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o
switch ( S2)
in the modulator #2 position.
Set the audio oscilla-
tor frequency to 30 cycles and modulate the exciter with
7
i14
DBM.
Next, adjust C119 so that the FM monitor reads 70% modu-
lation.
Distortion adjustment control ( R126)
is then adjusted
for best distortion.
If R126 is considerably away from the
proper adjustment point,
it may be impossible to obtain the de-
sired level of modulation or the waveform obtained may be com-
pletely torn up.
If such is the case,
adjust R126
.
for minimum
distortion while modulating somewhat less than 70%,
say,
a-
bout 50%.
Then, reset the level on the audio oscillator to
/14 DBM and adjust C119 and R126 as described above.
Then place
the modulator selector switch in the modulator #1 position, and
follow the procedure just described to adjust modulator #1.
In
this case,
however, the capacitor adjustment is C111 and the
distortion adjustment control is R115.
If it is impossible to reduce distortion at 30 cycles, it is
advisable to check just the audio portion of the exciter unit
and/or the audio oscillator itself.
The audio portion of the
exciter consisting of tubes V116 and V117 may be checked by
running test leads from TP120 and TP122 or TP123 to the input
of the distortion analyzer.
Distortion as measured nt TP120
should be well below . 5% nt any audio frequency.
If distor-
tion from the audio section is O.K. but overall distortion as
measured from the monitor is not,
then the waveforms of the
pulse circuitry should be checked.
Typical waveforms of V101
thru V106 are given on drawing
826 7991 001.
100% modulation should occur at an input level of approximately
/10 DBM from 30 to 1,000 cycles.
This input level will cause
an RMS audio voltage
at,1212D
of about 25
vnit.
If an input
level of /10 DBM does not generate an RMS voltage of about 15
volts at TP120,
then a defect in the audio section may be sus-
pected.
If sufficient RMS voltage exists at TP120 and the ex-
citer will not modulate 100%, thena.defect in the modulator or
previous stage should be suspected.
In any FM system worse distortion occurs at the lowest modu-
lating frequency.
In other words,
if distortion is 1% at 30
cycles then the distortion can be expected to be
better
at
all
higher modulating frequencies.
Occasionally;
a high distor-
tion figure may result between 10,000 and 15,000 cycles.
The
fault will not generally lic in the modulator stage,
however,
it
could lie in the audio section.
u
-14-
M6095 Exciter
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