Troubleshooting Relay Problems; A4 Analog Input Pca Troubleshooting - Fluke 2640A Service Manual

For dc volt problems that affect all channels, look for faults in the dc buffer and Stallion
device. For dc volt problems on individual channels or groups of channels, check the
channel select (2640A only) and treeing relays.
For resistance problems, check the dc volts characteristics first. If there are no problems,
then the difficulty is not in the dc buffer circuitry. This would suggest a problem in the
ohms conditioning circuit or the A3U30 Stallion device.
For ac volt problems, check the dc volts characteristics first. If there are no problems,
then the difficulty is probably in the ac-to-dc conversion circuitry.

Troubleshooting Relay Problems

Both the 2640A and 2645A use mechanical reed relays for signal switching, although the
2645A uses solid-state relays for channel selection. The mechanical relays have a life of
100,000,000 operations. If you use your instrument 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 instrument is not subject to these extremes, then
relay failures become less likely. An example of a relay failure is 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, applying
+1V dc to the even channels and -1V dc to the odd channels can assist you in signal
tracing. Crossed relays, for example, might cause a 0V reading where you expected +1V.

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 instrument. At normal room
temperatures, the voltage across A4R2 is a nominal 1V dc. If the measured voltage is
0V, 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.
To avoid electric shock, disconnect all channel inputs from the
instrument 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 instrument to stabilize in an ambient temperature of exactly 22*C and then
adjust A4R3 for a reading of 1.00V dc across A4R2.
Diagnostic Testing and Troubleshooting
WARNING
Troubleshooting the Instrument
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