Count Rate Circuit

TM 11-6665-245-34

across the meter. The capacitor is charged by the

is set so that small magnitude signals (noise) are

pulse and then discharges through R33 and the

eliminated.

meter to provide a steady reading on the meter.

c. Once Q2 is cut off, Q3 and Q4 turn on pro-

Capacitor C24 is switched into the meter circuit

viding a positive going pulse to be formed across

on the 1M and 1K positions of the selector switch

resistor R21.

and combines with C23 to change the time cons-

2-6. Count Rate Circuit

tant required on those ranges.

e. The output of U1D is used to energize the

(Figure 2-6)

headphones (when used) and provides the "click"

a, The count rate circuit consists of NOR gate

noise in the earpiece for each pulse received from

U1A, NOR gates U1B through U1D connected as

the inverter-buffer stage.

simple inverters, and transistor Q5 which is used

in a feedback loop to control U1A. Prior to any

2-7. Power Supply and Regulator

signal (positive pulse from the buffer inverter cir-

(Figure 2-7)

cuit Q4), the inputs (pins 1 and 2) to NOR gate U1A

The power supply and regulator circuit uses the

are both low, the output (pin 3) is high and tran-

voltage input from the two radiacmeter batteries.

sistor Q5 is conducting.

The output voltages which operate the

b. The positive going pulse from the buffer-

radiacmeter circuits are: minus 950, minus 9, and

inverter starts the circuit into operation. When pin

plus 3.8 volts dc. These voltages are generated

2, U1A, goes high, pin 3, its output goes low. The

from a blocking type oscillator circuit using step

voltage change is coupled directly to the base of

up voltage windings to generate an ac voltage

Q5 through timing capacitor C21. Q5 is then cut

off and its collector goes high holding the output

which is rectified for the dc voltages.

of U1A low. This condition holds until timing

a. The blocking oscillator circuit uses tran-

capacitor C21 discharges through resistor R23

sistor Q6 as the oscillator. The feedback for the

enough to allow Q5 to turn on, causing pin 1 of

oscillator is provided by winding 4-5 of

U1A to revert to a low condition. This action

transformer T1 coupled through the transformer's

results in pin 3, U1A, first going low as the result

core. The feedback signal alternately cuts off Q6

of the inverter-buffer input pulse and then staying

and drives it into saturation. The length of time Q6

low until the timing capacitor discharges forming

is cut off is controlled by the charge on capacitor

a negative going pulse on the input of U1B

C17 and C18. The charge path for C17 and C18 is

through U1D.

t h r o u g h and controlled by transistor Q7. Tran-

c. The negative going pulse at pin 3, U1A, is

sister Q7, in turn, is controlled by the differential

started by the inverter-buffer pulse. Its length is

amplifier pair of transistors Q8 and Q9 to set the

determined by the discharge time of C21 when S1

charge (voltage of C18). Controlling the voltage on

is in the position shown. When S1 is in other

C18 then controls the time Q6 is cut off, which

ranges, the value of the timing capacitor changes

controls the frequency of the oscillator and the

and so does the length of the output pulse. These

three power supply outputs.

different pulse lengths are used for different

b. Three ac output voltages are taken from

ranges and cause the meter to read correctly as

transformer T1. There is a separate winding bet-

scales are changed.

ween pin 6 asnd either pins 7 through 11 for the

d. Outputs from U1B and U1C are tied

high voltage supply and a common winding (pins

together and connected to the indicating meter

1, 2, and 3) to provide the low voltage outputs.

through potentiometer R26, resistor R27, switch

Both low voltage outputs use voltage doubler type

S1, and resistor R33. Since the meter cannot res-

rectifier arrangement to obtain their outputs. The

pond to the pulse, capacitor C23 is connected

high voltage rectifier arrangement is a voltage

tripler.

2-4

Change 1