SRS Labs SR560 manual Calibration And Repair, Offset Adjustment, Front End Replacement

CALIBRATION AND REPAIR

OFFSET ADJUSTMENT

The SR560's front-panel offset adjustment provides an easy way for the user to null the amplifier's DC offset. Use the COUPLING pushbutton to light the GND LED. Now, regardless of the SOURCE setting, the input to the amplifier is grounded internally. Insert a small screwdriver through the front- panel OFFSET hole and adjust the offset potentiometer until the DC offset of the amplifier (e.g. as viewed on a DVM) is zero. Finally, return to the desired coupling.

CALIBRATION

There are four pots, which are used to calibrate the instrument. The pots adjust the front-end CMRR (Common Mode Rejection Ratio) and offset. These pots are located close to the front of the instrument, and may be accessed by removing the bottom cover.

These pots should be adjusted to optimize the CMRR or null the offset when the front- end FET is replaced. Two of the pots adjust the CMRR and offset when the front-end gain is x10, and two adjust the CMRR and offset when the front-end gain is x2. The x10 gain pots must be set first, followed by the x2 gain pots.

First, the front panel offset pot must be set to zero:

•Adjust front panel Offset pot to read 0 VDC on pin 14, U407.

Next adjust the offset and CMRR for the case where the front-end gain is x10. View the amplifier output on a scope and perform the following adjustments:

•Couple = GND, Gain = 5 k, LOW NOISE: adjust P103 to null DC and output.

Now use a function generator as the source of a common mode signal:

•Apply 1 kHz 1 Vpp sine to both the A and B inputs.

•Couple = DC, source = A - B: adjust P102 to null sine wave output.

Now adjust the offset and CMRR for the case where the front-end gain is x2. View the amplifier output on a scope and perform the following adjustments:

•Couple = GND (remove signal from A and B inputs), Gain = 5 k, HIGH DR: adjust P104 to null DC and output.

•Apply 1 kHz 1 Vpp sine to both the A and B inputs.

•Couple = DC, source = A - B: adjust P101 to null sine wave output.

NOTE: In the above procedures, the gain of the front-end (x10 or x2) is determined by the selection of LOW NOISE or HIGH DYNAMIC RESERVE.

FRONT END REPLACEMENT

The most commonly damaged component is the front-end FET (U106, National Semiconductor Corp. P/N NPD5564). It is located in an 8-pin DIP socket behind the relays near the input BNCs. If the instrument exhibits a constant overload, excessive drift or noise, or large input bias currents, it is likely that this component has been damaged.

When replacing the FET, be certain that all eight pins are inserted into the socket, and observe the orientation of pin #1. After replacement adjust the CMRR and offset per the calibration procedure. More severely damaged front-ends may require replacement of op-amp, U102.

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