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Test 303.1 — LO OHM PATH

Bank DC/OHM

Inputs Open

Expected Value 7 volts

Limits 0.7 volts

Fault Message 7V SOURCE

Description

This test uses the ohms circuit. The +7V reference is switched to REF-

BOOT by closing U133 (/7V line low). Q123 and Q125 are turned on by set-

ting the OHMA control line high. +14V is applied directly to R194. Since

Q123 is on, +7V appears on the other side of R194. As a result, the voltage

drop across R194 (7.06k

Ω

) is 7V. A current of 1mA therefore ﬂows through

R194, Q125, Q119, CR114, and Q120 (/LOWOHM control line low).

The current (labeled OHM) then ﬂows through R304, U107, VR106, and

VR105 to LO. The +7V reference is routed through Q104, to BUFCOM, and

on to the A/D MUX with a gain of

1. Measure +7V at AD_IN.

Bit patterns

Bit pattern Register

87654321

—U106—

110v1111

87654321

—U109—

00101111

87654321

—U134—

0v10011v

—U130—

10111101

87654321

—U121—

01100100

ACDC_STB

MUX_STB

IC pins: Q8=11, Q7=12, Q6=13, Q5=14, Q4=7, Q3=6, Q2=5, Q1=4

Troubleshooting 2-37

Keithley 2000 Repair Manual

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