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78
Service Manual
3-26. Troubleshooting
The procedures provided in these paragraphs will help isolate problems with the meter.
When troubleshooting the Fluke 78 Automotive Meter, follow the precautions listed on
the "Static Awareness" sheet to prevent damage from static discharge.
3-27. Surface Mount Assemblies
The Fluke 78 Automotive Meter incorporates surface-mount technology (SMT) on the
printed circuit assembly (pca). Surface-mount components are much smaller than their
predecessors, with leads soldered directly to the surface of a circuit board; no plated
through-holes are used. Unique servicing, troubleshooting, and repair techniques are
required to support this technology. The information offered in the following paragraphs
serves only as an introduction to SMT. We do not recommend that you attempt a repair
based only on the information presented here.
Since sockets are seldom used with SMT, "shotgun" troubleshooting cannot be used;
isolate a fault to the component level before replacing a part. Surface-mount assemblies
are probed from the component side. The probes should contact only the pads in front of
the component leads. With the close spacing involved, ordinary test probes can easily
short two adjacent pins on an SMT IC.
Due to the limited space on the surface of the circuit board, component locations are not
labeled. Therefore, this service manual is a vital source for component locations and
values. Figures provided in Chapter 5 of this manual provide component location
information. Also, remember that chip components are not individually labeled; keep
any new or removed component in a labeled package.
Surface-mount components are removed and replaced by reflowing all the solder
connections at the same time. Special considerations are required.
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3-28. Power Supply Related Troubleshooting
The two regulator circuits are interrelated; a malfunction in either the common regulator
or the Vdd regulator may cause a problem in the other. Refer to Tables 3-11 and 3-12 for
descriptions of power supply components and voltage levels. To isolate the problem
regulator circuit, disconnect the battery, and drive Vdd - Vss = 6.3 V with a power
supply. This procedure tests the common regulator independently of the Vdd regulator.
Now check for DGND - Vss = -3.2 V +/-0.2 V. If this test is successful, the problem lies
with the Vdd regulator; refer to Vdd Regulator Troubleshooting later in this chapter. If
this test is not successful, the problem lies with the common regulator; continue with the
Common Regulator Troubleshooting. Note that if the common regulator works or has
been repaired, check both supplies with the 9 V battery supply.
3-18
The solder tool uses regulated hot air to melt the solder; there is no direct contact
between the tool and the component.
Surface-mount assemblies require rework with wire solder rather than with solder
paste. A 0.025-inch diameter wire solder composed of 63% tin and 37% lead is
recommended. A 60/40 solder is also acceptable.
A good connection with SMT requires only enough solder to make a positive
metallic contact. Too much solder causes bridging, while too little solder can cause
weak or open solder joints. With SMT, the anchoring effect of the through-holes is
missing; solder provides the only means of mechanical fastening. Therefore, the pca
must be especially clean to ensure a strong connection. An oxidized pca pad causes
the solder to wick up the component lead, leaving little solder on the pad itself.
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Table of Contents
Troubleshooting
