Keithley 2000 Repair Manual

Test 101.2 — TESTCAL
Bank A/D
Inputs Open
Expected Value (101.2) - (101.1) = 76275970 counts
Limits 1800000
Fault Message NO 7V AT A/D
Description This A/D test uses the default conditions of the ADC word and the ACDC
word. This sets up the front end of the instrument to a stable configuration.
The MUX word is applied to register U130 which sets the lines of U163 as
follows; A0 and A1 low, A2 high. This bit pattern selects the S5 input, which
connects REFHI to the D output.
REFHI is then connected to op amp U166 which is configured for
× 1 gain
with feedback through mux switch U129 pin 2 to 3. The buffered value of
REFHI is then connected to the A/D at A/D-IN.
A conversion is taken in the form of A/D counts and compared to the value
taken in test 101.1. The value in counts of test 101.2 minus the value in
counts of test 101.1 yields a value that is compared to a value by-design for
REFHI. If this value is within the limits, the REFHI reference, which is 7
volts, is considered acceptable. Measure 7V at A/D-IN. Failures could be the
MUX (U163), or the reference circuit (U141) and the associated circuitry.
Bit patterns
Bit pattern Register
QQ
87654321
—U106—
110v1111
QQ
87654321
—U109—
00101111
QQ
87654321
—U134—
1v10000v
—U130—
11001101
QQ
87654321
—U121—
01110010
ACDC_STB
MUX_STB
IC pins: Q8=11, Q7=12, Q6=13, Q5=14, Q4=7, Q3=6, Q2=5, Q1=4
2-26 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