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3.11. "EXCITER" lAMP INDICATION
A lamp labelled EXCITER is mounted on the front panel of the Power Amplifier Type
2707
.
Lighting of this lamp indicates that something is wrong at the Exciter. Drive to the
Exciter Head is removed when this lamp is lit.
a) Make sure that the Exciter Head is properly mounted on the Exciter Body
.
b) Make certain that the Exciter Cable is connected both to the Power Amplifier and to
the Exciter Head
.
c) Check if dirt or a piece of paper blocks the Field Supply Air Inlet Screen. If so, clean,
allow 20 minutes for the Exciter to cool, and turn on again
.
d) Check the MOTOR fuses (see Fig .2.5). If any of these are blown, turn off the
POWER switch on the Power Amplifier, clean the Field Supply Air Inlet Screen, re
place the fuse, wait 20 minutes for the motor to cool down and turn on. If the fuse
blows again see "Care and Maintenance", Chapter 8.
3.12. OPERATION IN HIGH AMBIENT TEMPERATURES
3.12.1. 4801 Body
The cooling system for the 4801 is designed to limit component temperatures to normal
values for ambient temperatures as high as
104°
F
(40°
C). Operation with ambient tem
peratures as high as
122°
F (50°C) is possible if input power to the Driver Coil is reduced
by limiting Driver Coil currents slightly
.
For ambient temperatures of
104°
F to
122°
F (40°C to
50°
C) reduce the setting of the
CURREI\JT LIMIT controi by 1
%
for each 2
°
F above 104°F (1
%
for each
1°
C above
40°
C)
.
The available force is reduced in proportion to the reduction in current.
3.12.2.4805 Body
The above rules also apply to the Type 4805 when adequate cooling is provided (see sec
tion 2 .3)
.
However, if there is no cooling, then the CURRENT LIMIT should be set to half
of the value shown on the back of the Head and reduced proportionately for increased
ambient temperatures .
3.13.
REDUCTION OF RESIDUAl MOTION
The residual motion is the motion of the moving element when the input signal to the
Power Amplifier is zero. The residua I motion of the system consisting of the Power Am
plifier Type 2707, the Exciter Body Type 4801, and a matching Exciter Head is typically
about 0,05 mms-
2 .
This low leve I of background motion is insignificant for most appli
cations
.
It is traceable to Amplifier noise and fan motor residua I unbalance. The proportion of the
latter depends on whether the Amplifier OUTPUT IMPEDANCE is set to
"
Low" or
"
High". This is because the suspension provides some isolation of the table from vibra
tions of the Exciter Body, and the amount of vibration isolation is influenced by the im
pedance in the drive coil electrical circuit.
25
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