Fluke Biomedical 956A-201-M2 - Manual (Page 66)

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When an input pulse exceeds both the high and low discriminators, the high (U93A-5) clocked by the
transition through the low discriminator is reset by the low (U91A-1) resulting from the transition through
the high discriminator. This action causes no pulse to be generated at U93B-9.
Signal Multiplexer
The signal mutiplexer comprised of U101, U102, and U94 allows the MPU to select either the radiation
pulse or the frequency output representing the high voltage to be input to the gross counters. When
counter enable is active (high), the signal detection circuit output (representing radiation) is routed to the
gross counters. When HV SELECT is active (high), the High Voltage frequency, HVf, is routed to the
gross counters. The outputs connected to pull-up resistor R81, are open collectors allowing the most
significant bit of the counters to force this node low, effectively terminating the pulse input to the counters
and indicating an overflow condition.
Anti-Jam Circuitry
The anti-jam circuitry allows for the detection of rapid increase in pulses (due to a rapid increase in
radiation at the detector) and provides a bit to the sensitivity select register. A detector will reach a point,
in a very high radiation field, when it will no longer provide pulses, but conducts continuously. The
absence of pulses would normally indicate a low radiation field, when in actuality this is not the case. The
purpose of the anti-jam circuit is to detect that this situation is about to occur, and to indicate it to the MPU.
The MPU will then shut down the high voltage.
The input to the anti-jam circuit is provided by the low discriminator output (U91B-6). JP7 selects detector
type, 1-2 for scintillation detectors and 2-3 for G-M type. Q3 turns ON/OFF with input pulses, which allows
C39 to charge to an average DC level. VR12 (adjustment range 0 to 1.6 volts) is used to adjust the trip
threshold on comparator U92-2. When the repetition rate of the input pulse causes C39 to charge and the
DC level to exceed the threshold, comparator output U92-1 (low in normal operation) is forced high. When
this occurs, U96A-2 goes high (U96A-1 is high after power-up) U96A-3 goes low and U96B-4 goes high.
Diode D9 effectively latches this circuit in the jam mode. That is, if C39 discharges (due to absence of
input pulses) and U92-1 goes low, D9 becomes forward biased which holds U96-2 high. The high, now on
U96B-4 causes Q4 to turn on driving Q5 on, forcing U96C-8, & 9 node to ground. In normal operation,
JP6 is in position 1-2 allowing high current flow through F1 (1/20 Amp fuse) causing it to blow. R82 will
now hold U96C-8, & 9 node at ground, causing U96C-10 (anti-jam bit) to be active (high). At this point,
normal operation can only be achieved by replacing fuse F1. Jumper JP6 -position 2-3 is for test purposes
only and allows fuse F1 to be removed from the circuit and R79 provides pull-up to + 5 volts. In this mode,
cycling of power resets the anti-jam circuit. R93/C41 on U96A-1 provides a delay from power-up to inhibit
false tripping of the anti-jam circuit.
High Voltage Supply
The high voltage is utilized by a G-M detector (typical range 500 volts to 650 volts). The adjustment range
of the HV supply is 300 Vdc to 1800 Vdc. The HV output is short circuit proof in that it will current limit the
oscillator section within ten seconds of the output being shorted. The board plugs into the main circuit
board at the J8 connector.
R5 and associated circuitry provide the DC voltage adjustment to U1-C. The output U1-C-8 will vary under
control of R5.
Operation amplifier U1-A drives transistor Q1 that in turn drives the oscillator section transistor Q2, the
transformer T1 primary and feedback windings, and associated circuitry. As R5 is adjusted to increase the
high voltage, U1-C-8 voltage increases which causes U1-A to increase transistor Q1 base current. This
increases the emitter/collector current, raising the voltage on the emitter. As this control voltage
increases, the voltage developed across the transformer primary also increases. The transformer
secondary increases in voltage, which causes the high voltage output to increase. The voltage quadrupler
operation is illustrated in Figure 3-7.