Com-Tech 400 Amplifier Service Manual
14
Electrical Checkout Procedures
Test 12: Output Power
Spec at 8 Ohm Stereo: ≥ 215 Watts/Ch at 0.1% THD
Spec at 4 Ohm Stereo: ≥ 230 Watts/Ch at 0.1% THD
Spec at 70-V Mode, Stereo: ≥ 210 Watts/Ch at 0.1% THD
Initial Conditions: Controls per standard
Procedure: Load each channel to 8 ohms. Inject a 1 kHz
sine wave and measure at least 41.47 VAC at the
output of each channel. Load each channel to 4 ohms.
Inject a 1 kHz sine wave and measure at least 30.33
VAC at the output of each channel. Switch each
channel to the 70-V mode. Load each channel to 25
ohms. Inject a 1 kHz sine wave and measure at least
70.7 VAC at the output of each channel. All power
measurements must be at less than 0.1% THD.
Test 13: Reactive Loads
Spec: No oscillations. Safe with all types of loads.
Initial Conditions: Controls per standard.
Procedure Capacitive: Load each channel to 8 ohms in
parallel with 2µF. Inject a 20 kHz sine wave with 30
VAC output for 10 seconds.
Procedure Inductive: Load each channel to 8 ohms in
parallel with 159µH. Inject a 1 kHz sine wave with 15
VAC output for 10 seconds.
Procedure Torture: Load each channel with the primary
(red and black leads) of a PS-U transformer (CPN D
7040-5). Each channel in the 70-V mode. Inject a 10 Hz
sine wave with an output level sufficient enough to
cause 3 to 5 flyback pulses for 10 seconds.
Procedure Short: Inject a 60 Hz sine wave into the input
and adjust for 5 VAC at the output. Short each chanel,
one at a time, for 10 seconds.
Test 14: ODEP Limiting
Spec: No oscillation on ODEP limiting wave form; either
channel controls limiting in Parallel Mono mode.
Initial Conditions: Controls per standard; rag or other
obstruction blocking fan blade so it does not turn.
Procedure: Load the amplifier to 4 ohms per channel.
Inject a 60 Hz sine wave into each channel and adjust
the output to 15 VAC. Allow the amplifier to heat up
until you observe a wave form similar to figure 5. Allow
the amplifier to cool for a few miniutes. Switch the
amplifier to Parallel Mono mode and remove the load
from channel 2. Inject a 60 Hz sine wave into each
channel and adjust the output to 15 VAC. Allow the
amplifier to heat up and observe limiting on both
channels.
Test 15: LF Protection
Spec: Amplifier mutes for dangerous subsonic fre-
quencies.
Initial Conditions: Controls per standard.
Procedure: No load. Inject a .5 Hz 6 Vp-p, or a 2 Hz 6
VAC sine wave into each channel and verify that each
channel cycles into mute.
Test 16: Signal to Noise Ratio
Spec: ≥ 100 dB below 8 ohm rated continuous average
20-20kHz power.
Initial Conditions: Controls per standard. Short inputs.
Procedure: Load each channel to 8 ohms. Measure
less than 415µV at the output of each channel (20 Hz-
20 kHz bandpass filter).
Test 17: Turn off Transients
Spec: < 80mV across an 8 ohm speaker load
Initial Conditions: Controls per standard.
Procedure: Load the output with an 8 ohm speaker.
From an on condition, turn off the amplifier and monitor
the output noise at the time of turn off.
Note: Turn off noise may increase significantly if the
amplifier is cycled off and on.
Figure 5. ODEP Limiting Wave Form
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