TM 11-5826-225-35
measurements with the ohmmeter leads con-
2-9. Additional Troubleshooting Data
nected one way and then reverse the leads.
a. Dc Resistances of Transformers and Coils.
The dc resistances of transformers and coils
measurements, check a receiver known to be
are given in the chart below.
in good operating condition. Since all low-
power transistors of the pluck-out type op-
erate on very low voltages, a capacitor seldom
causes trouble. If an IF transformer is sus-
pected of having an open winding, remove the
associated transistor before making resistance
m e a s u r e m e n t s . For all resistance measure-
ments, use Multimeter TS352/U. Set multi-
meter on the RX100 range.
If the receiver is inoperative in all
Caution:
operating modes (no sound or bearing indica-
tion) and a shorted voltage regulator filter ca-
pacitor is suspected, check the resistance of
capacitors C88, C116, and C146 (figs. 2-6 and
2-7) before applying power.
e. Voltage Measurements. M a k e voltage
measurements in accordance with figure 217.
T r a n s i s t o r s in the push-pull RF amplifier,
oscillator, bfo, and IF amplifier have collector
load impedances which present the proper load
at the resonant frequency but present such a
low dc resistance that no collector voltage can
be measured. In these transistor stages, a slight
shift in transistor characteristics may change
emitter and base-bias voltages from that in-
dicated in figure 217. If this occurs, try an-
other transistor. When making voltage meas-
urements, use Multimeter ME-26/U or a vtvm
with equivalent characteristics,
f. Waveforms. U s i n g O s c i l l o s c o p e A N /
USM140, make waveform checks and compare
them with the waveforms given in figure 2-18.
Waveform checks will help to isolate trouble
in the balanced modulator, synchronous filter,
agc circuit, and the servo amplifiers. Do not
connect the oscilloscope directly to the transis-
tor pins or wires. Use a low capacity probe.
g. Illustrations. Use the main schematic dia-
gram (fig. 5-10) in conjunction with the stage
and interunit schematic diagrams to circuit
trace and isolate the faulty part. For parts
2-32
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