Fluke 2680A, Руководство по обслуживанию

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268XA
Service Manual
6-44
Troubleshooting Relay Problems
Both the PAI and FAI use mechanical reed relays for signal switching, although the FAI
uses solid-state relays for channel selection. The mechanical relays have a life of
100,000,000 operations. If you use your device in long-duration, high-speed
measurement runs at high voltages and high common mode voltages, then the relays may
start to act up after a few years. This includes failure to open, failure to close, excessive
contact resistance, and so forth. If your device is not subject to these extremes, then relay
failures become less likely. An example of a relay failure occurs when the reading on one
channel affects the readings on another channel (beyond the normal effects of cross-talk
for ac measurements). This is especially true for dc volts, resistance, or thermocouple
measurements. If a group of channels is causing problems, particular for channels 1 to 10,
and 11 to 20, then it may be a bus problem (bank1 and bank2). If the banks are
interacting, the treeing relays A3K21 to A3K24 may be at fault. You may find it easier to
measure relay conditions by removing the A3 A/D Converter PCA and measuring at the
bottom of the PCA. However, removing the PCA could unstick a stuck relay and
complicate relay troubleshooting. Also, for relay troubleshooting, ap1 V dc to the
even channels and –1 V dc to the odd channels can assist you in signal tracing. Crossed
relays, for example, might cause a 0 V reading where you ex1 V.
A4 Analog Input PCA Troubleshooting
The A4 Analog Input PCA is essentially a passive assembly of terminal blocks, with the
exception of a small active network formed around A4Q1 that provides a temperature
reference for thermocouple measurements. If thermocouple measurements are out of
tolerance, A4Q1 may be suspect.
To verify the operation of A4Q1, connect multimeter test leads across A4R2 (marked and
clearly visible in the open module) and power the device. At normal room temperatures,
the voltage across A4R2 is a nominal 1 V dc. If the measured voltage is 0 V, A4Q1 is
probably open and should be replaced. For this test, you may find it easier if you remove
the cover portion of the Universal Input Module by gently lifting one of the tabs the form
the cover hinge and removing the cover.
XW
Warning
To avoid electric shock, disconnect all channel inputs from the
device before performing any troubleshooting operations.
The A4Q1 circuit is calibrated at the factory by adjusting the potentiometer A4R3. Do
not disturb this adjustment unless you have replaced A4Q1. If you have replaced A4Q1,
allow the device to stabilize in an ambient temperature of exactly 22*C and then adjust
A4R3 for a reading of 1.00 V dc across A4R2.
Troubleshooting Calibration Failures
The paragraphs in this section describe troubleshooting actions when there is a calibration
failure. Calibration of the device through the computer interface is described in Chapter 5
of this manual. Generally, a calibration failure is indicated by a Device Dependent Error
and a "!>" prompt after a CAL_STEP? command. These indications occur if the analog
input varies from what the device expects to see by more than +/-5% or +/-15%,
depending on the calibration step.
Before suspecting a fault with the device, verify that the calibration is being conducted
properly.
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Check the connections between the source and the device. Are all the connections in
place?