9
The Servo 120a Protection Circuitry
As noted in the “Guided Tour” section of this manual, the Servo 120a's front panel
Protection LED indicates the activity of the relay speaker connection circuitry. When
the Protection LED is lit, this circuitry is inactive, and all connected speakers are muted
(provided with 0 volts DC), thus protecting them and preventing any audible “thump”
from occurring.
The following conditions will cause the Protection LED to go on:
Initial power-up:
For approximately five seconds after initial power-up, the relay
speaker connection circuitry is deactivated and the speaker output is muted. If every-
thing is operating normally, you will hear an audible click at the conclusion of this
brief period, as the circuitry is activated and the Servo 120a begins delivering signal
to connected speakers (at which point the Speaker LED will light if the Speaker on/off
switch is in its “in” position). It is normal for the Protection LED to fade gradually after
the amplifier is powered off.
Overheating:
A temperature sensing device in the Servo 120a will cause the relay
speaker connection circuitry to be activated (and the Protection LED to go on) when-
ever the operating temperature of the unit rises above a safe level. To guard against
this problem, make sure the Servo 120a receives adequate ventilation on all sides
(see page 8 for more information); in extreme environmental conditions, you may also
want to consider the use of cooling fans.
Severe overcurrent conditions:
This occurs whenever the signal being output
from the Servo 120a rises to a level above 20% THD (Total Harmonic Distortion).
Shorted speaker cables:
This will occur if, due to faulty wiring, the hot and ground
signals being output by the Servo 120a short to one another.
Output impedance drops below 2 ohms:
This can occur if the Servo 120a is con-
nected to inappropriate speaker systems (see the “Setting Up and Using the Servo
120a” section in this manual for more information).
•
DC voltage detected at speaker output:
The most likely cause of this is an internal
failure.
In general, any time the Protection LED lights up (other than during the approximately
five seconds following initial power-up), there is reason to be concerned. If this
occurs, turn the Servo 120a off immediately and carefully check all wiring and external
devices in order to locate and correct the condition that caused the LED to light up in
the first place.
WARNING: If the Protection LED fails to go out (and you fail to hear the accom-
panying audible click) approximately five seconds after power-up, turn the
Servo 120a off immediately and check all external devices and wiring for pos-
sible shorts or other defects.