functioning properly.
Sometimes it is necessary to
(3) If it is necessary to unlce a wiring harness
a
substitute a number of tubes before one is found that will
to remove a defective wire, replace the
function satisfactorily.
wiring harness after replacement.
(4) Do not use a large iron to unsolder small
58. Power Supply Circuit Failures
capacitors and resistors. These parts may
a. Since the operation of the entire signal generator
be damaged permanently by the application
is dependent upon the proper functioning of the power
of an excessive amount of heat.
supply, it is wise to check the operation of this circuit first
(5) Do not permit drops of solder to fall into the
before proceeding with the more complicated and time
set; they may cause short circuits.
consuming procedures. A heavy overload on the power
(6) It is very important to make well-soldered
supply probably will cause F1 or F2 to blow. Repeated
joints, since a poorly soldered joint may
fuse failures indicate that trouble exists in the power
cause troubles difficult to find.
supply or in the circuits fed by the power supply. If the
(7) When a part is removed from the high-
signal generator is operating satisfactorily over an
frequency (hf) circuits, it must be replaced
extended period of time, an occasional fuse failure should
by a part with identical physical and
not be viewed with alarm as it may be the result of a
electrical properties. A part which has the
momentary surge. Check the power supply in accordance
same electrical value, but differs in physical
with the following instructions:
size may cause trouble in hf circuits. Give
b. If the fuses are found to be intact, measure the dc
particular attention to proper grounding when
voltage at pin 5 of tube V12 for a reading of +400 volts.
replacing a part. In all cases, ground the
Also, check for +200 volts dc at pin 6 of V12, and +150
part at the same point at which it was
volts de at pin 5 of V13.
grounded previously. Failure to observe
(1) If the voltages measured differ appreciably
these precautions may result in decreased
from the specified values, check and, if
output,
misalignment,
or
spurious
necessary, replace V12, V13, V14, or V15.
oscillations.
(2) If the trouble is not corrected by tube
(8) Do not disturb the settingsof the various
replacement, make voltage and resistance
trimmer capacitors, tuned inductances, or
measurements of the entire power supply
adjustment potentiometers, unless it has
circuit.
been
definitely
established
that
a
(3) It is entirely possible that the defect in the
misadjustment is the cause of trouble.
remaining circuitry of the signal generator is
(9) After replacing a part in a circuit, check the
the cause of the low- or high-voltage output
operation and calibration of the circuit or
of the power supply. Check for burned or
circuits affected in accordance with the
overheated resistors in other parts of the
procedures given in paragraph 88. Adjust
unit, and make voltage and resistance
and aline, if necessary.
measurements of all tube sockets and
terminal boards.
57. Checking Parts
(4) A fuse failure along with a malfunctioning
Many of the parts used in the signal generator are
signal generator is evidence that trouble
manufactured to close tolerances. For proper operation
exists in the power supply or B+ circuits.
of the signal generator, it is necessary that these parts
Starting with the power supply circuits, make
remain within the required exacting values. Therefore,
a complete resistance check of all the
the ordinary means of checking parts will not suffice in
circuits in the signal generator (figs. 37 and
some instances.
a. If tolerances are exacting, use a precision bridge
to check capacitors and resistor
59. Troubleshooting Chart
b. A good indication, using the best tube tester
The following chart is supplied as an aid in locatin
g
available, is not always conclusive evidence that a tube is
trouble in the signal generator. Listed in the chart are
good. At the high frequencies present in the signal
abnormal indications which a repairman may encounter
generator, direct tube substitution in the rf circuits is the
while making a few simple tests.
only effective means of determining whether a tube is
38
