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Notch Filters
Set the reference frequency to 60.0 Hz (50.0 Hz).
It is convenient to use the SYNC output of the
signal generator as the reference input if it is
available. Connect the sine output of the signal
generator to the A input and set the input selector
to A. With the SENSITIVITY at 100mV, adjust the
amplitude of the input signal to 100 mV (full scale).
Set the LINE NOTCH to IN, the SENSITIVITY to
10mV, and the DYN RES to LOW. Connect the
scope to the SIGNAL MONITOR output on the
rear panel. Set the scope to AC coupled, 0.2V/div,
10mS/div. Trigger the scope externally using the
reference input signal.
The LINE NOTCH frequency and depth are
adjusted by the pair of 20 turn potentiometers
located under the middle two holes in the signal
shield (row 4 on the circuit board). Using a small
screwdriver, carefully adjust one pot until the line
output on the scope is minimized. Then adjust the
other pot until the output is minimized. Iterate
between the two pots until there is no further
improvement. Set the SENSITIVITY to 5mV,
2mV, and 1mV, repeating the adjustments at each
sensitivity.
Repeat this procedure using a reference
frequency of 120.0 Hz (100.0 Hz) and the LINEX2
NOTCH filter. The LINEX2 NOTCH is adjusted by
the pair of 20 turn potentiometers located under
the back two holes in the signal shield (row 5 on
the circuit board).
Replace the top panel.
Replacing the Front-End Transistors
Both the voltage and current front end transistors
(Q101 and Q102) are 2N6485 (IMF6485) dual
JFETS. These transistors are selected at the
factory to meet the noise specifications.
This section outlines their replacement procedure
in the event that they become damaged during
use.
1) Remove the AC power cord from the unit.
2) Remove top and bottom panels.
3) Release the signal shields by removing the
four screws which hold it onto the circuit
board. Be careful not to lose the nuts.
Carefully slide the shields back and then lift
them out.
4) The input transistors are located on the main
board, just behind the input selector switch.
Q101 is the voltage (A, A-B) front end, and
Q102 is the current (I) front end. Desolder
and replace the appropriate transistor.
5) Replace the signal shields. Be careful to
check that the shields do not touch any circuit
board traces around their edges.
6) Replace the top and bottom panels.
7) If Q101, the voltage front end has just been
replaced, the Common Mode Rejection needs
to be readjusted using the procedure
described in the Amplifier Adjustments
section.