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INTRODUCTION
Assembly of your M-2666 Digital Multimeter Kit will
prove to be an exciting project and give much
satisfaction and personal achievement. If you have
experience in soldering and wiring technique, you
should have no problems. For the beginner, care
must be given to identifying the proper components
and in good soldering habits. Above all, take your
time and follow the easy step-by-step instructions.
Remember, "An ounce of prevention is worth a
pound of cure".
THEORY OF OPERATION
A block diagram of the M-2666K is shown in Figure 1.
Operation centers around a custom LSI chip. This IC
contains a dual slope A/D converter, display, latches,
decoder and the display driver. A block diagram of
the IC functions is shown in Figure 6. The input
voltage, current or ohm signals are conditioned by the
function and selector switches to produce and output
DC voltage between 0 and +199mV. If the input
The input of the 7106 IC is fed to an A/D (analog to
digital) converter. Here the DC voltage amplitude is
changed into a digital format. The resulting signals
are processed in the decoders to light the appropriate
LCD segment.
Timing for the overall operation of the A/D converter
is derived from an external oscillator whose
frequency is selected to be 40kHz. In the IC, this
Voltage
V
Divider
Ohms
V/
Converter
Range
COMM
Switches
Converter
mA
mA
Current
Shunts
Figure 1 Simplified Block Diagram
The meter kit has been divided into a number of
sections to make the assembly easy and avoid
major problems with the meter operation.
Section A - Meter display circuit assembly.
Section B - DC voltage and current circuit
Section C - AC voltage and current circuit
Section D - Resistance & buzzer circuit assembly.
Section E - Capacitance and transistor testing
Section F - Final assembly.
signal is 100VDC, it is reduced to 100mV DC by
selecting a 1000:1 divider.
100VAC, then after the divider it is processed by the
AC converter to produce 100mVDC. If current is to be
read, it is converted to a DC voltage via internal shunt
resistors. For resistance measurements, an internal
voltage source supplies the necessary 0-199mV
voltage to be fed to the IC input.
V
Function
Switches
VAC
AC
VAC/mA AC
mA
frequency is divided by four before it clocks the
decade counters. It is further divided to form the
three convert-cycle phases.
clocked at about three readings per second.
Digitized measurements data is presented to the
display as four decoded digits (seven segments) plus
polarity.
determined by the selector switch setting.
-1-
assembly.
assembly.
circuit assembly.
Should the input be
DC
A/D
Analog
Converter
Data
and
Display
Driver
Decimal
Display
Point
The final readout is
Decimal point position on the display is
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