SRS Labs SR560 - Manual (Page 22)

CALIBRATION AND REPAIR
18
SR560 OFFSET ADJUSTMENT
PROCEDURE
Required equipment:
Digital volt meter
Oscilloscope
4 BNC cables
1 BNC tee
Function generator
Small slotted screwdriver
Phillips screwdriver
Remove the bottom cover of SR560 to
expose the component side of pc board.
1. Turn off the SR560. Hold down the
"Reset" button, and turn the unit back on
(this sets the unit back to the default
settings).
2. Using a Digital Volt Meter, adjust the front
panel offset pot (located between "A''
and "B" input BNC's on front panel) to
read zero volts between pin 14 of U407
and ground (output BNC shield). Do not
use the chassis as ground.
There are 4 potentiometers located on the
bottom right side of the pc board
(Viewed from the component side, with front
panel facing forward). These are:
P101 = low gain CMRR adjust
P102 = high gain CMRR adjust
P103 = high gain offset adjust
P104 = low gain offset adjust
3. Set the SR560 coupling to "GND", and
the gain = 50,000.
4. Connect the oscilloscope to the 50 Ohm
output on the SR560.
5. Adjust P103 for zero volts on the 50 ohm
output, using the oscilloscope.
6. Set the SR560 gain = 1000 and Source
to "A-B''.
7. Set the function generator to square
wave, Freq = 1 KHz, amplitude =
500mV pp.
8. Using a BNC tee, and 3 BNC cables, put
the square wave into channels A and B.
9. Set the SR560 coupling to "DC".
10. Adjust P102 to null the square wave on
the oscilloscope.
11. Set the coupling to "GND" and gain =
50,000
12. Readjust P103 for zero volts on the
oscilloscope.
13. Set SR560 gain = 50 and coupling to
"DC".
14. Set the oscilloscope to AC coupling.
15. Using the digital volt meter, measure the
voltage from pin 6 of U105 to ground
(output BNC shield).
16. Adjust P104 for zero volts on the meter.
17. Adjust P101 to null the square wave on
the oscilloscope.
You might have to readjust P104 and P101
several times. The end result should be
zero volts on pin 6 of U105, with the
smallest amplitude square wave that you
can achieve on the oscilloscope.
BATTERY REPLACEMENT
After three to five years or about 1000
charge/discharge cycles, the sealed lead-
acid batteries degrade. When the battery
operation time shortens, or if the unit stays
very warm for more than a day after it is
plugged into the line, the batteries may
require replacement.
The three batteries are a standard size
which are available from several different
distributors. All are 12 VDC with a charge
capacity of about 2.0 Amp-hours, and
measure 7.02" X 1.33" X 2.38". Two of the
batteries are wired in parallel to provide the
high current required for the positive supply.
When replacing the batteries, take care to
observe the polarities!
FUSE REPLACEMENT
There are three fuses on the back panel of
the instrument. The fuse located inside the
power entry module will blow if the unit
draws excessive line current. Replace this
fuse with the value indicated for your line
voltage.