Open-Circuit Ohms Tests - Keithley 2002 Repair Manual

Troubleshooting

2.11.11 Open-circuit ohms tests

Test 304.1 —Open-circuit ohms
Type
Fault message
Description
Drawing reference
Components
Bit patterns
2-56
Pass/Fail
Open circuit ohms
This is the first test in the series that uses the ohms source. Most of the 300 and 500 series tests
that follow also use the ohms circuit.
The 7V reference REFHI signal generated by U219 and associated circuitry is connected to buff-
er U254 and U255, and the outputs of U254 and U255 are connected to U230. The 7V reference
value from U254 is switched at 600Hz through U230 to capacitor C234. REFBUF is connected
to the bottom of C22, and the 7V from C234 is switched to the top of C227 to create a 14V ref-
erence at OHMREFHI. The OHMREFHI to REFBUF lines are the isolated reference signals
used to generate the current for the ohms source. The ohms source is generated by OHMREFHI,
buffered by U231 and Q251 and is switched in with the /OHMA line at U220 and U223 to U233
pin 2. REFBUF is connected to U233 pin 3 through U232 pins 2 and 3. This circuit setup applies
7.2V across R270, 1kΩ resistor to generate 7.2 mA. In this test, there is no path provided for
current to flow outside of the ohms source. In normal operation, the current would flow out of
the ohms current source from the parallel path of Q211 and Q212, through Q213, R322,CR217,
K201, R334, R342, and out of input HI through the resistor being measured and back to com-
mon. Current flow in this setup with no resistor connected is downward through the parallel path
of Q211 and Q212, through CR213, VR204, and Q214 to common. The voltage drop across the
combination of VR204 and Q214 is sensed by the path that routes through Q213, R322, CR217,
K201, R334 and R342. This point is the HI input terminal to the DMM. This signal is then
switched through the Q246, Q245, Q244 path to BUFFIN and then to A/D_IN as in test 303.1.
Analog Board; 2002-100
U219, U220, U223, U233, U254, U255, 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—
01100000
—U203—
11110100
—U411—
11111011
Q Q Q Q
87654321 87654321
Register
—U809— AD_STB
11100111
—U207— MUX_STB
00001111
—U221— R1_STB
01110111
—U406— R2_STB
00000100