TM 55-6670-200-14&P
Section Ill. REPAIR AND REPLACEMENT
c. Incidental phase shift is reduced by the
4-10. Repair.
selection of C107.
No repair of component parts is recommended.
d. Complete failure of the oscillator-amplifier
assembly should first necessitate replacement of
4-11. Replacement.
all tubes. Oscillator failure may be spotted quickly
with the oscilloscope. Tube failure, an open C105
Replace any defective part.
a.
capacitor (which primarily controls the fre-
b. Replace all defective tubes with one from
quency), or an open winding in T3, as well as too
the spare tube kit furnished with the equipment.
low a voltage (red lead), may cause a dead oscil-
lator. A continuous jitter with an apparent con-
stant frequency usually signifies a beating of the
bly to conform with specified resistance.
oscillator with the sixth or seventh line power
harmonic and may be rectified by varying capac-
itor C105 between 0.4 and 0.6 mfd. A variable ca-
Refer to figures 4-4, 4-5 and 4-10.
pacitance box should be used to determine a new
NOTE
value.
Before proceeding with testing of oscil-
e. Operation of amplifier may be easily
lator-amplifier assembly, refer to note in
checked by observing the signal with an oscil-
loscope on plates of V1, V2 and V3. Serious dis-
components.
tortion usually indicates a failure in T1. Excessive
noise may be indicative of improper shielding of
a. The plate-tuned oscillator (approximately
C1, the critical capacitor leading into the first
400 cycles) provides voltage through T3 to the
stage with its relatively low signal level.
cells, zero set (R19, R20, and R21 ), and the com-
plex calibration circuit. A reference signal is also
f. It should be pointed out that inaccuracies
carried on the grey lead to the phase-sensitive de-
of the system cannot ordinarily be traced to the
tector circuit of the amplifier between R112 and
oscillator-amplifier since it is a null balance cir-
R113.
cuit, with the exception of errors induced by an
insensitive system.
b. The 80-db amplifier is fed the algebraic
The meter may be a cause of error or RE-
sum of the voltages from the cell, calibration cir-
PEATABILITY discrepancies. It may easily be
cuit, and zero set through input transformer T1.
(Voltage of the calibration circuit is 180 out of
checked by tare zeroing at 500 pounds and then
phase with that of the cell and zero set circuit.)
pegging the meter left, returning the dial to 500,
This voltage is amplified through the four stages
and noting if any needle displacement persists.
Repeat by pegging meter right. This error could
and the output transformer. The meter is in turn
driven by phase detector V3 and its direction is
be caused by damaged bearings.
determined by the sign of the difference in volta-
h. A variable reading induced by cycling the
ges between the opposing cathodes and the refer-
load dial with a cell under a constant load may be
ence voltage. Gain is controlled through the feed-
caused by improper connections in the calibration
back circuit consisting of potentiometer R117 and
circuit or by load switch S4.
1st cathode of V2.
