Fluke Portable Multimedia Player 8842a User Manual

Instruction Manual

6-70

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 500

Ω

; the

off resistance should be greater than 10 M

Ω

.) 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.

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 

µ

C 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 

µ

C. 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 

µ

C; 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.

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.

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.)