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Test 301.2 — HI OHMS

Bank DC/OHM

Inputs Open

Expected Value 12.4 volts

Limits 0.5 volts

Fault Message 13.3 REFBOOT

Description

This test is the same as 301.1 except that the +13.3V ohms reference is

tested. The +13.3V reference is generated by the same circuit as the +7V ref-

erence. 14V is routed through Q130 and then applied to a 1K/10K divider

which is part of R271. The +13.3V divider output is routed through analog

switch U133 (/.7V control line low) to op amp U123. The remainder of the

path is the same as test 300.1.

The expected voltage at AD_IN would be +13.3V except that at the source

lead of Q104 (labeled SOURCE) there is a clamping circuit. Back-to-back

11V zener diodes VR105 and VR106, and photo-coupler U107 clamp the

voltage at the SOURCE node to about +12.4V. Measure +12.4V at AD_IN.

Bit patterns

Bit pattern Register

87654321

—U106—

110v1111

87654321

—U109—

00101111

87654321

—U134—

0v01001v

—U130—

10111101

87654321

—U121—

10000100

ACDC_STB

MUX_STB

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

2-34 Troubleshooting

Keithley 2000 Repair Manual

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