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viii. While watching the scope, turn the gain control to the 12 o'clock position
and watch for the offset to change. If the trace moves in the positive
direction rotate VR2 in the clockwise direction. If the trace moves in the
negative direction rotate VR2 in the counter-clockwise direction. Adjust
VR2 until the trace is again on the zero line.
ix. Return the gain control to minimum gain. At this point the DC servo may
have developed a correction voltage in an attempt to zero the offset
generated while at the higher gain. Therefore the trace might not return
exactly to the zero line. What is important here is the
relative offset
between low and high gain settings. Turn the gain control quickly
between minimum and 12 o'clock then back to minimum and watch the
change in offset. Adjust VR2 so that the trace stays still while turning the
gain up and down. This step provides a coarse adjustment. To make a
fine adjustment repeat this step but turn the gain control all the way to
maximum and check that the offset is constant to within plus or minus
5mV
x. Repeat steps vii and viii for channels 2 through 8.
xi. The input offset does drift with temperature and having the lid off will
allow the circuitry to cool. It is advisable to replace the lid and let the
preamplifier warm up again (about 20 minutes) and then recheck your
adjustments.
