their output. This SSB signal is then RF clipped by the clipper and after
buffering, is again filtered by a third ceramic filter to remove undesired
harmonics induced by the clipping process. The resulting clipped signal has a
much higher average to peak power ratio than the original signal before
clipping. This processed SSB signal is converted back to audio by the product
detector circuitry. Use of the same local oscillator for the balanced modulator
and product detector assures that no audio frequency shift is produced. The
processing also reduces some of the lower audio frequencies resulting in a
rising response which adds to the processing effectiveness.
During audio pauses when the output from the audio compressor falls below
the input threshold of the threshold detector circuitry, additional attenuation
is inserted in the audio path by the FET audio attenuator circuitry to reduce
background noise pickup.
VI. ALIGNMENT AND TROUBLESHOOTING
The following procedures may be used to align the SP75 if there is good reason
to believe alignment is required. There are only two adjustments in addition to
the normal microphone gain and output level set-up.
1)
2)
3)
Turn unit on.
Attach a counter to test point
13.
Refer to the photo in figure 5. Adjust the
variable capacitor C44 for a frequency of 459.0
kHz.
Remove frequency
counter connection.
Connect an RF voltmeter or scope capable of measuring an RF voltage of
approximately 10 to 25 mV to test point 6. Disconnect all audio inputs and
select the TAPE or PATCH switch position. While observing the scope or
RF voltmeter adjust R26 for a minimum RF indication.
This completes the alignment procedure.
If the SP75 fails to work properly the following signal levels may be checked.
Refer to the schematic, figure 6, and pictorial, figure 5.
With an input signal of 10
mV at the
microp
phone gain, R5, set to maximum, the
followir
values are r.m.s, AC volts.
9
1) At test point
2,50
mV.
2) At test point 3,200
mV.
.
3) At test point 4,1.0 volt.
4) At test point
5,70
mV
with clipping
conb
5) Approximately 25
mV
at test point 12.
VII. SERVICE DATA
The
SP75 will be checked and aligned at the
fz
no evidence of tampering. Transportation
cl
repairs will be made on a time and material
factory for authorization before
returningl
yo
Address your requests for authorization to:
R. L. DRAKE COM
540 Richard Str
Miamisburg, OH
1
A’ITN: Customer Service
Telephone: (Area Code 5
Telex: 288-01
Y
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