Maintenance
7-4
2. Remove the two screws securing the top cover, and re-
move the cover from the instrument.
3. Select the amps function, and place the instrument on
the 2pA range.
4. Enable zero check, but leave zero correct disabled.
5. Locate the offset adjustment pot (R314) on the elec-
trometer board (see Figure 7-2). The pot is accessible
through a small hole in the shield closest to the rear of
the instrument.
6. Adjust R314 for a reading of 0.0000 ±1 count on the dis-
play.
7. Replace the top cover unless the following input current
adjustment is to be performed.
Figure 7-2
Input offset and current adjustment locations
7.4.8 Input current adjustment
Use the following procedure to null out any input current in
the input stage:
1. Disconnect all input signals from the Model 6512. Place
the optional protection cap (CAP-31) on the INPUT
connector.
2. Remove the two screws securing the top cover, and re-
move it from the instrument.
3. Place the Model 6512 in the amps function, and select
the 2pA range.
4. Enable zero check and zero correct in that order.
5. Disconnect floating sources, and connect a ground link
between the COM and chassis ground binding posts.
Disable zero check, but leave zero correct enabled.
6. Wait several minutes until the reading on the display set-
tles down; about 15 counts (1.5fA) p-p of noise is normal.
7. Locate the input current pot (R348) on the electrometer
board. It is accessible through a small hole in the shield
(see Figure 7-2).
8. Carefully adjust R348 for a reading of 0.0000 ±15
counts on the display. Iterative adjustment may be nec-
essary.
9. Replace the top cover, and secure it with the two screws
removed earlier.
7.4.9 Calibration program
The Model 6512 must be placed in the calibration program
in order to perform the front panel digital calibration proce-
dures. Select the calibration program as follows:
1. Turn off the instrument for at least three seconds if it is
presently turned on.
2. Press and hold the PROGRAM SELECT button, and
then turn on the power.
3. The instrument powers up as normal, but the CAL pro-
gram is accessible in the program menu.
4. Select the calibration program by pressing PROGRAM
SELECT repeatedly until the following message is dis-
played briefly:
CAL
5. A normal reading will now be displayed except that the
exponent decimal points will be displayed to indicate
that the instrument is in the calibration mode.
With the instrument in the calibration mode, perform the fol-
lowing procedures to calibrate the Model 6512 from the front
panel.
7.4.10 Amps calibration
Calibration of the amps function should be performed in the
following order: 200pA, 20nA, 20µA and 20mA ranges.
Once these ranges are calibrated, the remaining ranges are
automatically calibrated. Use the AMPS (active) function of
the Model 263 to source current.
FRONT PANEL
ELECTROMETER
BOARD
INPUT CURRENT
ADJUSTMENT
(R348)
INPUT OFFSET
ADJUSTMENT
(R314)
REAR PANEL
Summary of Contents for 6512
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