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Model 450A
Section IV
Paragraphs 4-15 to 4-21
)
is increasing line-frequency ripple as the line volt-
age is decreased. Incorrect or unstable voltage level
can be due to incorrect or unstable reference voltage
obtained from V7. High ripple at all line voltages is
an indication of poor electrolytic filters or weak V6.
4-15.
MEASURING AMPLIFIER STAGE GAIN.
4-16. The typical amplification factor for each stage
in the amplifier is gi ven below.
Gain is meas ured
by applying 0.01 rms volts at 1000 cps to the ampli-
fier INPUT terminals with the amplifier GAIN switch
set to 40 DB.
The 4000 Voltmeter is then used to
measure the resultant signal level at the input and
output of each stage, each time dividing the output
by the input voltage.
VI
V2
V3
E.
E out Gain E.
E out Gain E.
E
out
Gain
In
In
In
0.01 0.043 4.3
0.43
1.1
25.6 1.1
l.0
0.91
4-17.
AMPLIFIER ADJUSTMENTS.
4-18. The amplifier contains three adjustable com-
ponents which are used to obtain specified amplifier
performance with the normal variations in replace-
ment tubes and parts values.
Resistor R27 adjusts
the dc heater voltage applied to VI and V2. Resistor
R6 adjusts amplifier gain at middle frequencies. Ca-
pacitor C 11 adjusts amplifier gain at high frequencies.
Instructions for making each adjustment are given
in the following paragraphs.
4-19. ADJUSTING V1 & V2 HEATER VOLTAGE.
4-20. The heater of VI and V2 are supplied with dc
power to reduce line-frequency ripple in the ampli-
fier output.
The power is obtained from a full-wave
selenium rectifier bridge through an adjustable series
resistor, R27.
Resistor R27 permits resetting the
heater voltage as the rectifier ages and its internal
resistance increases. The adjustment must be made
at six-month intervals and when the rectifier is re-
placed. To adjust R27 proceed as follows:
a. Remove amplifier bottom plate; connect ampli-
fier to power source, turn on and allow 3-minute
warmup.
b. Measure the dc voltage from the positive termi-
nal of C7 to chassis. This voltage must be 6.3 volts
when the line voltage is 115 volts.
c.
If
necessary, adjust R27 to obtain 6.3 volts. After
a 24-hour run-in, recheck voltage to see that it has
settled.
d. Measure the ac voltage across C7.
If
it is greater
than 150 millivolts, check the capacity of C7.
e. The adjustment is completed; replace amplifier
bottom plate and return amplifier to normal service.
4-21.
ADJUSTING AMPLIFIER GAIN.
4-22. Amplifier gain at low and middle frequencies
for both the 20 and 40 db positions of the GAIN switch
is set by a potentiometer on top of the amplifier
chassis.
The adjustment per-mitt;' setting the gain of
either range exactly, or diViding any small error
equally between the two ranges.
Amplifier gain at
high frequencies for both the 20 and 40 db settings is
set by a trimmer capacitor on the bottom of the ampli-
fier chassis.
This adjustment pennits setting the
gain of either range at some selected high frequency
to equal the low-frequency gain, or pennits .dividing
any small error between the ranges, and frequencies.
Both adjustments are required after replacement or
aging of VI, V2, V3, R3 or C12. To adjust the gain
of the amplifier proceed as follows:
a. Connect amplifier and test equipment as shown
in figure 4-1 using the 400D Voltmeter to alternately
measure the input and output signal voltage levels
from the amplifier.
VOLTMETER
4000
@
@
o
00
o
o
o
AMPLIFIER
450A
OUTPUT
/ ' ; . / '
INPUT
/ ' / ' / '
-
---------
...,.",/
-
-
-
-
~-
CONNECT LEADS HERE TO
MEASURE AMPLIFIER
OUTPUT.
CONNECT LEADS HERE TO
MEASURE AMPLIFIER INPUT.
o
USE OPEN WIRE LEADS LESS THE
ONElI) FOOT IN LENGTH FOR
MEASURING I AND 2MC GAIN.
Figure 4-1. Test Setup for Measuring Amplifier Gain
ADJUSTABLE LINE
TRANSFORMER
o
TO
POWER
SOURCE
~------------~O-U~T~PU""~~
: : : -
--
- -
- -~
o
HEWLETT- PACKARD
MODEL 650A
OR EQUIVALENT
FOR MEASURING 10 CPS -2MCS GAIN
)
00001-1
4-3
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