Chapter Five: Maintenance and Troubleshooting Troubleshootin
75
Internal +5 V Voltage Reference
In the event that the voltage output from the internal +5 V source is not correct, measure the
input to the source.
1. Connect the high side of the voltmeter to the junction of R67 and VR1, located on the
rear of the Main PC board.
2. Reference the meter to the TP1 ground, located in the center of the Main PC board.
The proper input voltage is +1.23 to +1.25 Volts. With this input, the potentiometer
R66, located on the rear of the Main PC board, can be adjusted to produce a +5 V output
(actually, factory set to +5.1 V).
An incorrect input voltage may be caused by a defective reference VR1. An incorrect output
may be caused by a defective amplifier U9A, also located on the rear of the Main PC board.
Contact MKS Instruments for assistance.
External Ratio Amplifier
Failure of the 247 unit to work properly with an external voltage being used as a ratio signal may
be caused by a defective amplifier or by an incorrect setting on dipswitch S15.
The external ratio amplifier accepts signals corresponding to a full scale voltage of +5 V or
+1 V, controlled by the settings in dipswitch S15 on the Main PC board. For a +5 V full scale
input, ensure that dipswitch S15 is configured as listed in Table 14, page 64. For a +1 V full
scale input, ensure that dipswitch S15 is configured as listed in Table 15, page 65. Refer to
How
To Change the Dipswitch Settings
, page 64, for more information.
The input voltage required to produce the +5 V or +1 V output is applied to the 247 unit through
pin 7 on Interface connector P5. Both the input voltage and the meter measuring the output
should be referenced to pin 1 on Interface connector P5. Refer to Table 8, page 33, for the
complete pinout of Interface connector P5.
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