TM 11-6625-524-14-4
h. R e f e r e n c e amplifier: E m i t t e r follower transistors, 3Q3 and 3Q4, are con-
n e c t e d to the DC modulator driver's output. Amplifier transistors, 3Q5 and 3Q6, are
d i r e c t - c o u p l e d to the interstage transformer (3T1). The output of transformer 3T1 is
a p p l i e d to the synchronous demodulator's reference source transistors.
i . Synchronous demodulator: The demodulator's transformer (3T2) compares the
m o d u l a t e d signal from the post amplifier's output to the reference amplifier's out-
put. Transistors 3Q7 and 3Q8 provide a phase sensitive reference source for the de-
m o d u l a t o r . The resulting rectified DC output signal voltage appears at 3T2, Pin 2
a n d its polarity corresponds to the original signal's output from the probe. A
filter capacitor (2C6) removes any AC components while diode 2CR7 dampens and sup-
presses below SET REF level excursions.
j . O u t p u t linearizing network: T h e network corrects for the non-linear response
o f the probe's cartridge diodes. D i o d e s of similar characteristics as the cartridge
2CR6, along with resistors 2R21 and 2R22 and capacitor 2C5 achieve a linear ampli-
f i e r o u t p u t c h a r a c t e r i s t i c . The network functions to expand the lower end and com-
press the upper end of the meter scales. Upper end (HI) and lower end (L0) cali-
b r a t i o n controls are used in conjunction with the linearizing network.
Thermis-
t o r , 2 R T 1 , c o r r e c t s f o r v a r i a t i o n s i n a m b i e n t temperature extremes.
k. C a l i b r a t i o n c o n t r o l s : Each measuring range is equipped with three (3) cali-
b r a t i o n controls. These controls are located on the voltmeter's rear mounted cali-
b r a t i o n controls board and are screwdriver adjustable. Easy access to the controls
is obtained by removing the rear panel cover plate.
T h e "MID" calibration controls are used to adjust the voltmeter's overall DC feed-
back voltage. Proper adjustment is made when the meter reads 60% of full scale.
Each range's " M I D " feedback network consists of a variable resistor ("MID" control)
a n d two (2) resistors in series with it; one (1) located on the calibration con-
t r o l board and the other in the probe.
The "HI" and "L0" calibration controls are used to adjust the 100% and 30% of full
scale meter readings, r e s p e c t i v e l y . These two (2) controls are part of the output
l i n e a r i z i n g network and provide accurate linearization correction adjustments.
F o r a better understanding of the output linearizing network and calibration con-
t r o l s c i r c u i t r y , F i g u r e 4-2, linearizing correction network, details the 0.001V full
scale range. A brief description follows:
The rectified DC signal voltage, which appears at the synchronous demodulator's out-
p u t (3T2, Pin 2) is applied to the output linearizing network and calibration con-
trols. Capacitor 2C6 adds filtering while diode 2CR7 dampens and compresses below
SET REF excursions.
For readings in the mid meter scale region (50% to 70% of full scale) the response
w i l l b e n e a r l y l i n e a r . T h e signal splits into two (2) paths; one (1) through the
path
output linearizing network of 2R22, 2R21, 2CR6, 1M1 and 5R1 and the second
through the DC feedback circuit of 4R9, 4R33, 1RT7, 1RT5, 1RT3, 1RT2 and 1RT1, probe
resistor (500K) and 3R9. The high resistance of 4R1, 4R35, 2RT1, 2F24 and 2CR5 will
w i l l prevent current flow through the "HI" network.
T h e effect of 2CR6 will be to allow current flow through itself and thus bypass the
" L 0 " network. This action keeps the reading nearly linear in this region. The "MID"
,
c o n t r o l (4R9) is adjusted for 60% of full scale indication.
For readings in the upper meter scale region (70% to 100% of full scale) the effects
of 2CR5 and 2CR6 become minimal allowing partial current flow through the "HI"
l i n e a r i z a t i o n correction network. This will act to compress readings in this region.
T h e "HI" control (4R1) is adjusted for 100% of full scale indication.
For readings in the lower meter scale region (30% to 50% of full scale) the resis-
tance of 2CR5 and 2CR6 becomes very high allowing partial current flow through the
" L 0 " linearization correction network. This will act to expand readings in this
region. The " L 0 " control (4R17) is adjusted for 30% of full scale indication.
Change
1
4-3
