Overview Of Circuits; Power Supply; Cpu; Front Panel - Stanford Research Systems SR124 Operation And Service Manual

5 – 2

5.1 Overview of circuits

5.2 Power Supply

5.3 CPU

5.4 Front Panel

5.5 Main board

The following sections correspond to schematic pages at the end of
the manual.
The SR124 power supply board uses low-voltage AC power from the
shielded transformer to power the instrument. Separate power nets
are created for the oscillator, the main signal board, and the CPU
Front-panel boards. A single "star ground" is established on the
power supply board, where it is connected to the instrument chassis.
The CPU board contains the microcontroller that configures all in-
strument hardware of the SR124. Non-volatile memory on the CPU
board stores user settings. The fully stopable CPU oscillator is also
on this board.
The front panel board contains the LED indicators and button con-
trols. All numeric displays are driven from static shift registers,
whose values are only updated in response to a user command.
The Main board contains the front-end voltage and current preampli-
fiers, programmable filters, the PSD (square-wave mixer), the output
filters, and the reference output attenuator.
The input JFET, Q1101, is quite sensitive and may be damaged by
overvoltage, including overvoltage from a static discharge (ESD).
Symptoms of a damaged JFET can be excess noise or signal loss.
Instructions for replacing the input JFET are as follows:
1. Equipment needed:
Replacement JFET (obtained from SRS)
BNC grounding cap or 50
Voltmeter capable of showing millivolts
Test leads with small clips, to attach to test point loops
Small adjusting screwdriver
terminator
SR124 Analog Lock-In Amplifier
Circuits