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
QQ
87654321
—U106—
110v1111
QQ
87654321
—U109—
00101111
QQ
87654321
—U134—
0v10011v
—U130—
10111101
QQ
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
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