Ohms Tests - Keithley 2002 Repair Manual

2.11.13 Ohms tests

Test 306.1 — 9.6µA ohms source
Type
Fault message
Description
Drawing reference
Components
Bit patterns
Test 306.2 — 1.92µA ohms source
Type
Fault message
Description
Drawing reference
Pass/Fail
9.6uA ohms source
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.
Analog Board; 2002-100
U220, U223, U226, U232, and associated components.
—U811—
00001101
—U224—
00010111
—U400— —U432—
01111011 10000000
*Bits associated with register IC terminals as follows:
Q Q Q Q
87654321 87654321
IC pins: Q8=11, Q7=12, Q6=13, Q5=14, Q4=7, Q3=6, Q2=5, Q1=4.
Pass/Fail
1.92uA ohms source
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.
Analog Board; 2002-100
Bit pattern*
—U810—
00000011
—U206—
01100000
—U203—
11010110
—U411—
11111011
Q Q Q Q
87654321 87654321
Troubleshooting
Register
—U809— AD_STB
11100111
—U207— MUX_STB
00001111
—U221— R1_STB
11100111
—U406— R2_STB
00000100
2-61