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CALIBRATION
Six pots exist which may be used to calibrate
various voltages on the SR570. Two are used to
adjust the battery voltages, while the rest are
used to null out offsets in the front-end amplifier
stage. The bottom panel of the SR570 must be
removed to access the pots.
Pots P701 and P702 adjust the battery voltage
levels. The batteries must be disconnected to
make these adjustments. To adjust the positive
supply voltage, adjust P701 while measuring the
voltage at U701, pin2. For the negative supply,
adjust P702 while measuring U702, pin 3. The
recommended voltages are +/- 14.0 V and
should have been set at the factory.
Pots P103 and P104 are used to ensure that the
applied input bias voltage is completely
subtracted from the front-end output.
P104 should be adjusted first. Turn off the
SR570. Using an ohmmeter, measure between
U107, pin 3, and U105, pin 4, and adjust P104
so that the resistance is equal to that measured
across R105.
Turn on the SR570. Select a 1 mA input offset
current and 1 mA/V sensitivity from the front
panel. The SR570 should otherwise be in its
default start-up state (Low Noise, no filters, etc.
see p. 4 )
Measure the voltage at U106, pin 1.
Now select a 3V bias from the front panel.
Adjust P103 until the output at U106, pin 1 is
the same as that measured without the bias.
Op amp input offset voltages can be nulled with
pots P101 and P102.
To null out the offset for U108 (Low Noise
input), attach a "shielded open" to the input of
the SR570 and select the 1 mA/V sensitivity.
The unit should otherwise be in the default start-
up mode. Adjust pot P102 to give 0 volts at
U108, pin 6.
To adjust the Low-Drift mode op amp, repeat
the procedure as above (with Low Drift mode
CALIBRATION AND REPAIR
and 1 µA/V sensitivity selected), adjusting P101 to
give 0 volts at U107, pin 6.
FRONT END REPLACEMENT
The most commonly damaged components are the
front-end input op amps U107 (Analog Devices
AD546) and U108 (Analog Devices AD743). Both
are located under the metal shield near the front of
the PCB. If the unit is constantly overloaded, or
doesn't amplify any signals, chances are one of these
op amps has been damaged. Switching between gain
modes while referring to the op amp allocation table
in Appendix B should determine which device needs
to be replaced.
When replacing an op amp, make sure that all eight
pins make firm contact in the socket and that the
orientation for pin 1 is observed (match the notch on
the IC with the notch on the socket). After
replacement, the op amp input offset voltage should
be nulled out using either P101 or P102 (cf.
Calibration).
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. Take care
to observe battery polarities when replacing!
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. The replacement should be a standard 1A
"slo-blo" fuse.
17
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