Fluke Biomedical 945A - Manual (Page 74)

9. Select type “A” and the following test menu should appear on the screen:
SUBTEST: A
997 PREAMPLIFIER TESTS
0 - VICO LOOP TEST
1 - BAUD RATE TEST
2 - ANALOG OUTPUT
3 - PROG. GAIN AMP OFFSET ADJ. (Z5)
4 - AUTO ZERO ADJUSTMENT
5 - ECS TEST
6 - HV TEST
R - RESET
SUBTEST?
10. Type “3” to select the “PROG, GAIN OP AMP OFFSET ADJ”.
11.
Disregard the text that appears on the display, it’s for a previous version of 977 that was removed
from service. Step 6 accomplishes the message displayed on the screen. Allow the detector to
stabilize for two minutes before continuing.
12. Adjust the R11 potentiometer on the 977-200-15 board until the DVM reads 0 Volts ±1 mV.
Clockwise adjustment increases the voltage, while counter-clockwise adjustment decreases the
voltage.
13. Turn
OFF the power to the 946 UDR and move jumper JP1, on the 977-200-15 Board, back to the
A - B position.
14. Turn
ON the power to the 946 UDR and repeat steps 8 and 9 to return to the TEST menu.
15. Set your DVM range to 2 volts and type “4” to select “AUTO ZERO ADJUSTMENT”. Wait two
minutes to allow the detector to stabilize before continuing.
16.
The DVM should display a voltage close to -250 mV. Adjust potentiometer R12 on the 977-200-15
board until the DVM reads -250 mV ± 3 mV. Clockwise adjustment increases the voltage, while
counter-clockwise adjustment decreases the voltage. If the voltage cannot be adjusted into spec.,
one of two things may be the cause. If “4” was entered more than once, the detector’s analog to
digital converter will increment. To check this, turn the 946 UDR OFF and then ON to reset the
ADC. Repeat steps 8, 9, 15, & 16. If the auto zero value still cannot be adjusted to spec., the
problem lies in the detector’s electrometer circuitry.
17.
After the adjustments have been made, turn the 946 UDR power OFF and return the 977-210-10
JMP1 jumper block to the A - B position to enable the VICO communications loop.
18.
Reset the 977-210-10 DIP switch settings to their original positions. Refer to the switch positions
listed below if required.
Switch 1: (ON / CLOSED)
Switch 2: (OFF / OPEN)
Switch 3: (OFF / OPEN)
Switch 4: (OFF / OPEN)
7-4