Pro Intercom MS300 Instructions Manual

MS300

3-Circuit

Intercom

Instructions

A. Connections:

AC:

Before attaching the IED power cable, check that the voltage setting is correct for your area
and that the fuse is the right value. (110/120V- 2A @250V; 220/240V - 1A@250V). Assure that
the front panel power switch is in the Off position. Connect the IED cable to the MS300 and
the power source. Switch the unit On at the Front panel. The top LED in the Status column
on the front should light - green. The three lower LEDs will also remain green unless there
is a problem on that circuit, in which case the LED will turn red.
There is room for a spare fuse in the IED inlet.

Master Station

Comms circuits:

Your groups of stations should now be connected to the appropriate XLR jack, A, B or C.
If you have more cable runs than the number of XLR jacks for a particular circuit, you can

easily increase the available jacks with an SB1 Adaptor or with Y-cables. It is essential that
you check all your station interconnect cables for correct phasing. If Pin 1 is grounded
anywhere other than in the MS300 it will create a serious problem. The same is true with Pin
1/3 reversals which can easily happen when switching genders. Some mis-wiring will still
allow a cable to work with a microphone, but will render an intercom system useless.

Setting the DIP switches:

There are 8 switches in the DIP group. In the MS300 only 3 are active.
No’s 5 thru 7 lift the 200W termination circuit from circuits A, B and C. The standard position
for this switch is Down, with the termination circuit connected. The most common of the
rare times that the termination circuit would be lifted is when a circuit is being connected to
another intercom system which already has a termination circuit.
When the front panel ‘Link’ switch is used, the MS300 automatically adjusts the overall
termination so that it remains at 200W.

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The left section of the front panel contains controls which affect all circuits.
The ‘mains’ switch connects or completely disconnects the MS300 from the AC source.

The 4-pin male XLR is used to connect a headset, a handset or a gooseneck microphone.
Pins 1 & 2 are the input, with pin 1 being ground. Pins 3 & 4 are the output with pin 3 being
‘low’. When a gooseneck microphone is being used, the speaker and controls on the right
side of the chassis are used to listen. Communication is duplex, simultaneous talk/listen no
matter what device is connected. If a handset is used and it has a push-to talk bar, the bar
should be locked On or internally wired for constant On. If a gooseneck microphone is used,
particular attention should be paid to instructions regarding Sidetone, below.

The Status Bar of LEDs contains four green LEDs. The top LED indicates that the power
supply section of the MS300 is delivering 24VDC to the operating section. The green LEDs
below it are bi-color and turn red if there are any problems on Circuits A, B or C. The
most likely problems are a short circuit between the power pin (2) and ground in one of
the interconnecting cables, or a pin 1/3 reversal in the wiring which often happens when
attaching different gender connectors to opposite ends of a cable. The fact that a microphone
cable works with a microphone does NOT assure that it will work in an intercom system. An
out-of-phase microphone will still work, if poorly. An out-of-phase intercom system will not
work at all and may damage components. If the problem on the circuit is serious enough
to damage the MS300, the circuit will shut down until the problem is removed. The other
circuits will not be affected.

Below the status bar is a switch which allows you to link the circuits, either A+B or ALL.
This will put all the stations on the linked circuits on a common circuit as may be desirable
during a sound check or rehearsal. The normal position is Off.

Above and to the right of the Link switch is a button marked Override. Pro Intercom
Loudspeaker stations have a built-in circuit which restores their front panel controls to
preset functions, regardless of how they may have been adjusted locally. Units turned Off
are turned On, volume is restored to an audible level, etc., all determined by internally set
jumpers. The Override button activates that circuit. For example, a Loudspeaker station that
has been turned down locally could have its volume restored and an important message
heard.

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Override

Active

The next control in this segment is marked with a symbol: ‘Sidetone’. You experience
Sidetone every day when you use the telephone. It overcomes the brain’s conviction that if
you can’t hear yourself, nobody else can either. It is a small amount of the audio from your
own microphone fed back into your earspeaker. You need just enough Sidetone and no
more. The amount varies from person to person.

Sidetone is used in two ways with an MS300:

First, if you are using a headset, tiny adjustments of the little recessed control can make
your listening more comfortable. Remember, Sidetone only affects what you hear, it does
not change the level that others on the circuit hear. The second, and most dramatic use of
Sidetone is to permit the MS300 to be used with a noise-canceling gooseneck microphone
and the built-in loudspeaker in a full simultaneous talk/listen mode (duplex) hands-free.
Reducing Sidetone also reduces the level of your own voice coming out of the loudspeaker,
thus delaying the onset of feedback. This allows you to raise the level of the audio coming
from those with whom you are communicating

.

Using a noise-canceling gooseneck microphone cuts back on the amount of audio coming
from the loudspeaker and the environment which is getting back into the audio path via
the microphone. The result is that the MS300 can be used in a fairly quiet environment
to talk and listen to other stations without a headset and without pressing buttons. The

Sidetone control requires very tiny adjustments to nd that perfect spot where speech is

loud enough, and feedback does not occur. The default position of the control is 12 o’clock
which is usually acceptable for most headsets

.

The last control in this segment is a button marked Mic Type.
This control sets the circuitry to accommodate either a dynamic or an electret microphone,
whether it is part of a headset/handset or is a gooseneck microphone. An LED indicates
which setting is active. It is essential that you use a professional communications headset,
handset or microphone with the MS300. The earspeakers in the headset should be between
200 and 400W, and the microphone should be either a dynamic from 150 to 250W, or an
electret between 1.2 and 1.8 kW. Inexpensive headsets commonly sold for connecting to
computer sound cards are not acceptable for this purpose.

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Individual Circuit Controls

Each circuit has a control section on the front panel like the one illustrated above. Unless the
Link Switch (see previous section) is in use, each circuit operates completely independently
of the others.
The rotary control marked Local Level controls the volume of the audio on that circuit as
heard in the MS300 user’s headset. It does not affect the level that anyone else on that
circuit hears. Each remote station controls its own level.

The Signal button lights the signal lamps on all stations connected to that circuit. Those
stations can also signal the MS300 operator. In both cases the bright yellow LED next to
the button is lit. The Talk button functions momentarily by simply pressing and releasing it.
Tapping it quickly will lock it ON while another tap turns it OFF.

Power Distribution

The MS300 contains a powerful and sophisticated power supply. The upper limit of its
ability to supply current is determined by the toroidal transformer that is at its heart, and its

associated rectier. The transformer is able to supply (after rectication to DC) 2.4 Amps

@24VDC. This is roughly enough current to drive 80 belt packs with their signal lamps
constantly lit.

Each of the three circuits has its own regulator and is regulated at 1.0A. This means that if
all 3 circuits were loaded to draw their full output. The total would exceed the transformer’s
capability by 0.6A. In this event the circuitry would simply begin to reduce the voltage

available from 24VDC down towards 18VDC. The system would not fail and 18VDC is sufcient

to provide beltpack performance close to normal. However, common sense suggests that
any one circuit should be limited to 0.8A. (2.4/3) This represents about 27 beltpacks or 8
LS3 loudspeaker stations. In the very unlikely circumstance that a greater load than this is
required, the stations could be spread across 2 circuits and the link switch used to put them
in communications with one another.

The fact that each circuit has its own regulator means that each circuit can be monitored
separately and that a fault in one circuit will not affect the others.

The Power Supply components are mounted on a massive heat sink and a temperature
sensitive cooling fan starts up if the power supply section gets warm.

In conclusion: A huge intercom system can be built around an MS300 without fear of

failure

.

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