Troubleshooting Cha
ter Five: Maintenance and Troubleshootin
70
Flow Amplifiers
Failure of the 246 unit to work properly with an external voltage being used as a ratio signal may
be caused by a defective amplifier.
To check the output of the SET POINT CONTROL:
1. Prepare a 15 pin “D” connector (which will mate with J8) by placing a jumper from pin 2
to pin 8.
This allows the 5 V set point voltage to be used to simulate an input. Refer to Table 8,
page 34, for the pinout for connector J8.
2. Connect a digital voltmeter (DVM) to this jumper and analog ground and adjust the front
panel SET POINT CONTROL until the DVM reads 5.000 Volts.
3. Set the SET POINT SOURCE SWITCH to FLOW and the FLOW CONTROL SWITCH
to ON.
The DVM should read approximately 5 Volts on pin 10 in connector J8, and on pin 6 in
connector J9.
To check the output of the ZERO CONTROL:
1. Verify that the ZERO CONTROL works by using it to set the voltage on pin 10 in
connector J8 to 5.000 Volts.
If you cannot achieve the voltage, the problem most likely is with the first stage
amplifier (U4A). Contact MKS Instruments for assistance.
2. Check the output of the second amplifier stage (U5).
The output should be twice the output voltage measured at pin 10 in connector J8, and at
pin 6 in connector J9. This can be found easily on the input pin of the gas correction pot
R11 (J7-3).
3. Turn pot R11 to “1” and verify that approximately 1 Volt appears at pin 2 in connector
J9 and in the microjack connector J10 (Flow Output).
If you cannot achieve the correct voltage, check R11 and U6 for problems.
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