Keithley 2002 Repair Manual

Troubleshooting
2-61
2.11.13 Ohms tests
Test 306.1 — 9.6µA ohms source
Type
Pass/Fail
Fault message
9.6uA ohms source
Description
The 9.6µA ohms source is switched in with the /OHMD line at U220 and U223. U232 pin 2 to
3 connects REFBUF to U233 pin 3. (For a more detailed circuit setup description, see test
304.1.) 7.2V appears across R277. The 9.6µA current will all flow through Q211 (9.6µA
through R291 does not produce a high enough voltage drop to turn Q212 on), Q213, R322,
CR217, K201, R334 and R342. This point is the HI input terminal to the DMM. Q246, Q245,
Q244, and Q242 are all on, allowing the current to be shunted around the 9.9M Ω leg of the input
divider. The current then flows through the 100k Ω leg through Q226 and U244 pin 11 to pin 10
to common. The voltage at BUFFIN is applied to the input buffer to BSCOM and to U226,
which is configured for X1 gain.
Drawing reference
Analog Board; 2002-100
Components
U220, U223, U226, U232, and associated components.
Bit patterns
Test 306.2 — 1.92µA ohms source
Type
Pass/Fail
Fault message 1.92uA ohms source
Description
The 1.92µA ohms source is switched in with the /OHMD line at U223 and U220. U232 pin 10
to 11 switches in R276, basically forming a 5:1 divider between buffered OHMREFHI and REF-
BUF. 5.76V (relative to REFBUF) or 12.96V relative to common 2 appears at U233 pin 3. 14.4V
- 12.96 = 1.44V appears across R277. (For a more detailed circuit description, see test 304.1.)
The 1.92µA current will all flow through Q211 (1.92µA through R291 does not produce a high
enough voltage drop to turn Q212 on), Q213, R322, CR217, K201,R334 and R342. This point
is the HI input terminal to the DMM. Q246, Q245, Q244, and Q242 are all on, allowing the cur-
rent to be shunted around the 9.9M Ω leg of the input divider. The current then flows through the
100k Ω leg through Q226 and U244 pin 11 to pin 10 to common. The voltage at BUFFIN is ap-
plied to the input buffer to BSCOM and to the A/D MUX, which is configured for X1 gain.
Drawing reference
Analog Board; 2002-100
Bit pattern* Register
—U400—
01111011
—U811—
00001101
—U224—
00010111
—U432—
10000000
—U810—
00000011
—U206—
01100000
—U203—
11010110
—U411—
11111011
—U809—
11100111
—U207—
00001111
—U221—
11100111
—U406—
00000100
AD_STB
MUX_STB
R1_STB
R2_STB
*Bits associated with register IC terminals as follows:
QQQQQQQQ
87654321 87654321 87654321 87654321
IC pins: Q8=11, Q7=12, Q6=13, Q5=14, Q4=7, Q3=6, Q2=5, Q1=4.

Keithley 2002 Specifications

General

Brand	Keithley
Model	2002
Category	Multimeter
Language	English