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The Fluke 26-III and 79-III models are identical except for the model number. This model adds some extra features not available on the 77-III (and its identical twin the 23-III), like frequency and capacitance measurement, 4000 counts vs. 3200, RMS response, and a faster update speed. It also has a special 40-ohm resistance range that provides 10 milli-ohm resolution, aimed at electricians who may want to measure and compare contact resistances on relays and switches. It’s not a bad tool for the electronics workbench either, provided you do not need precise measurements of current in the micro-amp range. It does however have 1μA resolution in its lowest current range (4mA).
This unit arrives in a completely non-functional state. It doesn’t look too bad on the outside, but attempting to power up with a fresh battery results in absolutely nothing on the display. Disassembly and a quick visual inspection, with particular attention to the battery connections, reveals no apparent damage to either the components or either side of the printed circuit board (PCB). Both fuses are blown, but that’s about par for the course with a used multimeter.
One thing of particular note about the construction of this model, is that the microprocessor at the top is completely covered with a grounded metal shield. It obscures the pins of the quad flat-pack (QFP) chip.
Diagnosis begins by checking the current draw of the meter when powering from a DC power supply set to 8.5V. It measures 120μA, which is way lower than expected if this meter were actually running. When turned on, there appears to be a regulated 2.98V from the COM jack to the positive battery terminal. This means there’s at least one voltage regulator working in the power supply section (wherever that is, I’m working with no schematic here.) But there must be no load on the supply, as if something is disconnected.
Closer inspection reveals a possible problem up near the microprocessor and under the shield. The shield has to be removed for a better look. It appears we have a small area of major crud. This is isolated and a long way from the battery, so it would be hard to blame it on battery leakage. In any case it appears something corrosive has come into contact with the PCB in this area.
Time for scrubbing with a stiff brush and isopropyl alcohol (IPA). After the crud is removed, it becomes obvious that a copper pad near the microprocessor has eroded, leaving a trace broken and disconnected. The pins of the microprocessor appear to be OK, despite a little discoloration. There is a large 4-pin SMD component (clock crystal) in the area that will make repairs difficult, so it gets removed. More cleaning and examination shows that there is also another area of erosion on that same trace.
Repairs to the broken trace are made with 30ga wire-wrap wire, stripped of its kynar insulation and tinned. The 4-pin SMD crystal is re-soldered. Checking the supply current now shows a more healthy 1.05mA is being drawn. This is very promising.
The shield is re-soldered on, and the LCD re-installed. The meter boots up and displays all zeros on VDC! It measures resistance correctly, but checking with a 5.000V reference shows the calibration is off. There is a pretty good chance that I may have disturbed the calibration pot with my brush while I was doing the initial cleaning. Recommended calibration voltage is 3.5V, so a quick adjustment at that level restores calibration. After cleaning the plastic parts and giving the scratchy lens a polish, this meter is ready for service again.
DMMCheck calibration check results for this Fluke 26-III:
Reference
Reading
Notes
DC
5V
5.00
1mA
1.000
AC
5V
5.03
1mA
1.003
Ohms
100Ω
100.1
0.2 with leads shorted
1K
1.000
10K
10.00
100K
99.8
Page 29: Cat ii, Min max range hold h hz rel, 4 1/2 digits 1 seconds, True rms multimeter, Figure 6. testing a diode, Typical reading, Forward bias reverse bias
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Making Measurements
21
Testing Diodes
Caution
To avoid possible damage to the meter or to the equipment under test, disconnect circuit power and discharge all high-voltage capacitors before testing diodes.
Use the diode test to check diodes, transistors, silicon controlled rectifiers (SCRs), and other semiconductor devices. This function tests a semiconductor junction by sending a current through the junction, then measuring the junction’s voltage drop. A good silicon junction drops between 0.5 V and 0.8 V.
To test a diode out of a circuit, set up the meter as shown in Figure 6. For forward-bias readings on any semiconductor component, place the red test lead on the component’s positive terminal and place the black lead on the component’s negative terminal.
In a circuit, a good diode should still produce a forward- bias reading of 0.5 V to 0.8 V; however, the reverse-bias reading can vary depending on the resistance of other pathways between the probe tips.
MIN MAX
RANGE
HOLD
H
Hz
REL
mA
A
mV
V
V
OFF
!
!
A
COM
V
mA
µ
A
1000V MAX
400mA MAX
FUSED
10A MAX
FUSED
PEAK MIN MAX
µ
A
CAT II
+
Typical Reading
MIN MAX
RANGE
HOLD
H
Hz
REL
mA
A
mV
V
V
OFF
!
!
A
COM
V
mA
µ
A
1000V MAX
400mA MAX
FUSED
10A MAX
FUSED
PEAK MIN MAX
µ
A
CAT II
+
Forward Bias
Reverse Bias
4 1/2 DIGITS
1 Seconds
87
Fluke 87 Iii Multimeter Manual
TRUE RMS MULTIMETER
What Is A True Rms Multimeter
III
4 1/2 DIGITS
1 Seconds
87
TRUE RMS MULTIMETER
III
iy9f.eps
Figure 6. Testing a Diode
Fluke 87 True Rms
Fluke 26 Iii True Rms Multimeter User Manual 95652