Ludlum Measurements 12, Technical Manual

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Model 12 Technical Manual
Section 8
Ludlum Measurements, Inc. Page 8-1 July 2017
Technical Theory of Operation
MAIN BOARD (Drawing 464 × 275 with 3 sheets in
back of manual)
Input
Detector pulses are coupled through C16 to emitter follower Q4. R42
provides 3.7 volt bias. R41 couples to detector high voltage. CR3 protects
the input from high-voltage transients.
Amplifier
A self-biased amplifier provides gain in proportion to R15 divided by R14
and R47. Transistor (pin 3 of U4) provides amplification. U6 is coupled as
current mirror to provide a load for pin 3 of U4. The output self-biases to 2
Vbe (approximately 1.4 volts) at emitter of Q1.
Positive pulses from Q1 are coupled to the discriminator U8. Amplifier gain
is controlled by R47.
Discriminator
Comparator U8 has a fixed discrimination of 15 millivolts. Any pulse above
15 millivolts causes the output of U8 to go low. This negative pulse is
coupled to pin 5 of U9A for meter drive and pin 12 of U9B for audio drive.
The pulse is also available at pin 3 of P2 for special applications.
Audio
U9B develops a 22-millisecond pulse for each discriminator pulse if audio
switch is ON, allowing pin 13 of U9B to be high. Pin 10 of U9B is normally
low, preventing U12 from oscillating. When pulse goes high, U12 oscillates,
driving the unimorph speaker. U7B is normally closed, but during low
battery state, pin 5 of U7B opens, allowing R30 to pull pin 3 of U12 high,
which causes a steady tone even though the audio is
OFF
.
Section
8