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Powering up the c200
Before
switching on the power, make sure that the preset R13 is set
for maximum resistance and no speakers are connected to the
output of the amplifier. The Vbe multiplier Q6, must also
be thermally coupled to the main heatsink to prevent thermal
runaway.
With
the input shorted to ground, slowly increase the output of
the power transformer to the working voltage of 40-0-40Vac
with a variac. This should be done with an oscilloscope connected
to the output of the amplifier.
If
the amplifier is properly assembled, there should be no oscillations
at the output as you gradually power up the amplifier's transformer.
With the supply rail voltages at +-53.5V, the DC operating
points should read TP1=1.65V; TP2=1.0V; TP3=0.84V; TP4=0.26V
and TP5=0.79V.
Once
this is established, slowly increase the bias of the output
into Class AB to 100mA per output transistor. A DMM across
an emitter resistor (0.39 ohms) should read 39mV. With the
oscilloscope set to 5mV resolution, you should be able to
reproduce Fig 1.
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Small
Power Testing
Switch off the amplifier and connect a 10 watts 8 ohms dummy
load to the output. Remove the input short and connect the
signal generator to the amplifier. Set the oscilloscope to
50mV resolution and slowly power up the amplifier again. Inject
a 10KHz square wave for a peak to peak reading of 250mV. You
should be able to reproduce Fig 2. There should be no signs
of ringing on the square wave. Rise time should read 2.4us
from 10%-90% of 250mVp-p.
Increase the input signal to reflect 10Vp-p
as seen in Fig 3. Rise time and square wave should remain
unchanged.
Reset
the signal generator for 20KHz square wave output. Repeat
the above procedure for an output of 10Vp-p and you should
be able to replicate Fig 4. Note the absence of overshoot
and ringing and fairly wide bandwidth.
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- THD & Full Power Testing
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