TP8 = + 7 to + 9 V
2. TP6 - TP9 = + 5 V ± 0.5
3. TP12 - TP10 = - 270 V ± 10% (use an electrostatic voltmeter)
3. TP6 - TP4 = 1.1 V ± 0.2 V (see the adjustment for R19 for information)
The control functions are discussed in the following paragraphs. Refer to the internal circuit board for
component locations.
NOTE