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
CAUTION
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 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.
3.Thoroughly dry all PCAs using one of the following methods:
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.
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.)
