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4.
When
measuring hum and noise, all inputs
should be disconnected from the amplifier
and the level controls set to minimum
or
to
maximum, preferably minimum.
5.
Whenever possible avoid ground loops in the
test equipment caused by connecting the out-
put ground to the input ground.
Never
con-
nect the ground of the cable going to the load
back to the input ground.
Ground loops are especially obnoxious when
measuring distortion.
An
I
-M
distortion an-
alyzer, for example, has its input and output
terminals tied to a common ground. Such a
test should use
an
ungrounded output return,
with the output lead(s) wrapped around the
well-shielded and grounded input cable.
6.
Always
monitor
the
test
oscillator when
measuring frequency response. Use a wide-
band
AC
voltmeter; or use the same meter
for both
input
and
output
level measure-
ments,
if
the meter's frequency response is
known not to
be
dependent
on attenuator
settings.
7.
Accuracy in measuring voltages for comput-
ing wattage is critical. For example, a 2%
voltage error together with a 1 % resistance
error can result in an error
of
1.8
watts
power into 8 ohms.
8.
Residual distortion and noise levels should
be
fully known for all the test equipment in
order to accurately evaluate the amplifier.
9.
Never attempt to measure damping factor by
placing abnormal loads on the output.
D-F
measurements taken during clipping, or any
other form
of
overload, are meaningless.
The preferred method
is
to apply an exter-
nally generated current to the output termi-
nals and measure the resultant voltage
at
the
terminals. A convenient current
is
one am-
pere -
as
the resultant voltage
wiH
read di-
rectly in ohms for
I
Zo
I.
Damping Factor is
defined
as
Iii , where I ZL! is typically 8
ohms. A
convenient
generator for the
lA.
current
is
that amplifier channel not under
test. A non-inductive resistance of 8 ohms
- coupled between both channels' output ter-
minals - will provide
lA.
when 8 volts are
impressed across the resistor (by that chan-
nel not under test).
10. Never measure hum and noise when in the
presence
of
strong magnetic fields. The am-
plifier should
be
at
least 4 inches away from
any large metallic objects
or
shield plates
for a reading to
be
meaningful.
11. Noise measurements should
be
taken with a
band-pass filter of 20-20KHz. For audio pur-
poses
the measurement of noise above 20KHz
is meaningless.
4.3
SERVICE
Should service other than routine fuse replace-
ment ever
be
required, it
is
recommended that the
unit
be
returned to the factory in the original pack-
ing (or replacement,
if
damaged).
For
warranty
service the machine must
be
returned to the fac-
tory or warranty stations. The
CROWN
warranty
is detailed
on
page
10.
Because of the level of circuitry sophistication of
the D-60, only the most competent technicians
should
be
allowed to service it.
Many
of
the parts are standard items stocked by
most supply houses. However, there are several
which appear to
be
standard parts but are actually
different. Although standard parts may
be
used in
an
emergency, best results will
be
with factory
parts. A number of the parts are available only
from
CROWN.
When
ordering parts,
be
sure
to
give the ampli-
fier serial number
as
well
as
the part number and
description.
Rated
firms will
be
billed, otherwise
shipments will
be
C.0.D.
Before returning
an
amplifier to the factory for
servic
e,
authorization must first
be
obtained from the
service manager. All shipments must
be
sent by
UPS
or truck freight. prepaid and insured at total value.
The factory will return your serviced unit by
UPS
or
truck freight, collect, and will add C.0.D. charges in
the event that the cost is
not
covered by registered
warranty.
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