The CMRR is adjusted by the single turn potentiometer located at A1 under the single hole at the front of the signal shield. (The shield is the aluminum box on the left side of the main board). Using a small screwdriver, carefully adjust the potentiometer to minimize the 100 Hz output on the scope. Set the DISPLAY to R,Ø and the sensitivity to 5µV and minimize the R output on the Channel 1 meter.
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,
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.
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