7.1.2 Finding the Faulty Component
WARNING
HAZARDOUS VOLTAGES MAY BE PRESENT INSIDE THE 2105
Experience has shown that apparent malfunctions are often the result of misinterpretation of
the specifications or operating procedures of the unit. Check to be sure that the cables and
other test equipment are in good order before attempting to repair the 2105.
Knowledge of circuit operation is a prerequisite for efficient fault finding in the 2105. This
section is divided into two parts. The functional description gives basic descriptions as to the
functions of various circuits in the Power Analyzer. The detailed descriptions explain circuit
operation down to component level.
7.2 Functional Descriptions
A block diagram of the Model 2105 is shown in Figure 7-1. Power for the load under test is
connected to one set of rear panel terminals and passes through the instrument to a second set
of terminals where the external load under test is connected. A 0.01
Ω
current shunt is installed
between one input terminal and one output terminal. The inputs to the voltage amplifier are
connected to both power line terminals. The inputs to the three-stage current amplifier are
connected across the current shunt.
The voltage amplifier gain is controlled by the voltage range relays so that it has a full-scale 5-
volt output at the selected range. The gains of the three stages of the current amplifier are 25,
10 and 10, respectively. The outputs of each current amplifier stage are individually selected by
the current range relays for the 0.2, 2 and 20 ampere ranges.
The signal applied to the true RMS converter is determined by the position of the amps/volts
selector relay. The 5-volt full-scale output of the voltage amplifier is the same for the four
ranges and the output of the RMS-DC converter will pass through the scaler when the voltage
amplifier output is selected. The scale factor is controlled by the voltage range relays, which
selects the correct voltage to be applied to the RMS-DC converter. If a current amplifier output
is selected, scaling is not required.
The DVM is a true dual-slope, integrating digital voltmeter. The full-scale voltage applied to its
input is 0.2 volts in the current ranges and 0.3, 1.5, 0.3 and 0.6 volts in the four voltage ranges
of the Model 2105. The voltage measured by the DVM is latched to its internal registers at the
end of the measurement cycle. During the time of the next input measurement, each digit of
the previously measured voltage is sequentially applied to the inputs of the decoder/driver.
While the decoded data is present at the output of the decoder/driver, the display digit selector
energizes the appropriate display digit. Thus, the display is multiplexed from a single BCD
output of the DVM and at such a rate that it appears to be continuously illuminated.
29
