Crown DC-300A Instruction Manual page 19

devices is good practice. Refer to Section 4.2, par. 5
for testing precautions.
An extremely common form of this problem is en-
countered when using electrostatic loudspeakers or any
other kind of load that is joined to the AC power mains.
Capacitive coupling through the load's supplies may allow
the amplifier's output to be fed through the AC mains
and into the grounds of input equipment resulting in a
system oscillation.
To combat the problem, first try reversing the speaker
leads on all channels if possible. If this does not solve
the problem, try grounding the power amplifier to the
AC ground with its 3 wire olua. (A around loon mav result
through.an
FM tuner
with
an earth-grounded
antenna
system when deploying the 3 wire plug. The use of .005 uF
disc capacitors in series with the FM antenna leads
will eliminate this problem.) All hookup (interconnecting)
cables should of course be as short as reasonably possible,
and a turntable baseplate ground should always be
brought to the phono preamp with a separate ground
wire, never via the input cable ground. If the oscillation
still persists, start removing input devices, working
towards the amplifier until the oscillation disappears.
This will identify the point of feedback. If an offending
piece of equipment is found it should be inspected for
unnecessary AC line-to-ground capacitances such as line
filters, etc. If possible such should be removed. Such
devices should never be found in the load, although it
is true that some RF loudspeakers have used such filters
to reduce RFI.
2.6 CONNECTING POWER
The amplifier is furnished with a three-wire AC plug
as standard equipment. Adapters are readily available
commercially for adapting this to a two-wire system
if necessary.
15
When testing the amplifier, the line voltage must be the
peak equivalent to a sinusoid of the indicated line voltage
when at full load. Line regulation problems can intro-
duce serious errors in the measurements on an
amplifier of this size.
Only a competent technician should attempt alteration of
the line voltage connections.
In order to change the voltage, it is first necessary to
carefully detach the bottom cover from the unit. On the
back of the board-mount subchassis a terminal strip
with solder-on jumpers is used to make the line-to-
transformer primaries connections (see Fig. 2-9).
ALL VIEWS WITH BOTTOM PLATE REMOVED
I20V
I28 V
See Note
240 V
248 V
256 V
See Note
NOTE: The 128V and 256V connections require the
changeover of the RED wire from TB-3 to TB-2. There-
fore, the Front Panel, control circuit-board and shield
must be carefully removed for access. When replacing
shield, use care in locating so that on-off switch
locates properly in Front Panel opening!
FIG. 2-9
TABLE OF LINE VOLTAGE CONNECTIONS