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NOIJ,dIUCSUC TVIISNSC
Input Impedance:
Accuracy:
Frequency Response:
DC Current
Range:
AC Current
Range:
Ohmmeter:
Meter:
Power Supply:
A-C Power Requirements
:
Size:
Weight:
1 megohm
*\Vo of.full scale (with sinusoidal input voltage)
25 Hz to 2 MHz (extendable to 250 MHz with EICO RF Probe PRF-1l)
0to 1, 3, 10, 30, 100,
and
300
ma, and
1amp
RMS - same as d-c current
P-P - 0 to 2.8, 8.5, 28, 85, 280, and850ma, and
2.8 amps.
0. 2 to 1000 megohms in seven ranges
6", 200
1t"
a movement
Transformer isolated, silicon- rectifier , zener regulated; three
NEDA No. 16049-volt batteries for portable use. One 1.5-volt
D-size cell for ohmmeter use.
105-132
volts
, 50f60 Hz, 1.
4 VA
7-Lf2" high, 8-Lf2" wide, 5" deep
6lbs,3oz,
CIRCUIT DESCRIPTION
Basic Metering Circuit
The Model 242 citcuirt.,ahown
in simplified form in figure 1, consists of an FET source follower
stageQl, driving a differential amplifier formed by lilPN silicon transistors Q2andQ3. Regardless
of the parameter being measured(a-c or d-c voltage or current, or resistance), the meter reading
is a function of the d-c input appliedto FET Ql. The high input impedanceof the FET prevents it
from loading tlre inputs at the probe, as srell as the switching and attenuating circuits at the front
end of tlle TVOM.
Figure 1. Basic Metering Circuit
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OPERATION
Preliminary Adjustment
Before applyinga-c or battery power to the_[dodel
242, place t]e instrument on a level surface and
checktlrat the meter pointer rests on tJIeleft-hand zero scale marks. If not, using a fine screw-
driver, vERY CAREFULLY adjust the meter zero screw (locateddirectly below the meter
movementon the front panel) until ttle pointer doesrest onthe zero mark.
NOTE
If possible, it is preferable that the meter Iugs be
shorted to one anotherfor this mechanical adjust-
ment. Be sure to remove the short before proceeding.
A-C Power Application
a. WitI tie slide switch at the rear of t]te cabinet set to LINE andthe Multi-Probe andground
leads inserted in their respective receptacles on the front panel, pIW tlre Iine cord into a 105-132
volt a-c 50-to-60 Hz outlet. Notethat for safety sal(ethe Model 242is equippedwith a 3-wire plug.
If youdo not have a 3-wire receptacle, use a 3-to-2 pin adapter, but be sure to connectthe ground
lead onthe adapterto a nearby ground.
CAUTION
Do NOT plug the Model 242into a source of higher
voltage or dilferent frequency (or into a d-c outlet)
'
for the instrument will be damaged.
b. Rotatethe MODE switch out of the OFF position. The front panel neonlamp will l8ht to
indicate that power is applied.
Battery Operation
a. When
using the Model 242as a portable instrument, set t}le slide switch at the rear of tJre
cabinetto BATTERY andmovetlle MODE switch out of t]le OFF position.
NOTE
The front panel neon lamp does not normally light
during battery operation. It will light ottly if the
Iine cord remains plugged into an a-c outlet.
D-C Voltage Measurements
a. Rotatethe endof the Multi-Probe until the detent locks in the DCposition. (TheMulti-
Probe endcanbe rotated either way.)
b. Setthe FUNCTIONswitch to VOLTS andthe MODE switch to either -DC or +DCdepending
on the polarity of the voltage to be measured
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c. Set the RANGE switch so that full scale is higher than the cunent to be measured. If in
doubt, always start at the HIGHEST
range andwork downward. Note that all d-c current readings
are madeat the two qlpermost black scales (0to 30 and0 to 100). Use the 0 to 30 scale for
RANGEswitch settings of 3MA, 30MA, and300MA. Use the 0 to 100scale for RANGEsu'itch
settings of lMA, 10MA, 100MA, and 1A.
d. Short the ground lead to the Multi-Probe tip andadjust the ZEROADJ control until the
meter indicates zero.
e. Connectthe commontest lead andthe test probe IN SERIES
witlr tlle circuit being tested.
The test probe shouldbe connected
to the POSITIVEendof the circuit andtlre common
test lead to
the NEGATryE end. Use a RANGEswitch setting that results in at Ieast l/4-scale pointer
deflection,
A-C Current Measurements
A-c current is measuredsimilar to d-c current. Sett}le Multi-Probe to AC/OHMS/MA andthe
FITNCTIONswitch to MA. In this case, however, position the MODE switch to AC. Use t}|e TWO
IOWEST SCALES
for all a-c current measurements: the 0-1 scde for RANGEswitch settings of
lMA, 10MA, 100MA, and1A; the 0-3 scale for RANGEswitch settings of 3MA, 30MA, and300MA.
In this case, Ieadpolarity is not importent.
Ohmmeter Measurements
a. Rotatethe endof the Multi-Probe until thb detentlocks in the AC/OHMS/MA position.
b. Place tne FUNCTIONswitch in the OHMS
position andthe MODE s$'itch to either -DC or
+DC. The meter shouldindicate up-scale.
c. Set t}te RANGE switch to RX100 and short the ground lead to tlle probe tip. Adjust tle
ZEROADJ control until the meter indicates zero. f)o not change
the zero adjustmentfor any '
other RANGEs$'itch setting.
d. Removethe ground-leadfrom the probe tip, tlen adjust the OHMS
ADI control until the
meter indicates exactly on the extreme right handmark ( @ ) on t}le top scale.
e. Remove
power from the equipmentunder test to avoid damagingthe Model 242.
f. Connectthe groundlead and
probe tip across t]le resistance to be measured. Settlre
RANGEswitch for the most appropriate deflection. Multiply the reading on t}le OHMSscale by the
factor indicatedby the RANGE switch setting.
g. Whennot in use, place the FUNCTIONsurltchanrayfrom OHMSto avoid draining the
internal D cell unnecessarilv.
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positive voltage indicated (meter deflection to the right) is obtainedby subtracting half the RANGE
selector valuefrom tlre d-c voltagereading on the appropriate scale. The va.lueof t}le negative
voltage (meter deflection to tl|e left) is obtaiiredby subtracting the d-c voltage reading on the
appropriate scale from half the RANGEsetting. (Thepolarities are reversed if the -DC function
position is used.
)
CAUTION: Althoughthe Model 242is protected against meter burn-out under normal operating
conditions, repeatedoserloads may impair the accuracy of the meter movement
that results from slamming the needleagainst the limit stops. For this reason, on
either ac or dc, ahpays
start with a range setting GREATERt}lanthe voltageyou
er$ect to measure. Always pay attentionto polarities \rthenmeasuring d-c
voltage.
DB Measurements
To avoidovercrowding of the meter scales, there is no dB scale ontlle meter. Another reason
for the absence
of a dB scale is that there are manydifferent reference levels in use, andeach
reference level results in a different scale. Figure I is a graphfor oneacceptedlevel--nemely
0.775 volt across 600ohms resistive Ioad (1 milliwatt). Using t]lis graph will allow youto
convenientlyconvert rms a-c voltagesto dB readings. Thesereadings are also referred to as
DBM.
Figure 9
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NOTE
For maximum accuracy, select a voltage that produces
almost full-scale deflection. It is preferable to calibrate
on the 30V or 100V ranges, although any range may be used.
D-C Calibration
someknownlevel of dc, suchas a single fresh dry cell (1.55 volts) or an accurate metered
adjustabled-c power supply suchas the EICOModel 1025must be used' Proceed as follows:
a. set t}le Multi-Probe to DC.
b. Setthe FUNCTIONswitch to VOLTS andthe MODE switch to -DC'
c. Setthe RANGEswitch to the 3V position'
d. Connect
the Multi-Probe tip to the negativeterminal of ttle battery or power supplyandthe
ground leadto the positive terminai. Adjust R24(accessiblethroryh t}reUPPER hole on the left
lide of the cabineti until the meter reads the correct value, This one adiustment suffices for a.ll
ranges andfor both -DC and+DCmodes.
Ohmmeter Function.
No separate calibration is required for the OHMS
function range.
To checkfor ohmmeter leakage(if any), proceed as follows. This also checksthe conditionof the
ohmmeter battery.
1. Place the FUNCTIONswitch to OHMS.
2. place the RANGEswitch in the R x I position. The meter pointer shoulddeflect to the
right, Operatethe OHMS
ADJ control for an e:ract
full-scale indication. If your meter doesnot
deilect full-scale, replace the 1-1/2-volt ohmmeter battery.
3. Rotatethe Multi-probe tip to ttle Ac/oHMs/MA position. short the tip to the groundlead.
The meter shoulddrop to zero. Adjust the ZEROADI until it does. It may be necessaryto repeat
this procedure until tlie meter pointlr rests at bothfull scale andzero as the probe tip is grounded
andringrounded. Onbattery oferation, the meter may not be capable
of being zeroed- Thia
indicates defective g-volt batteries.
4, Isolate the probe tip andset the RANGEswitch to R x 1. Thepointer shouldindicate
enactly full scale.
S. Without touchingthe OHMS
ADt control, place th€ range switch in theI v lM position. If
the meter pointer dropJaway from the full-scale position, there is excessiveleakagein the
ohmmeter circuit andit shouldbe thoroughly checked.
r8
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