TM 11-6525-586-45
greater than IV rms, and distortion is less than
(4) Connect oscilloscope to jack A2J1.
10%. (This can be judged by the eye.) In this
(5) Supply 25 volts dc to the IF assembly
case, make the connections of RA and CA perma-
by connecting TP2 on signal generator (inside,
nent and reassemble the IF assembly.
rear, bottom of unit) to A2FL1 on IF assembly,
Case II: An oscillation is observed but at the
and by connecting a lead between the chassis of
wrong frequency, too little amplitude or too high
the signal generator and the housing or ground
distortion.
plane of the IF assembly.
(a) To correct distortion try increasing
NOTE
RA, and then alternately increasing and decreas-
ing CA.
Use alligator clips and always disconnect 25 volts
d c (TP2) when making a circuit change.
(b) To correct low amplitude alternately
raise and lower CA.
(6) From figure 5-6, select a value of RA
(c) To correct appearance of 3rd or 5th
and CA for the desired frequency of operation.
harmonic, increase CA and then, if this is not
Example: If the 4.3 MHz crystal is being re-
sufficient, alternately increase and decrease RA.
placed by a 2.5 MHz crystal, an RA of 220 ohms
Case III: Circuit does not oscillate.
and a CA of 800 pf would be selected from the
(a) Verify once again that circuit is op-
homograph.
erational by switching to another crystal.
(7) Resistor RA is installed in place of the
(b) Return to crystal under test and if it
47-ohm resistor on switch A2S1, at the switch
is 1.3, 1.4 or 1.5 MHz try attaching one end of
position corresponding to the crystal which is
a 0.01 uf capacitor to the crystal socket terminal
being replaced. In the example given in step (6)
that goes to A2E1 which, in turn, goes to the
above, the 220-ohm resistor will be soldered in
wafer on the switch furthest from the knob, and
place of the 47-ohm resistor at the first position
the other end to the ground plane area on the
of the switch.
foil side of the circuit board A2A1.
(8) Capacitor CA is soldered on one side
(c) Once an oscillation has been obtained
to the terminal coming out of the crystal socket
proceed as in Case II.
which goes to A2E3 which, in turn goes to the
(d) After proper operation at the desired
middle wafer of switch A2S1. The other lead of
frequency is obtained, insure that the oscillator
CA is soldered to the ground plane area on the
will oscillate each time B+ is removed and then
foil side of the circuit board A2A1. The unit is
reapplied. Do this by removing and then connect-
now ready for test.
ing the clip lead to TP2 and observing oscillo-
d. Testing the Modified IF Assembly.
scope.
(1) Connect 25 volts dc.
(2) Turn A2S1 to an unmodified crystal po-
step (d) above, increase the value of resistor RA
sition, to verify that the circuit is operational.
and proceed as in Case II.
(3) Turn A2S1 to the modified position and
(f) For other frequencies try varying CA
observe oscilloscope:
above and below the value obtained from the
Case I: Waveform observed is at the correct fre-
graph. Then, if unsuccessful, change RA, and
quency (not a 3rd or 5th harmonic), amplitude is
again vary CA.
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