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either adjust the DIRECT CURRENT OUTPUT to give zero indication on
the current meter or, using this control, centre the table in its vertical travel.
For manual testing set the signal source frequency to the desired value
and increase the AMPLIFIER GAIN until the required test levels are
established. For automatic sweep testing full instructions are to be obtained
from the relevant instruction manuals.
3.3.3. Warning Lights
If the DISPLACEMENT LIM IT is set at less than twice the maximum
test level, check that the Displacement circuitry does not trip at the low
frequencies. Adjust the DISPLACEMENT LIMIT as necessary.
If any of the red front panel lamps light, determine the cause, and
remedy the situation. Return the AMPLIFIER GAIN to "Reset" and recom-
mence the run-up.
Should the CLIPPING lamp light, the signal level must be reduced until
the light is extinguished. Full output voltage is available to 10kHz and
output voltage decreases in inverse proportion to frequency between 10kHz
and 100kHz.
If the TRANSISTOR lamp lights during a test, monitor the voltage wave-
form and the vibration levels. If no degradation has occured and if the
amplifier is operating reasonably below maximum rating, the test, if vital,
can be continued. When the test is discontinued, turn down the AMPLI-
FIER GAIN control to the "Reset" position and the POWER switch to
"Off". Remove the top-cover of the amplifier and check the fuses on the
component board mounted to the fan-cooled assembly (Fig.3.8) and the
power transistors mounted on the heat-sinks.
3.3.4. Fault Rectification
The schematic circuit diagram of the power output stage is given
(Fig.3.7) and shows that the protection fuses can be checked in situ by
measuring across each fuse in turn with an
Ohm-meter.
An un-blown fuse
will indicate as a short circuit and a blown fuse as approximately 250Q for
the NPN transistors, depending on the polarity of the Ohm-meter con-
nection.
19
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