INTRODUCTION
This instruction sheet describes the operation of the Sigtronics
SPA-4S intercom system. For information on SPA-4S installation
see the separate SPA-4S INSTALLATION INSTRUCTIONS.
The SPA-4S series of aircraft intercoms incorporate voice acti-
vated (VOX) intercom with “transmit through the aircraft radio
capability” using your push-to-transmit (PTT) switches. In addi-
tion they allow for stereo music input.
CONTROLS
Three controls are provided on the SPA-4S units:
ON/OFF Switch - Turns the intercom “on” or “off”.
INTERCOM VOL - Volume Control - Controls the intercom volume.
(Except in OFF mode when the intercom is switched off)
INTERCOM SQ - Squelch Control - Sets the intercom turn-on
threshold for voice activated intercom (VOX) mode.
INTERCOM OPERATION
CAUTION - As is standard practice with all aircraft avionics equip-
ment, be sure that the aircraft radio master switch is turned off
when you start up the aircraft engine.
SQUELCH SETTING - To use the SPA-4S intercoms voice acti-
vated (VOX) feature, the Squelch Control will have to be set. This
can be performed by the following procedure:
1. To set the Squelch controls it is helpful to have some back-
ground noise present and music sources off. With aircraft
electrical power on, set the SPA-4S ON/OFF switch to the “ON”
position. Turn the SPA-4S unit volume (VOL) control to the 10
o’clock position.
2. Turn the Squelch control all the way counter-clockwise. Now,
without speaking, rotate the Squelch control clockwise just
until you hear the background noise in your headset. Next,
rotate the Squelch counter-clockwise a small amount and
wait (approximately 1 second) until the background noise
disappears. Finally, make small adjustments until your voice
triggers the unit at comfortable speaking levels. This proce-
dure is necessary because the squelch is a “Fast-on, Slow-off”
system.
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.
In most aircraft the squelch controls will not need to be set again
unless you change the number or type of headsets used with the
system. In some high noise aircraft it is better to set the squelch
during climb or cruise.
SPA-4SN PUSH-TO-INTERCOM - Install a separate momentary
switch at each headset position. To use the push-to-intercom
feature, turn the squelch control on the intercom fully counter-
clockwise. Then to activate, press the push-to-intercom switch(s).
See the separate Installation Instructions Figure 8 for switch
wiring instructions.
VOLUME SETTING - The Volume control should be set to the
minimum level for best performance. Intercom Volume levels set
too high allow excessive background noise to enter the headset.
This promotes whispering into the headset microphone. Exces-
sive background noise and whispering combine to degrade what
is called the “signal to noise ratio”. This decreases intelligibility
and interferes with proper Squelch (VOX) operation.
SPA-4S OPERATING INSTRUCTIONS
Specialists in “SOUND” Management
Sigtronics Corp. 178 East Arrow Highway, San Dimas, CA 91773 (909) 305-9399
Made in the U.S.A.
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