Fluke Biomedical 190N - Manual (Page 15)

Service Information
General
4
4-1
WARNING
Section 4
Service Information
An electrical shock hazard exists between the high
voltage supply and ground.
4.1 General
The 190N contains 4 functional elements:
1. Model 190 Survey Meter
2. Probe Adapter Module / Preamplifier Assembly
3. Moderator Assembly
4. BF
3
Tube
See the Model 190 manual for all items pertaining to the Model 190. The detector assembly is composed
of a polyethylene cylinder, other neutron moderating and absorbing materials, and a BF
3
tube. The Probe
adapter module on the Model 190, the cable, and the preamplifier in the moderator housing couple,
shape, and discriminate the pulses from the BF
3
tube to the Model 190.
4.2 Circuit Description
Refer to the basic Model 190 manual, part number 190001, for all items pertaining to the Model 190.
The 190PA Probe Adapter/Preamp consists of two circuits boards that are interconnected with a shielded
multi-wire cable. Sheets 1 & 2 of the circuit diagram 190123 shows both of these circuits. In sheet 1, the
first amplifier stage is a charge sensitive amplifier that integrates the radiation produced detector pulse on
capacitor C5. Resistor R5 discharges C5 exponentially. The second amplifier stage shapes the pulse into
approximately 1 microsecond in width.
In sheet two of the circuit diagram, the preamplifier pulse is capacitor coupled to a comparator circuit that
has an amplitude discrimination level of about 80 millivolts. The 93C46 device is an EEPROM that is used
for storage of calibration factors, probe ID and other similar data specific to the operation of the system.
The Model 190 provides high voltage, typically 1100 V, for operation of the detector. Additional filtering of
the high voltage is provided by R1, R2, C2, C3, and C4 as shown is sheet 1 of 190123.
4.3 Calibration and Adjustments
There are no calibrations or adjustments to be made to the detector assembly. For calibrations and
adjustments to the Model 190, see the Model 190 manual.