TABLE OF CONTENTS
PAGE
I.
General Description
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.
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1
II.
Specifications
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.;
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1
III. Installation
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2
IV. Operation
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6
V. Theory of Operation
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7
VI.
Alignment and Troubleshooting
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.
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8
VII. Service Data
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9
. . .
III
ILLUSTRATIONS
Fig. 1
Connecting SP75 to TR7 and PS7
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4
Fig. 2
Connecting SP75 to Systems Other Than
TR7 and PS7
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...a
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4
Fig. 3
Phone Patch Connection
to SP75
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5
Fig. 4
Tape Player Connection to SP75.
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5
Fig. 5
P. C. Board Pictorial, SP75
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11
Fig. 6 Schematic, SP75
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../
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12
I. GENERAL DESCRIPTION
The SP75 Speech Processor is designed to provide an increase in average
power and readability of a single sideband voice signal during weak signal or
high interference conditions. The SP75 is connected between the microphone
and microphone input of the single sideband transmitter, thus requiring no
modification of the existing transmitter or transceiver. A front panel switch
allows the processor to be switched in or bypassed as conditions warrant. Two
additional inputs, such as a tape player or phone patch, may be front panel
selected in place of the normal microphone input.
The amount of RF envelope clipping is adjustable between zero and twenty
decibels by a front panel control. A LED indicates proper audio input level.
Because of the pre-clipping audio compression, small changes in voice levels
do not widely affect the clipping level.
Muting circuitry reduces gain during speech pauses, thus reducing undesirable background noise pickup and allowing VOX operation with the processor
on.
The SP75 was designed for optimum performance using the Drake model 7077
microphone and the Drake TR7 transceiver or Drake 4-line transmitters. However, excellent performance can be obtained with most other transmitters
which have a microphone input impedance of approximately 50 K ohm or
greater. Microphones other than the Drake 7077 may be used successfully.
However, microphones with a more sharply rising frequency response may
produce a less pleasant response than a microphone with a flatter response.
II. SPECIFICAT
Processing Type:
RF Clipping
Range
ONS
Preclipping audio compression followed
by RF envelope clipping at the processor
intermediate frequency.
Adjustable 0 to 20
dB
from front panel
control.
Input Level (Microphone Input):
3.5
mVV
minimum for full processing. Gain
adjustable to
accomodate
up to 300
mV
maximum.
1
Input Level (Tape & Patch Inputs):
Input Impedance (Microphone):
Input Impedance (Tape
&
Patch):
Output Level w/Processing:
Output Impedance:
Muting (Microphone Input Only):
Frequency Response:
Distortion:
Supply Requirements:
Size:
Weight:
15
mV minimum for full processing,
30 mV maximum.
1
Megohm
50 K ohm
0-50 mV
adjustable into 50 K ohm load.
50 K ohm
10 to 20
dB
attenuation during speech
pauses.
400-6000 Hz
@
-6
dB
Less than 5% T.H.D. @ 1
kHz,
20
dB
clipping.
11-16
VDC @ 95
mA
7” L x
6-l/4”
W x
2-l/4”
H
(17.8 cm x 15.9 cm x 5.4 cm)
1.4 lb.
(.63
kg)
III.
INSTALLATION
Locate the SP75 at a convenient location. If the SP75 is placed on top of the
TR7 transceiver, the microphone cable from the rear of the SP75 can be routed
along the side and underneath the TR7. DO NOT block the power amplifier
cooling slots on the right side of the TR7.
The SP75 may be secured to the top of a RV7, MS7, or equivalent cabinet with
the two retaining clips provided. First, remove the screw-in mounting feet on
the SP75 and replace with the four number 6 screws provided. Place the four
stick-on rubber feet on the bottom of the SP75 next to the cover mounting
screws on the cover. Remove the top screws on each front extrusion end cap of
the MS7 or RV7 and the bottom screws on each SP75 end cap. Place a retaining
clip between the end cap and extrusion. Position so that the hole is properly
aligned and replace end cap screws. Repeat retaining clip installation on the
other side.
2
Plug the SP75 output cable into the TR7
phono plug on the SP75 supply cable into the
the PS7 power supply. The microphone ma:
input.
If another transmitter other than the TR7 is u
can be installed on the SP75 cable
plied. Connect the red wire to the P’M’ connect
the shielded cable to the microphone audio
black wires to the ground connection.
When using a transmitter/power supply
connect the positive SP75 supply (11-16 V)
cable plug. Connect the negative supply lead
Do not connect the negative supply lead to
as this shield is not internally connected in th
supply negative connection through the
transmitter or transceiver chassis. This is
from being introduced across the SP75
RF feedback condition.
In addition to the above supply connection t
prevent any possibility of creating undesira
SP75 which could result in RF feedback.
1) Any microphones or other inputs
2) When a phone patch or tape recorder are cc
3) See the following diagrams for examples.
Plug the SP75 output cable into the TR7 microphone input connector. Plug the
phono plug on the SP75 supply cable into the 13.6 volt accessory output jack on
the PS7 power supply. The microphone may then be connected to the SP75
input.
If another transmitter other than the TR7 is used, the proper mating connector
can be installed on the SP75 cable replacing the TR7 microphone plug sup-
plied. Connect the red wire to the PTT connection, the white center conductor of
the shielded cable to the microphone audio connection, and the shield and
black wires to the ground connection.
When using a transmitter/power supply combination other than the
TR7/PS7,
connect the positive SP75 supply (11-16 V) lead to the tip of the SP75 supply
cable plug. Connect the negative supply lead to the
chassis of the transmitter.
Do not connect the negative supply lead to the shield of the SP75 supply cable
as this shield is not internally connected in the SP75.
The
SP75 must receive its
supply negative connection through the output cable ground by way of the
transmitter or transceiver chassis. This is done to prevent ground currents
from being introduced across the SP75 output cable which could result in an
RF feedback condition.
ADDITIONAL GROUNDING NOTES
I
In addition to the above supply connection the following must be observed to
prevent any possibility of creating undesirable ground currents through the
SP75 which could result in RF feedback.
1)
Any microphones or other inputs connected to the SP75 MUST NOT be
grounded through any other path other than through the SP75 input
connector.
2) When a phone patch or tape recorder are connected to the SP75, they should
NOT be grounded elsewhere, such as through a three wire AC supply cord. If
the patch or recorder must be grounded, then the audio connection to the
SP75 may need to be isolated by a transformer.
3) See
the following diagrams for examples.
3
POWER SWITCHING OPTION
As supplied from the factory (jumper option A) the SP75 is always connected
the DC supply regardless of the position of the ON/OFF pushbutton. This is
true even though the green “ON” LED is switched. This is done to allow
instant IN/OUT audio switching. If it is desirable to eliminate all power
consumption when the ON/OFF pushbutton is OFF, the power jumper can be
moved as shown in the schematic,
jumper
option B. When this change is
made, there will be a slight delay before the processor is operational when the
unit is turned ON. Unless battery power is used it is usually desirable to leave
the jumper in the factory supplied position. See figure 5 for power jumper
location.
IV OPERATION
Before operating with the SP75 it may be necessary to set the SP75 microphone
gain and output audio levels. Once these adjustments are made it should not be
necessary to readjust them unless a microphone with a different output level is
used. The SP75 set up procedure is as follows:
1)
2)
3)
4)
With the SP75 bypassed, (“ON” button out) adjust transmittei microphone
gain for normal transmitter operation. Select the microphone input by
depressing the MIC pushbutton.
Set the SP75 clipping
level
to 0
dB.
Turn
SP75 on by depressing “ON” pushbutton. While speaking normally,
advance the SP75 microphone gain, accessible through SP75 rear panel,
with a small screwdriver until the green “AUDIO” LED on the SP75 just
stays on while speaking. Do not advance-beyond this point. NOTE: Observ-
ing from the rear panel, full CCW setting is minimum gain. Advancing the
potentiometer CW increases gain. The LED should not blink while speaking
continuously, but should start dropping out if you back away from the
microphone a few inches farther than normal and should go out completely
when you stop speaking.
Place transmitter in transmit.
-With
the SP75 at 0
dB
clipping level,
talk
normally into the
microphone
and advance the SP75 output level control,
accessible through rear panel, starting at minimum level
(CCW), for normal
6
c
.
transmitter operation With the TR7 this
level until the green TR7 ALC indicator just
advance output level beyond this point.
gain should not be changed
5) To use the processor with a tape or phone p
priate
microphone gain control from the settings
output level
general
operative in the TAPE or PATCH mode an
control on the tape player or patch.
Once the SP75 has been adjusted as per the a
operation. One of the three possible inputs car
appropriate interlocking pushbutton. Any
transmitter, bypassing the processor, by leavin
position. When the ON pushbutton is
The clipping control can be advanced to obtain
The best setting will depend on band
however 9 to 12
V. THEORY OF OPERATION
Refer to the schematic diagram. The audio in
phone preamp when the microphone input is
the gain
from the microphone preamp is then routed t
When selected, tape
sor. The audio compressor compresses the
more constant level to the RF clipper. This con
clipping of the softer speech passages.
The audio compressor output is fed through tl
and muting circuitry to the input of a MC1496
circuit. A double sideband output signal is gent
tor at an intermediate carrier frequency of
through two cascaded ceramic filters, leaving c
transmitter operation
With the TR7 this means advancing the SP75 output
level until the green TR7 ALC indicator just lights to full brilliance. Do not
advance output level beyond this point. NOTE:
Transmitter
microphone
gain should not be changed from the setting determined in step 1.
5) To use the processor with a tape or phone patch input, depress the appropriate pushbutton. Do not change the processor output level or transmitter
microphone gain control
from
the
settings
determined above. Adjust the
output level
on
the patch
or
tape
player
for proper audio level using the same
general procedure as step 3 above. The SP75 microphone gain control is not
operative in the TAPE or PATCH mode and the level
must be set by the
control on the tape player or patch.
Once the SP75 has been adjusted as per the above procedures it is ready for
operation. One of the three possible inputs can be selected by depressing the
appropriate interlocking pushbutton. Any input can be fed directly to the
transmitter, bypassing the processor, by leaving the ON pushbutton in the out
position. When the ON pushbutton is
depressed,
the processor is switched in.
The clipping control can be advanced to obtain the desired amount of clipping.
The best setting will depend on band conditions and the individual operator,
however 9 to 12
dB
is usually a good setting.
V. THEORY OF OPERATI’ON
Refer to the schematic diagram. The audio input is amplified by the micro-
phone preamp when the microphone input is selected. Gain control
R5
allows
the gain to be set properly for the particular microphone being used. The output
from the microphone preamp is then routed to the audio compressor stage.
When selected, tape or phone patch inputs are routed directly to the compressor. The audio compressor compresses the dynamic range thus providing
a
more constant level to the RF clipper. This compression also allows more RF
clipping of the softer speech‘passages.
The audio compressor output is fed through the clipping level potentiometer
and muting circuitry to the input of a MC1496 balanced modulator integrated
circuit. A double sideband output signal is generated by the balanced modulator at an intermediate carrier frequency of 459
kHz.
This signal then passes
through two cascaded ceramic filters, leaving only a single sideband signal at
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
phone gain, R5, set to maximum, the
values are r.m.s, AC volts.
9
1) At test point
2) At test point 3,200
.
3) At test point 4,1.0 volt.
4) At test point
5) Approximately 25
VII. SERVICE DATA
The
no evidence of tampering. Transportation
repairs will be made on a time and material
factory for authorization before
Address your requests for authorization to:
8
With an input signal of 10
mV
at the microphone input and with the micro-
phone gain,
R5,
set to maximum, the following audio levels should result. All
values are r.m.s, AC volts.
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 control at 20
dB.
5) Approximately 25
mV
at test point 12.
VII. SERVICE DATA
The SP75 will be checked and aligned at the factory for a nominal fee if there is
no evidence of tampering. Transportation charges are extra. Any necessary
repairs will be made on a time and materials basis. Please write or call the
factory for authorization before
returning
your unit for alignment or service.
Address your requests for authorization to:
R.
L. DRAKE
COMPANY
540 Richard Street
Miamisburg, OH 45342
ATTN:
Customer Service Department.
Telephone: (Area Code 513) 866-3211
Telex:
288-017
9
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