Keithley 2700, Service Manual

Page: 90 / 174

4-10 Troubleshooting
AMPS input
The ACA or DCA input signal is applied to the Current Shunt circuit, which is made up of
K103, R158, R205, and R338. For the 20mA DC range, 5.1
Ω
(R205/[R338 + R158]) is
shunted across the input. Relay K103 is energized (set state) to select the shunts. For all other
DCA ranges, and all ACA ranges, 0.1
Ω
(R158) is shunted across the input (K103 reset).
The ACA signal is then sent to the AC Switching & Gain circuit, while the DCA signal is
routed directly to the A/D MUX & Gain circuit.
Signal switching
Signal switching for DCV and OHMS is done by the DCV & Ohms Switching circuit. FETs
Q113, Q105, Q104, and Q108 connect the DCV or ohms signal to the X1 buffer (U113).
Note that the reference current for OHMS is generated by the Ohms I-Source circuit. For
4-wire ohms measurements, SENSE LO is connected to U126.
Signal switching and gain for ACV, FREQ and ACA is done by the AC Switching & Gain
circuit, which is primarily made up of K102, U102, U103, U105, U112, U118, U111, and
U110. Note that U111 is used for frequency adjustment. The states of these analog switches
vary from unit to unit.
Multiplexer and A/D converter
All input signals, except FREQ, are routed to the A/D MUX & Gain circuit. The multiplexer
(U163) switches the various signals for measurement. In addition to the input signal, the multi-
plexer also switches among reference and zero signals at various phases of the measurement
cycle.
When the input signal is selected by the MUX, it is amplified by U132 and U166. Gain is
controlled by switches in U129 and associated resistors.
The multiplexed signals of the measurement cycle are routed to the A/D Converter (U165)
where it converts the analog signals to digital form. The digital signals are then routed through
an opto-isolator to the MPU to calculate a reading.
Scanner card signals
Scanner card input signals are connected directly to installed scanner cards. Scanner card
output signals are routed internally to the INPUTS switch, which selects between the front
panel terminals and the scanner card outputs.