SRS Labs SR850 - Manual (Page 249)

8-3
SR850 Service
Adjusting the DC Offset and Common Mode Rejection
1. The DC Offset and CMRR adjustments affect each other and must be done together. This proce-
dure requires a 50
Ω
terminator, a BNC Tee and a few BNC cables. A small slotted screwdriver is
also required to adjust the potentiometers.
2. Turn the unit ON while holding the [<-] key down. This resets the unit. Let it warm up for at least
an hour.
3. Remove the four black screws which secure the top lid. There are two screws on each side. Slide
the top lid back about 1/4". Remove the lid by pulling the lid towards the back and upwards.
Remove the card cage cover by loosening the two screws at the center edge of the cover.
4. The adjustment pots are on the Analog board on right hand side of the card cage. Remove the
magnetic shield cover at the front of the Analog board. Simply pull the shield up using the finger
hole.
5. The adjustment pots are shown in the figure.
The Offset and Common Mode adjustment pots
are inside the shield which is mounted on the
analog board.
6. Press the following keys to set the frequency and display.
[REF/PHASE]
<Ref. Frequency> Select Reference Frequency
[1] [ENTER] Enter 1 Hz
[DISPLAY/SCALE]
<Type/Trace> Select Trace Number for Top display
[3] [ENTER] Enter 3 to display R
7. Connect a 50
Ω
termination to the A input. This shorts the input so the lock-in's own DC offset will
be measured.
Press
[GAIN/TC]
<Filter dB/oct> twice to select 24 dB/oct
<Sensitivity>
Rotate the knob to select 100 mV full scale
Line Notch Freq
Common Mode
Voltage Input Offset
Current Input Offset
Analog
Board
DSP
Board
2xLine Notch Freq
Line Notch Depth
2xLine Notch Depth
Front