1. EQUIPMENT NEEDED
i.
Oscilloscope with a DC coupled 10mV input range.
ii. Unbalanced output cable. (see Figure 1.)
iii. Plastic alignment tool or a small screwdriver.
iv. #2 Phillips screwdriver
2. PROCEDURE
i.
Remove the 12 #6-32 flat head screws from the top lid but leave the lid in
place.
ii. Set all of the preamplifier gain controls to minimum. Set the trim controls to
maximum. Turn on the 201 preamplifier and the oscilloscope and allow to
warm up for at least 45 minutes.
iii. Connect the output cable from channel 1 of the preamplifier to the
oscilloscope input.
iv. Set the oscilloscope vertical sensitivity to 10mV/division and input coupling to
DC.
v. Set the oscilloscope horizontal sweep rate to 0.5mS/division.
vi. Adjust the trigger for a stable sweep and the vertical position so that the trace
is on the zero line.
vii. Remove the top lid and locate potentiometer VR2. (See Figure 7.)
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 10mV
x. Repeat steps vii and viii for channel 2.
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.
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