Guard Crossing Troubleshooting; Internal Cleaning; Cleaning Printed Circuit Assemblies - Fluke 8842A Instruction Manual

8842A
Instruction Manual
It is safe to force one control line at a time high (+5V) or low (0V) to test the individual
switches in U808. (The on resistance of switches in U808 should be less than 500Ω; the
off resistance should be greater than 10 MΩ.) Forcing the control lines high or low
should cause the reading to change when the voltages in Table 6-27 are applied to the
input terminals at 100 kHz. If only certain ranges cannot be calibrated, refer to Table 6-
26 to find the suspected stage.

6-75. Guard Crossing Troubleshooting

To troubleshoot the Guard Crossing circuit, place the 8842A in the In-Guard
Troubleshooting Mode as described under Digital Controller Troubleshooting, earlier in
this section. This causes the In-Guard µC to send a test pattern to the IEEE-488 Interface
PCA via one-half of the Guard Crossing circuit. You should be able to observe the
waveforms shown in the left half of Figure 6-21.
To troubleshoot the second half of the Guard Crossing circuit, leave the 8842A in the In-
Guard Troubleshooting Mode. This causes the IEEE-488 Interface PCA to send a test
pattern to the In-Guard µC. You should be able to observe the waveforms shown in the
right half of Figure 6-21. The IEEE-488 Interface PCA sends the test pattern in response
to the test pattern sent by the In-Guard µC; therefore, the first half of the Guard Crossing
Circuit (which was tested in the previous paragraph) must be working properly before the
second half can be tested.

6-76. INTERNAL CLEANING

Failures due to electrostatic discharge can be caused by
improper handling of the PCAs and by the use of a vacuum
cleaner with static-inducing brushes. To prevent damage by
electrostatic discharge, observe the precautions described on
the Static Awareness sheet in front of this section.
If visual inspection of the instrument shows excessive dirt build-up in the instrument,
clean the appropriate section using clean, oil-free, low-pressure air (less than 20 psi). If
necessary, remove the option PCAs first.

6-77. Cleaning Printed Circuit Assemblies

If conditions warrant, individual printed circuit assemblies (PCAs) can be cleaned with
water-based commercial cleaning systems such as dishwashers. If such systems are used,
observe the following precautions:
1. Remove all shield covers (applies to the True RMS AC PCA) and socketed ICs.
2. Use Reagent Grade 2 or better water (de-ionized or distilled water) for the final rinse
3. Thoroughly dry all PCAs using one of the following methods:
A satisfactory cleaning method consists of holding the PCAs under hot running water
until they are clean. Follow this wash with a final rinse. (See consideration 2, above.)
6-70
in geographic areas with exceptionally hard water. During the final rinse, spray or
run the water so that the surface is thoroughly covered to remove all ionized material.
a. Preferably, the PCA should be dried in a low-temperature drying chamber or
infrared drying rack with a temperature range of 49°C to 72°C (120°F to 160°F).
b. If neither a drying chamber nor a drying rack is available, air dry the PCA at
ambient room temperature for at least two days.
CAUTION