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Gain controls are located
on the rear panel.
LED Indicators
The LED indicators can be used to monitor system operation and identify common problems.
POWER: A single GREEN LED, on left side of AC Power switch.
Normal indication: AC switch ON: LED will illuminate.
If no indication: Check AC power cord and AC outlet.
CLIP: Two RED LEDs, one indicator for each channel.
Normal indication: illuminates whenever the amplifier is driven beyond full power. The resulting distor-
tion corresponds to the brightness of the LED. Distortion that causes only brief flashing may not be audi-
ble.
During muting, the LED fully illuminates. This occurs during normal “On-Off” muting.
Abnormal indication:
Bright red illumination while the amp is being used indicates either thermal muting or a shorted output.
If the amplifier overheats, the fan will run at full speed, and operation should resume within one minute.
Allow the fan to run, and make sure the amplifier ventilation is adequate.
A shorted or overloaded output circuit will cause excessive Clip flashing and possible overheating.
If distortion is audible without a Clip indication, the problem is either before or after the amplifier. Check
for damaged speakers or overloaded signal source. The amplifier Gain control should be in the upper half
of its range to prevent input overload.
SIGNAL: Two GREEN LEDs, one indicator for each channel.
Normal indication: illuminates when the input signal is strong enough to drive the output to -40 dB from
rated 8 ohm power. As signal approaches full power, the LED will illuminate continuously.
If no indication: check Gain settings and increase gain if necessary. Check input connections and audio
source for signal. If the Clip LED illuminates with little or no Signal indication, check the output wiring for
shorts.
Abnormal indication: If the Signal LED illuminates with no signal input, there may be system oscillations
or some other malfunction. Disconnect the load and fully reduce the gain. If the signal LED remains on, the
amp may need servicing.
PROTECT (ISA 1350 only): A single RED LED directly below the POWER LED on left side of AC
Power switch.
Normal indication: not illuminated (NOTE!: If both rear panel AC circuit breakers are open, the PROTECT
LED and the POWER ON LED will not illuminate)
Abnormal indication: illuminated. If the PROTECT LED illuminates, the amplifier is in thermal protect or
one of AC circuit breakers has opened. If the amplifier is hot, leave the AC power switch ON so that the fan
cools the amplifier off. Normal operation will resume automatically after the amplifier has cooled suffi-
ciently. If an AC circuit breaker (rear panel) has opened, reset the breaker by pushing inward on its reset
button.
Gain Controls
Turn the gain controls clockwise to increase gain and counterclockwise to decrease gain.
The Gain controls are marked in dB of attenuation. There are 11 detents for repeatable adjustments. The
upper 6 steps are about 2 dB each, and settings should normally be made within this range. The range
below -10 dB should not be used for normal program levels, as the input headroom could be exceeded, but
can be used for testing at reduced levels. At the minimum setting, the signal is completely cut off.
The Gain controls can be adjusted by grasping the control shaft and rotating.
Clip and Signal indicators
for both channels.
Power on indicator.