16
PMD125-V, PMD250-V, PMD500-V
5.
USING THE EQUIPMENT
5.1 Switching
on
Before starting up the equipment, make sure that all the appropriate connections have been made to complete the system. Position the main
power switch (
4
) in the ‘
ON
’ position. The ‘
ON
’ (
2
) signalling lamp will con
fi
rm that the equipment has been switched on.
5.2 LED
Display
With the exception of the ON/OFF LED (
2
), the remaining LEDs (
3
) on the front panel, have two functions: the two LEDs marked ‘OVL’ and ‘HOT’
indicating overload and overtemperature conditions respectively will work. The LEDs can also function as output level indicators or indicators
of the operational status of the ampli
fi
er. For further information concerning operation of the LEDs on the display refer to the
‘Failure conditions’
table at page 15.
6. SERVICE
NOTES
6.1 Forced
ventilation
The
PMD
ampli
fi
ers have a fan for the forced cooling of the
fi
nal power stages and of the inside of the equipment. This fan, controlled by its own
power-supply and control circuit, is activated automatically when the heat-sink reaches a given temperature and turns off once the temperature
has returned to the normal level. In practice, in typical conditions of use for ampli
fi
ers of this size, such as the broadcasting of background
music with full-power voice announcement at intervals, and in normal climatic conditions, forced ventilation is not necessary and will therefore
not be activated. The fan takes in cool air through the slits on the front of the equipment and expels the heated air through the rear slits (
5
).
It is therefore essential that these slits should not be obstructed in any way
.
6.2 Overload conditions and protection devices
In addition to the usual protection consisting of fuses, the
ampli
fi
ers have an electronic protection device and a thermal protection device in order
to safeguard them from possible risks of damage. Applying a lower loading impedance than the rated value means requiring the equipment to
supply a power higher than it can do on a continuous basis.
This could cause damage to the
fi
nal power stages and to the output transformer.
In order to avoid these problems, the ampli
fi
er is equipped with protection circuits against overloads, with automatic reinstatement. The protection
circuit will be tripped immediately on the ampli
fi
er if any of the following cases occurs:
- short circuit on one of the loudspeaker outputs;
- loading impedance less than 50% of the rated value;
- power required of the loudspeaker system connected to the constant-voltage lines higher than the power that the ampli
fi
er is able to deliver.
Overload conditions are signalled by the fact that the ‘
OVL
’ lamp located on the front panel of the equipment will light up. The thermal protection
is also of the self-resetting type, and is tripped when the equipment reaches an excessively high temperature. This may be due, for example, to
the fact that the ambient temperature is too high or to the insuf
fi
cient ventilation.
When the thermal protection is tripped, the ampli
fi
er will stop working, all the LED’s will extinguish and only the fan will continue to run.
Summary of Contents for PMD Series
Page 27: ......
