Keithley 2000 Repair Manual

Test 601.2 — AMP SHUNT
Bank AMP/OHM
Inputs INPUT HI to AMPS Short
Expected Value 0.084 volts
Limits 0.02 volts
Fault Message AC AMP SHUNT
Description The previous test sets up the circuit for this test. There is a routine in soft-
ware that generates a waveform for the ACV tests. This is done by selecting
the 7V reference by closing analog switch U133 (/7V controlline low). The
reference is buffered by U123.
Control line OHMA line is high turning Q123 and Q125 on which gener-
ates a 1mA current source with R195, op amp U123, Q119, and associated
circuitry. The /LOWOHM control line of U133 is switched to toggle Q120
on and off to generate the 1mA AC current to the OHM node.
This test current is then switched through K101 (pin 4 to 3). Control line
/SETK2 is high and /RESETK2 is low. The test current goes through Q102,
Q101, the parallel combination of R115, L109, and R324, then to the INPUT
HI jack.
The switched current signal is applied to the circuit described in the pre-
vious test and a measurement is made. The input signal switching stops while
the A/D is taking the reading, then continues when the measurement is com-
plete. There are delays before the reading is taken to ensure that the ACV sec-
tion and filters have enough time to reach a charged stable reading. For this
test, the switched signal can be traced through the circuit described in the pre-
vious test. Measure 84mV DC at A/D_IN.
Bit patterns
Bit pattern Register
QQ
87654321
—U106—
101v0010
QQ
87654321
—U109—
11011111
QQ
87654321
—U134—
1v10110v
—U130—
10011101
QQ
87654321
—U121—
10110000
ACDC_STB
MUX_STB
IC pins: Q8=11, Q7=12, Q6=13, Q5=14, Q4=7, Q3=6, Q2=5, Q1=4
2-58 Troubleshooting

Keithley 2000 Specifications

General

Digits	6.5
DC Voltage Range	100 mV to 1000 V
AC Voltage Range	100 mV to 750 V
Resistance Range	100 Ω to 100 MΩ
Connectivity	IEEE-488 (GPIB)
Display Type	VFD