Rosemount 755, Instruction Manual

Page: 77 / 94

S
ERVICE AND
M
AINTENANCE
748183-H
7-3
Rosemount Analytical
December 1997
V
OLTAGE
T
EST
M
EASUREMENTS
B TO A
C OR D TO
GROUND
DIAGNOSIS CORRECTIVE ACTION
- + Normal NA
+ + U1 or U2 defective Replace case board
- - U1 or U2 defective Replace case board
+ - Detector defective Check detector per Section 7.3
If polarities are correct, set front-panel SPAN potentiometer R4 at maximum. The
output at pin 6 of U3 should be 7.5 VDC. Pins 2 and 3 of U4 should both be at 0.465
VDC, resulting in 5 VDC at the output of Q1 .
Checkout of the case circuit board is now complete.
F
IGURE
7-1. L
OCATIONS OF
C
ASE
B
OARD
T
EST
P
OINTS
A, B, C
AND
D
HOT
GN
N
E
U
T H
O
T
TB1
+ - +
MA MV
TB2
COM
P8
A
B
C
D
D C B A
D C B A
A
B
C
D
Alarm Option removed for clarity.
Case Board