Table of Contents
Advertisement
7.8
Input stage balancing procedure
If the input FET (Q308) or associated bias resistors are re-
placed, the following procedure should be used to rebalance
the circuit. This procedure may also be used if the offset ad-
justment potentiometer (R314) has insufficient range. The
procedure involves checking the number of counts of offset
with jumper W303 in position B.
Proceed as follows:
1. With the power off, remove the top cover and electrom-
eter board shield, as described in paragraph 7.6.
2. Place jumper W303 in position B.
3. Turn on the power, and allow the instrument to warm up
for one hour.
4. Select the volts mode and the 200mV range. Do not use
auto-ranging.
5. Enable zero check, but leave zero correct disabled.
6. Set the input offset adjustment potentiometer (see Fig-
ure 7-2) fully clockwise as viewed from the screw end.
The control is a multi-turn potentiometer, so consider-
able adjustment may be required.
7. Note the number of counts shown on the display, and
compare the value to the ranges listed in Table 7-16.
From this table, you can determine in which position
jumper W303 should be placed. For example, if the dis-
play shows between -1200 and +200 counts, jumper
W303 should be placed in position C.
8. Place the jumper in accordance with the results of step 7.
9. Turn off the power, and replace the electrometer board
shield.
10. Turn on the power, and allow the instrument to warm up
for one hour before performing the following adjustment.
11. Set the input offset potentiometer (R314) for a reading
of 00.00 ±1 count on the display.
12. Replace the top cover when the procedure is complete.
Table 7-16
Input stage balancing
Number counts with W303 in
position B and R314 fully clockwise
1400 to 2800
200 to 1400
-1200 to 200
7.9
When troubleshooting or otherwise working inside the in-
strument, care should be taken not to touch PC board traces
and open wires to avoid contaminating them with body oils
or other foreign matter. In particular, there are two areas
within the Model 6512 that have numerous high impedance
nodes where contamination could cause degraded perfor-
mance. These include the input amplifier area on the elec-
trometer board and the ranging amplifier section location on
the mother board.
The same general precautions apply when replacing parts in
these areas. When unsoldering and soldering parts, be care-
ful not to spread the flux around the board to adjacent areas.
After replacing parts, or if contamination is suspected, use
the following procedure to clean the affected area.
1. Using a squeeze bottle, carefully apply clean, uncon-
taminated methanol to the area to be cleaned. Use suffi-
cient solution to thoroughly wet the circuit board.
2. Using a small, clean brush, wipe the area thoroughly un-
til it is free of flux or contaminants. In some cases, it
may be helpful to tilt the board at an angle and brush
contaminants away from the affected area, allowing
contaminated residue and methanol to run off the board.
3. Wash the area again with fresh, clean methanol.
4. Once the area is thoroughly cleaned, is should be dried
with pressurized dry, clean air or nitrogen. Do not use
compressed air from an ordinary air compressor, as oil
particles in the air could contaminate the circuit board.
5. After cleaning or parts replacement check to see that any
components connected to the Teflon insulators are not
physically touching the board or adjacent parts.
6. Allow the board to dry for several hours in a 50°C envi-
ronment before use.
Position
of W303
A
B
C
Handling and cleaning precautions
Maintenance
7-17
Advertisement
Table of Contents
