SRS Labs SR510 - Manual (Page 38)

34
Calibration and Repair
This section details calibration of the instrument.
Calibration should only be done by a qualified
electronics technician.
********** WARNING **********
The calibration procedure requires adjusting the
instrument with power applied and so there is a
risk of personal injury or death by electric shock.
Please be careful.
Most of the calibration parameters are determined
by a computer aided calibration procedure after
burn-in at the factory. These calibration
parameters are quite stable and so will not need to
be adjusted. Calibration parameters which may
need field adjustment are detailed below.
Multiplier Adjustments
On the HIGH dynamic reserve setting, there can
be some reference frequency feedthrough. This
section describes how to null this unwanted
output.
This adjustment requires an oscilloscope and a
signal generator which can proved a 500Hz
reference signal.
Allow the unit to warm up for about 1 hour.
Reset the unit by turning it off and back on while
holding the REL key down.
Select voltage input A and connect a 50
Ω
terminator or shorting plug to the A input BNC
connector. Connect the 500 Hz reference signal to
the reference input. Set the SENSITVITY to 1mV
and DYN RES to HIGH. The PRE TIME
CONSTANT should be set to 1mS and the POST
TIME CONSTANT to NONE. Connect the scope
to the OUTPUT on the front panel. Set the scope
to 2V/div and 5mS/div. Externally trigger the scope
using the reference input signal.
After about 60 seconds, the scope display should
show a 500 Hz sine wave on a 30 Hz (500/16 Hz)
square wave. Remove the 4 screws holding the
top panel on. Slide the top panel back about half
way. Using a small screwdriver, adjust P402 at
location D2 to minimize the 500 Hz output. Adjust
P403 at location C2 to minimize the 30 Hz output.
Now set the both time constants to 1S. Adjust
P404 at location F4 to zero the output. This
adjustment has a range of 20% of full scale on the
HIGH dynamic reserve setting. (2% on NORM and
0.2% on LOW). This zeroes the DC output of the
unit on all dynamic reserve ranges.
Replace the top panel.
Amplifier and Filter Adjustments
This section describes how to adjust the Common
Mode Rejection and Line notch filter frequencies.
An oscilloscope and a signal generator which can
provide an accurate line frequency and twice line
frequency signal are required.
Allow the unit to warm up for about 1 hour.
Reset the unit by turning it off and back on while
holding the REL key down.
Remove the 4 screws holding down the top panel.
Slide the panel back about halfway.
CMRR
Set the reference frequency to 100 Hz. It is
convenient to use the SYNC output of the signal
generator as the reference input if it is available.
Connect the sine output of the signal generator to
the A input and set the input selector to A. With
the SENSITIVITY at 100mV, adjust the amplitude
of the input signal to 100mV (full scale).
Now set the input selector to A-B and connect the
signal to both the A and B inputs. Set the
SENSITIVITY to 20
µµµµ
V, the DYN RES to NORM
and the BANDPASS fiter IN. Connect the scope to
the SIGNAL MONITOR output on the rear panel.
Set the scope to AC coupled, 0.2V/div, and
10mS/div. Externally trigger the scope using the
reference input signal.
The CMRR is adjusted by the single turn
potentionmeter located at A1 under the single hole
at the front of the signal shield. (The shield is the
aluminum box on the left side of the main board).
Using a small screwdriver, carefully adjust the pot
to minimize the 100 Hz output on the scope. After
nulling the output, set the sensitivity to 2
µµµµ
V and
null the output again.