6. Resolder the red-black lead wire to the solder-lug
to which the green-black transformer lead wire is
already attached.
7. Reinstall the bottom plate.
8 . Remove the four screws from the nameplate on the
handle end of the amplifier. Turn the nameplate over
and reinstall. The nameplate nomenclature will now
be suitable for 120 volt use.
To change the PM300E back for use with 240 volt power,
reverse the above steps. Be careful to reconnect the yellow-
black and red-black transformer lead wires to an UNUSED-
UNGROUNDED solder-lug. A pictorial diagram of the trans-
former lead wire connections is located on the inside of
the bottom cover. The circuit diagram for both 120 volt and
240 volt operation is shown on page 10 of this manual.
PM300E6 Power Requirements:
The Model PM300E6 is supplied with a detachable three-
conductor A.C. power (mains) cable without a connector
on the “mains” end. To attach a connector, connect the
brown conductor of the cable to the “live” terminal of the
connector, the blue conductor to the “neutral” terminal,
and the green-yellow conductor to the “earth” (
) terminal.
The PM300E6 is designed to operate from 100, 120, 140,
200, 220, or 240 volts, 50 to 60 Hertz power (mains). An in-
tegral voltage-selector switch, located on the rear panel,
allows selection of the proper voltage.
As shipped from the factory, the PM300E6 is set to op-
erate from 240 volts.
The A.C. fuseholder, located nearest the voltage-selector
switch, contains a 1.6 ampere slow-blow fuse (1.6 AT)
which should be used when the unit is operated from 200
through 240 volts. When operated from 100 through 140
volts, this fuse should be changed to a 3 ampere slow-blow
type (3.0 AT). The fuseholder located farthest from the
voltage-selector switch contains a 5 ampere (5.0 A) fuse
which should not be replaced by other values under any
circumstances. Each of the three fuses mentioned is a
5mm x 20mm type, and two of each value are supplied with
the unit.
CAUTION
- DO NOT CHANGE VOLTAGE-SELECTOR
SWITCH SETTING OR ATTEMPT TO RE-
PLACE FUSES WITHOUT FIRST DISCON-
NECTING THE A.C. POWER (MAINS)
CABLE.
Ventilation:
The Power Master has been designed to operate in a
horizontal position and minimum heat rise in the amplifier
will occur when mounted in this manner. Forty-four and
one-half millimeters (1¾ in.) minimum clearance should
be provided on front, back, and the top of the amplifier
to insure adequate air circulation. The Power Master may
be operated in an ambient temperature range from -7°C
to 43°C (20°F to 110°F) for continuous duty without de-
rating.
Thermal Overload:
The Power Master is equipped with thermal sensing
switches on the heat-sinks of the output transistors. The
thermal switches are set to shut off AC power to the ampli-
fier when a temperature of 90°C (194°F) is attained on the
heat-sinks; the switches will automatically recycle and
return AC power when the heat-sink temperature reaches
73°C (164°F). A thermal overload light (II), located on
the front panel of the Power Master, will indicate if ther-
mal cycling has occurred. Thermal cycling may occur if
air is not allowed to circulate through the grilles of the
amplifier or if there is a prolonged short-circuit on the
speaker output.
Rack Mounting:
The accessory Rack Panel Kit, Model P300R, allows the
Power Master to be rack mounted in standard 19-inch audio
equipment cabinet racks. To insure adequate air circula-
tion, a 44.5 mm (1¾ in.) space should be provided at the
top of the amplifier. The rack mounted Power Master
requires seven inches of height plus the air space. The
Rack Panel Kit is to be installed by qualified service
personnel only.
Connecting a VU Meter:
An external VU meter may be connected to the speaker
wires of the Power Master with a resistor attenuator (see.
Figure 4, Page 5). Use a true VU meter (such as Simpson
Model No. 1349) and three resistors connected as shown.
The resistors should be ½-watt carbon 5%, or 1% if avail-
able.
With an 8-ohm speaker load (two VA300-S) zero VU is
50 watts. Output power for other VU readings is shown in
the table below.
V U
Power to 8-ohm load
+ 3
100 watts
0
50
- 3
25
- 7
10
- 1 0
5
FIGURE 4.
PM300 Series Power Master
Service Instructions
Amplifier Service (See Guarantee):
The Power Master uses components of the highest
quality, operating well within their respective ratings to
assure long life.
CAUTION:
There are no user serviceable
parts inside. Refer servicing to qualified service personnel.
Replacement Parts:
Parts that are readily
parts distributors are not
available through local electronic
shown on the accompanying Parts
List. Their values are shown on the circuit diagram. The
special custom made parts are shown on the Parts List.
The commercial alternates shown on the Parts List are
not necessarily equivalents, but may be used in the event
that direct factory replacements are not immediately avail-
able. To maintain the highest possible performance and
reliability, Shure factory replacement parts should be used.
When ordering replacement parts, specify the Shure Re-
placement Kit Number, description, product model number
and serial numbers.
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