Installation and Operating Instructions

Installation and Operating Instructions
Sigtronics Auto Squelch Intercom System
Installation and Operating Instructions Models SAS-440 and SAS-640
ATTENTION INSTALLER: To assure a trouble free
installation, please read the entire instructions through
once before beginning.
Once again Sigtronics is first. First with VOX, now first with
Auto Squelch. One touch and the VOX squelch level is
perfectly set to your headsets and the aircraft background
noise level.
All of our products are “systems designed” to provide reliable,
fail-safe products at the lowest possible cost. And we stand
behind our products to guarantee your satisfaction!
The Sigtronics Models SAS-440 and SAS-640 Auto
Squelch intercom systems are specifically designed for
permanent, panel mounted installation in aircraft. The SAS
units allow both pilot and co-pilot to operate the aircraft
radios and have the convenience of voice activated intracabin communication. Optional passenger connections are
provided that allow up to four additional intercom positions,
depending on model.
VOICE ACTIVATED feature allows “hands free”
intercommunication between pilot(s) and passengers. Start
speaking and the intercom instantly turns on to relay your
message clearly to the other headsets; stop talking and it
turns off to reduce background noise. There’s no need to
raise your voice or turn your head to communicate.
the model SAS-440. Model SAS-640 handles up to six
PILOT ISOLATE MODE - The pilot can set the intercom
so that he can transmit and receive on the aircraft radio(s)
independently from the rest of the intercom. The co-pilot and
passengers can still intercom among themselves and not be
heard by the pilot. Conversely, they wont hear the aircraft
radios or the pilot. This can be an advantage in areas of
busy radio traffic.
RADIO MONITORING capability allows radio output to be
heard by the pilot, whether the SAS unit is in “ALL”, “ISO”,
or “OFF” mode.
COMPATIBILITY - All Sigtronics Auto Squelch intercoms
operate with all standard aircraft radios and headsets.
FAIL-SAFE feature provides the pilot with the ability to
transmit on the aircraft radios even if the unit is “OFF”.
EASY UPGRADE - In most cases just enlarge one mounting
hole and re-route one wire in an existing SPA-400 or SPA600 installation and get the latest technology in aircraft
WEIGHT: 6.4 ounces (SAS-440 intercom unit with panel and
knobs). Jacks and wiring harness weigh 5.5 ounces.
AUTOMATIC SQUELCH - Another Sigtronics first. Just
press and release the squelch button and the intercom
perfectly sets the squelch level. In approximately one
second the unit monitors the current aircraft noise level via
all the headsets and adjusts the squelch for perfect voice
activated operation. No more manual adjusting.
SIZE: Panel 1” x 2.5”. Chassis - 1” high x 2.5” wide x 6”
deep. Can be mounted either horizontally or vertically in the
aircraft panel.
TRANSMIT CAPABILITY allows both pilot and co-pilot
to transmit to air traffic control via their push-to-talk
switches and headset boom mics. The intercom function is
automatically disabled during transmitting, so that only the
voice of the person transmitting can be heard.
ENVIRONMENTAL: Meets requirements of TSO-C50b.
INPUT POWER: 11 VDC through 32 VDC. Maximum current
drain 0.06 Amp (SAS-440).
DISTORTION: Less than 1% total harmonic distortion.
WARRANTY: The SAS units are constructed of high
quality components and carry a five year parts and labor
Specialists in “SOUND” Management
178 East Arrow Highway, San Dimas, CA 91773 ( 909 ) 305-9399
2. Center punch each hole at the cross lines. (The five holes
are in a straight line and equally spaced 0.4” apart).
To upgrade an existing SPA-400 installation to a SAS-440
or a SPA-600 to SAS-640 skip to the “UPGRADING A
SPA INSTALLATION TO A SAS” section on page 3. For a
completely new intercom installation continue below.
3. Drill 1/8” pilot holes in all five places.
4. Enlarge two holes to 3/8” and one hole to 1/2” per the
Hardware Supplied
Besides the intercom unit and these instructions, each SAS
system comes with the following hardware:
Headphone Output Jacks - Accept standard
0.250” aircraft headphone plugs.
1. Remove the knobs from the Volume and OFF/ALL/ISO
controls using a 0.050” Allen wrench. NOTE: DO NOT
REMOVE the nuts from the Volume, Squelch, or OFF/
ALL/ISO controls.
SAS-440 SAS-640
Microphone Input Jacks - Accept standard
0.206” aircraft microphone plugs. (U93 plug
compatible jacks can be used in place of the
jacks provided).
Mic Jack Insulating Washers, Flat
Mic Jack Insulating Washers, Shoulder
3. Install the printed SAS panel and lightly thread the two
4-40 screws through the holes in intercom panel. The
nuts on the Volume and OFF/ALL/ISO controls should fit
inside the 3/8” diameter holes.
Intercom Panel - lettered on both sides.
4. Tighten the two screws.
Intercom Control Knobs
Mounting Screws 4-40 x 1/2
5. Put the knobs on the Volume (VOL) and OFF/ALL/ISO
control shafts and tighten the Allen screws.
Drill Template - Adhesive backed hole size
pattern for drilling aircraft panel.
Aircraft / Intercom Interface Cable (4 feet long)
Single White / Green Wire with pin (4 feet long)
2. Insert the SAS unit from the rear of the aircraft panel
with the appropriate arrow on the unit chassis pointing
The SAS unit has been designed to mount either horizontally
or vertically in your aircraft panel.
The location selected for the SAS unit requires a minimum
front panel area of 2 1/2” by 1”. Depth required behind panel
is 6” plus cable access.
(See Figure 3)
CAUTION: Move the aircraft flight controls through the limits
of travel while observing the selected area and making sure
that the rear of the intercom and cable will not interfere with
any aircraft control components.
1. Locate the mounting areas. (One mic and one headphone
jack required for each headset). Again, make sure that
the jacks will not interfere with any aircraft control
2. Drill 3/8” diameter holes for headphone jacks and
3. Drill 1/2” diameter holes for the mic jacks and install with
the insulating washers supplied. (See Figure 3).
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Note: If the aircraft already has pilot headset jacks, they
can be used for intercom, however, the mic jack must be
re-wired as follows:
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1. Remove any existing wires from the tip, ring, and barrel
2. Connect the intercom White / Black wire to the ring
1. Position the adhesive drill template on the aircraft panel
in the selected area.
3. Connect one end of a ground wire to the barrel terminal
of the mic jack and connect the other end to Point “A”.
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4. Install insulating washers as necessary if the barrel of the
mic jack is mounted in metal.
not necessary, shielded audio wire can be used if desired.
This can simplify the wiring process.
For the headphone jack, just add the blue wire (pin 3) to
the tip terminal. No need to remove existing wires on the
headphone jack.
*1. The blue wire from Pin 3 must be connected to the aircraft
radio headphone output - NOT the speaker output.
*2. Connect all intercom mic jack grounds to a single aircraft
chassis ground point - Point “A” - as shown in Figure
4. (Use the black washers supplied to insulate the
intercom mic jacks from aircraft chassis ground). Note
this intercom central grounding point is used to eliminate
any unwanted electrical noises, such as alternator whine
or strobe noise, from being induced into the intercom
system through the grounds. All intercom mic jack barrels
must be insulated from ground where they are mounted
and connected back to Point “A” on their own individual
ground wire. Similarly, both intercom ground wires (pin
4) and the push-to- talk switch grounds must also be
connected back to Point “A”. It is not necessary, however,
to connect the headphone jack barrels to Point “A”. They
can either be grounded where they are mounted or some
place nearby.
Connections should be made as shown in Figure 4 and
indicated in Table 1. If longer wire lengths are required, use
a good quality hook-up wire - 22 gauge or larger. Although
TABLE 1 – See Wiring Instructions
White / Black
Pilot Mic Input
Ring Terminal of Pilot Mic Jack
White / Red
Pilot and Radio Transmit
Switch Input
Pilot Transmit Switch (PTT) & Radio
Key Input (Switch to Ground to
Blue *1
Pilot Headphone - Radio &
Intercom Outputs
Radio Headphone Output and Tip
Terminal of Pilot Headphone Jack
Black *2
Intercom Central Grounding Aircraft Chassis Ground
Point “A”
No Connection
Transmit Mic Audio Output
Ring Terminal of Aircraft Hand Mic
Jack or Mic Input of Aircraft Radio
or Audio Panel
Red *3
12 through 24 VDC Power
Intercom Circuit Breaker
Tan *4
Passenger #1 Mic Input
Ring Terminal of Passenger #1
Intercom Mic Jack
Tan *4
Passenger #2 Mic Input
Ring Terminal of Passenger #2
Intercom Mic Jack
White / Blue
Co-Pilot Radio Transmit
Switch Input
Co-Pilot Transmit Switch (PTT)
(Switch to Ground to Transmit)
White / Orange
Co-Pilot Mic Input
Ring Terminal of Co-Pilot Intercom
Mic Jack
White / Green
Co-Pilot & Passenger
Tip Terminal of Co-Pilot &
Intercom Headphone Output Passenger Headphone Jacks
Tan *5
Passenger #3 Mic Input
Ring Terminal of Passenger #3
Intercom Mic Jack
Tan *5
Passenger #4 Mic Input
Ring Terminal of Passenger #4
Intercom Mic Jack
*3. The red wire may be connected to either 12V (14V) or
24V (28V) power source. No switching or adjustments
are required to operate from either source.
*4. Tan wires (pins 8 and 9) are only used on installations
that require extra intercom positions.
*5. Provided on SAS-640 units only.
ADJUSTMENTS section on page 4.
chassis, jacks, and wiring harness are clear of all aircraft
operating controls and cause no interference with them.
Check out the SAS unit installation by following the
instructions below.
Chassis Mounting
The SAS-440 is specifically designed to easily replace a
SPA-400. Similarly, the SAS-640 can replace a SPA-600.
The panels are exactly the same dimensions (1” x 2.5”),
however the SAS units are 1 3/4” longer than the SPA units.
You will have to make sure that you have the extra depth
required behind your panel. The five mounting holes are
exactly in the same positions. Only the center hole will have
to be changed (enlarged from 1/4” to 1/2”).
Plug in all the headset mic and phone plugs into the
respective intercom jacks. Put on the pilot’s headset and
position the boom mic close to the mouth, as is the practice
with a hand-held mic. Voice clarity is best when the mic is
at one side of the mouth and 1/4” from the lips.
To assure that the aircraft radios, pilot’s headset, and
PTT switch are connected and functioning properly, turn
the SAS units OFF/ALL/ISO switch to the “OFF” position.
If applicable, set the aircraft audio panel to “Headphone”
position. Then turn on the aircraft radio(s) as usual, and
verify that the pilot can hear the radios and can transmit
using his push-to-talk switch and headset. Aircraft radio(s)
and audio panel should operate exactly as they did before
the SAS system was installed. Aircraft radio reception should
not be heard in the co-pilot or passenger headsets. There
should be no intercom between headsets with the SAS unit
turned “OFF”.
To upgrade, first remove the SPA unit from the aircraft panel.
This is done by unscrewing the two Phillips head screws
and the nut on the ON/OFF switch. Remove the printed SPA
panel. Pull the SPA unit out and unplug the white connector
from the wiring harness. Next, drill the center hole in the
aircraft panel out to 1/2”. Then to mount the SAS chassis
(see Figure 2 on page 2):
1. Remove the knobs from the Volume and OFF/ALL/ISO
controls using a 0.050” Allen wrench. NOTE: DO NOT
REMOVE the nuts from the Volume (VOL), Squelch (SQ),
or OFF/ALL/ISO controls.
Next turn the SAS unit OFF/ALL/ISO switch to the “ALL”
position. Set the SAS volume control to mid-position. Verify
that all headset positions can now intercom with each other,
including the passengers. Verify that both pilot and co-pilot
can operate the aircraft radio(s). In this mode all headsets
on the intercom will hear the aircraft radio(s).
2. Insert the SAS unit from the rear of the aircraft panel
with the appropriate arrow on the unit chassis pointing
3. Install the printed SAS panel and lightly thread the two
4-40 screws through the holes in intercom panel. The
nuts on the Volume and OFF/ALL/ISO controls should fit
inside the 3/8” diameter holes.
It may be necessary at this time to adjust the SAS unit
mic output to the aircraft radios. A small adjustable
potentiometer is provided inside the unit for this purpose. It
is accessible through a hole in the side of the SAS chassis.
It is marked “Mod. Adj.”, and can be adjusted with a small
blade screwdriver. In the event of over-modulation (garbled)
or reports of weak transmissions over the aircraft radio, an
appropriate adjustment can be made. Clockwise rotation
increases the output level to the aircraft radio mic input.
Counter-clockwise rotation decreases modulation level. This
adjustment sometimes needs to be made after the initial
installation of the intercom or if a new radio is installed. (The
output is set for unity gain at Sigtronics).
4. Tighten the two screws.
5. Put the knobs on the Volume and OFF/ALL/ISO control
shafts and tighten the Allen screws.
Wiring Change
The only wiring change to an existing SPA installation is to
rewire the co-pilot and passenger(s) headphone lines. You
will not need the 4 foot interface cable that comes with the
SAS system. Instead, you will use the SPA harness already
in the aircraft. Find the single white / green wire with female
pin (supplied). Push the pin into the vacant hole at pin 12
next to the white / orange wire in your existing SPA interface
harness. Make sure it is in as far as the other pins and does
not pull out. Then plug the cable on the SAS unit into the
modified SPA interface cable.
In an SPA installation, the tip terminals of the co-pilot and
passenger headphone jacks are wired to the blue wire (pin
3). They need to be disconnected from there and connected
to the white / green wire (pin 12) (see Figure 4 on page 3).
Make sure that the blue wire (pin 3) is still connected to
the tip of the pilot’s headphone jack as well as to the radio
headphone line.
After the unit is installed, again check that the SAS unit
If everything checks out, refer to the OPERATING
INSTRUCTIONS on page 5 for proper use and other
operating modes of the Sigtronics SAS intercom system.
If something does not work as above, carefully go over the
intercom wiring again. If OK, refer to the following sections for
solutions to two of the most common intercom problems:
Symptoms are:
Solution: Add a 1/4 or 1/2 watt resistor per Figure 6 above.
The value of the resistor most commonly used is 220
ohms, however, optimum performance can be achieved by
selecting the value right for your particular installation. The
resistor can be any value between 100 and 330 ohms and is
selected for the best balance between radio receive volume
and intercom volume.
1. Intercom audio volume is weak or non-existent when the
SAS unit is in “ALL” mode but fine in “ISO” mode and ...
2. Transmit function is normal on aircraft radio and ...
3. Receive function is normal on aircraft radio.
Most aircraft radios, when transmitting, provide what
is called “sidetone”. Without sidetone, you will not hear
yourself or your co-pilot transmit to ATC. This can be
distracting, especially during instruction, because no one
on the intercom will hear the outgoing transmission of the
radio conversation. Some aircraft radios, however, do not
provide this function. Also, some radios are designed to drive
only one headset. Therefore, if you have such a radio, the
sidetone may be extremely weak or nonexistent.
If something is still not right or you have any questions
regarding the installation and operation of the Sigtronics
SAS intercom or any other Sigtronics product feel free to
contact us directly. Technicians are available Monday though
Friday 8 am to 4:30 p.m. Pacific time.
If this is the case and the sidetone cannot be turned up in
the radio(s), a simple modification can be performed within
the SAS unit to simulate sidetone. The SAS requires partial
disassembly to accomplish this modification. First unplug
and remove the SAS unit from the aircraft. To disassemble
unit, carefully remove the four screws securing the bottom
cover. Remove the cover to expose the solder side of the
circuit board.
This concludes the installation check-out.
The Sigtronics Transcom SAS-440 / SAS-640 is a voice
actuated intercom for aircraft with “transmit through the
aircraft radio capability” using your push-to-talk switches.
Three controls are provided on the SAS units:
Solder in an insulated jumper wire as illustrated in Figure
5 on page 4. Replace the cover and the four screws to
complete the modification.
OFF/ALL/ISO Switch - Sets the intercom mode of
VOL - Volume Control - Controls the intercom volume. (Does
not affect aircraft radio volume. Set radio volume on radio
as normal.)
With the unit modified in this way, the intercom volume
control adjusts the level of the sidetone. Additionally, when
in “ISO” mode, the SAS unit will not provide sidetone even
with this modification.
SQ - Squelch Button - Automatically sets the intercom turnon threshold for voice activated intercom (VOX) mode.
Install the SAS unit into the aircraft panel and plug in the
CAUTION - As is standard practice with all aircraft avionics
equipment, be sure that the aircraft radio master switch is
turned off when you start up the aircraft engine.
Intercom operation and adjustment can be performed by the
following procedure:
1. With aircraft power on, set the SAS OFF/ALL/ISO switch
to the “ALL” position - the preferred mode for setting the
squelch. The SAS squelch level can also be set with the
SAS in the “ISO” mode, but the pilot headset audio will
not be utilized for squelch level setting.
(3) BLUE
2. Turn the SAS unit volume control to the 9 o’clock position.
Notice the intercom is now continually activated (hot
mic mode) and you should be able to talk between
This modification is used when a SAS intercom is connected
to a radio or audio panel which has a low impedance
headphone output (less than 100 ohms). This is most
common with some Narco equipment (CP-135,CP-136,
and Mark 12D, for example) and “home made” audio select
3. To adjust the intercom squelch control for voice activated
operation (VOX), press and hold the “SQ” button down.
Now announce to the other persons in the aircraft not
to talk because you need a moment to set the intercom.
When all are silent release the “SQ” button. The intercom
will immediately go quiet. In approximately one second
the intercom will briefly pop on. This tells you the squelch
has finished automatically setting. If someone did speak
while the intercom was setting, it will have to be set again.
This is because the intercom would have tried to set to
a level just above the persons voice level instead of just
above the aircraft background noise level.
to the radio. Only his mic will be live. The microphones
of the co-pilot and any passengers, will be muted when
the pilot transmits. Similarly in the “ALL” mode, when the
co-pilot transmits, only the co-pilots mic will go out on the
radio. When transmitting, the pilots will hear their own voice
via the aircraft radio sidetone return. (If the radio does
not provide transmit sidetone, then you will not hear your
voice. A minor modification to the SAS unit will enable it to
simulate sidetone. (See “SAS-440 / SAS-640 SIDETONE
MODIFICATION” on page 4)
Once set, the intercom stays silent until someone speaks
in their headset microphone (mic). Then it instantly turns
on and relays the conversation. After about a second of no
conversation the intercom goes silent again.
When the SAS is set to the “ISO” mode, only the pilot can
transmit and receive on the aircraft radio(s). The pilot will
not hear intercom and cannot talk to the other headsets.
The co-pilot and passengers still have intercom, but do not
hear the radios.
In most aircraft the squelch level wont need to be set again
until the next flight. In some high noise aircraft it is better to
set the squelch during climb or cruise.
Since the intercom is not needed during solo flights, it may
be turned “OFF”. The pilot will still hear the aircraft radio(s),
since this circuit is always active, and may transmit to ATC
via his headset and push-to-talk switch.
When the SAS intercom turns on, it automatically comes
up in “hot mic” mode (non voice activated mode). During a
flight, if you want to deactivate the voice activated mode and
return to hot mic mode, simply turn the OFF/ALL/ISO switch
to “OFF” and wait 3 seconds and then turn it back to “ALL”
or “ISO”. To facilitate automatic setting of the squelch level,
whenever the “SQ” button is held down, the intercom also
goes into hot mic mode.
If power to the SAS unit fails, the pilot can still transmit and
receive on the a/c radio(s) (assuming the radio(s) are still
functioning). Additionally, if a problem is ever suspected in
the intercom, simply turn it to “OFF”. In this mode you will
also be able to transmit and receive on the aircraft radio(s)
from the pilot’s position. The hand mic may also be used,
however, the SAS unit should be turned to “OFF” and you
should unplug your headset mic plug from the mic jack.
(Leave headphone plug in for radio reception).
When the SAS is in the “ALL” mode, transmitting from both
pilot and co-pilot positions is possible on a one-at-a-time
basis. As normal, the pilot selects the appropriate radio he
wishes to use. When the pilot presses his PTT switch, his
headset mic is automatically routed through the intercom
Sigtronics Corporation
178 East Arrow Highway
San Dimas, CA 91773
Phone: (909) 305-9399
E-mail: [email protected]
Web Site:
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