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F. INPUT OFFSET ADJUSTMENT:
The input amplifiers in the 801 are ultra precision laser trimmed devices which
have a very low input offset. However, even the smallest differential offset at the
input can result in an audible "click" when the gain control is turned. It should be
noted that this type of click will be audible when no signal is present. With no
signal present there is no signal amplitude to be changed unless
input offset
is
present. The DC servo circuit in the 801 corrects any DC errors that appear at
the output of the instrumentation amplifier stage. It can not correct the very
small offset at the inputs. This procedure nulls the input offset and minimizes
the clicking sound with no signal present. In contrast, there will almost always
be an audible click if turning the gain control while a signal is present. This is
because the gain control, being a stepped control, will make an instantaneous
change in amplitude when turned, which creates a transient signal.
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. 3/32" hex wrench
2. PROCEDURE
i. Remove the 9 #8-32 flat head screws from the top lid but leave the lid in
place.
ii. Set all of the preamplifier gain controls to minimum. Turn on the 801
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 for
serial #81037 and above or Figure 8 for serial #81036 and below)
