Keithley 2002 Repair Manual page 79

Troubleshooting
Test 308.3 — 2M overload ohms
Type
Fault message
Description
Drawing reference
Components
Bit patterns
2-66
Pass/Fail
2M Overload OHMS
The 9.6µA ohms source is switched in with the /OHMD line at U223 and U220. U232 pin 2 to
3 connects REFBUF to U233 pin 3, and 7.2V appears across R277. (For a more detailed descrip-
tion of the circuit setup, see test 304.1.) In this test, there is no path provided for current to flow
outside of the ohms source. Current flow is downward through the parallel path of Q211 and
Q212, through CR213, VR204, and CR214 to common. U238 sees the open-circuit ohms volt-
age of about 6.6V at pin 3. It is used in a unity-gain configuration, and its output, which is the
COMPLIANCE signal, goes to comparator U215. U232 pin 15 to 14 is open, and the divider in
this case is made up of R235 and R234. This divider places pin 5 of U215 at 5.7V. Since the
COMPLIANCE line at 6.6V is more positive than the 5.7V on /OVLD_OHM, the line is pulled
low at about -15V. All of the OVLD lines are latched by U202, parallel loaded into U201, and
serially input to the A/D board at J1026 pin 15. There is no A/D measurement for this test. The
pass/fail criteria is based on the value of the /OVLD_OHM bit.
Analog Board; 2002-100
U220, U223, U232, U233, 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.
Bit pattern*
—U810—
00000011
—U206—
01110000
—U203—
10101010
—U411—
11111011
Q Q Q Q
87654321 87654321
Register
—U809— AD_STB
11100111
—U207— MUX_STB
11001111
—U221— R1_STB
11101011
—U406— R2_STB
00000100