Stanford Research Systems SR124 Operation And Service Manual page 83

5.5 Main board
SR124 Analog Lock-In Amplifier
2. Work should be performed at an ESD-controlled workstation.
Be sure the technician is grounded before opening the instru-
ment.
3. Turn o power to the unit, and remove the AC power cord.
Disconnect all other cables from the SR124. Remove the top lid
of the instrument by removing the 2 large black screws from
either side of the lid, and the 6 small black screws from the top
of the lid. Slide the lid slightly backwards, and then lift away
from the instrument.
4. Locate the input JFET, marked as Q1101 on the main circuit
board. You can find Q1101 directly behind the A I and B inputs,
about 5 cm back from the edge of the board. Q1101 is socketed,
as shown in Figure 5.1. Using your thumb and forefinger,
carefully remove the JFET.
Figure 5.1: The front-end JFET (Q1101), installed.
5. The replacement JFET will be installed in the same socket.
Please note that two of the socket positions, aligned with the
front and back of the instrument, are left unpopulated. It is
important that the pins of the new JFET all be seated correctly
in the remaining positions. Refer to Figure 5.2 for the unpop-
ulated positions. When fully seated, the JFET should look like
Figure 5.1.
6. After replacing the JFET, the o set voltage must be trimmed to
near zero. Power must be applied to the SR124 with the top
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