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Sqfety Precouliong

When testing electronic equipment, there is always a danger present. Un-expected high voltages can be present at unusual locations in defective equip-ment. The technician should become familiar with the device thar he isworking on and observe the following precautions.

1. An isolation transformer should always be used on equipment having rhechassis tied to one side of the AC power line.

2. When making test lead connections to high voltage poinrs, remove rhepower, if this cannot be done, be sure to avoid contact with other equip-ment or metal objects. Place one hand in your pocket as a safety pre-caution and stand on an insulated floor to reduce the possibilityof shock.

3. Discharge filter capacitors before connecting test leads to rhem. Ca-pacitors can store a charge that could be dangerous to the technician.

4. Be sure your equipment is in good order. Broken or frayed tesr leadscan be extremely dangerous and can expose the technician to dangerouspotentials.

5. Remove the test leads immediately after the test has been completed toreduce the possibil i ty of shock

6. Do not work alone when working on hazardous circuits. Always haveanother person closeby in case of accident. Remember, even a minorshock can be the cause of a more serious accident, such as falling a-gainst the equipment, or coming in contact with higher voltages.

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DESCRIPTIONAsthe state of the artin electronics progresses, moretransistors, FETs,and integrated circuits are being used in radio, television, and industrialmarkets. Sencore engineers have kept up with todays pace and have de-veloped a newField Effect meter to meetthe moredemandingrequirementsof solid state and integrated circuit servicing.

The FE20 HI-LO meterfeatures newlow AC and DC ranges of . l volts fullscale and alow power ohms circuitallowing for the first time accurate re-sistance measurements in transistor and integrated circuits.

Just compare these features and see why the FE20 is the most advancedmeter on the market today:

* Highinpug impedance on all ACand DC ranges (15 megohmDC, 12 meg-ohms AC).

* True peak to peak AC ranges.* Zero center ranges on DC for solid state servicing.* Db scale for audio and lab work.* DC current ranges for general testing.

1 Lo* power (low apptied voltage) ohms ranges for solid state servicing.* Mirrored scale meter for greater accuracy in readings.* Meter protected from overloads.* Circuits protected from accidental application of wrong vohages.* All steel carrying case with cover for greater durabitity and

-portability.

* Long life "C" cell power.

SPECI FICATIONSDC VOLTS

Ranges: 0 to .1, .3, l , 3, 10, 30, 100, 300, and 1000 ful l scale- . 0 5 t o . 0 5 , - . 1 5 t o . 1 5 , - . 5 t o . 5 , - 1 . 5 t o 1 . 5 , _ 5 r o 5 ,-15 to 15, -50 to 50, -150 to 150, and -500 to SOOZeroCenter Scale Ranges.

Input Resistance: 15 megohms shunted by 90 pf in probe "NORM"position; or shunred by l0 pf in probe "120K ISOLATION"position.

Accuracy : +L .5 /oAC Rejection: 30 db Minimum

AC VOLTS

Ranges: (RMS)0 to .1, .3, L, 3, 10, 30, 100, 300, and 1000 Ful l scale

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OPERATIONSHIELDED TEST I-.,EAD AND PROBE ON THE FE2O

The FE20 HI-LO Meter is equipped with a shielded tesr lead to allow ac-curate measurements on AC in the presence of strong electrostatic andelectromagnetic fields. The special probe has a 120K isolation resisrorto isolate the cable capacitywhen making DC voltage measurements in crit-ical circuits such as RF, IF or sweep circuits. For all normal readingsincluding DC voltage, the probe switch is left in the normal position. ftreonly time that the switch is placed in the isolare position is when it is nec-essaryto isolate cable capacityfrom the circuitbeing measured. When theswitch is in the isolate position, the DC voltage reading on rhe FE20 wi1 beonly 0. 6/oJo*. T9 ilol"te position should onty be used wirh rhe DC voltageand AC voltage at 60 Hz functions as much greaier error will result on otherfunctions.

CONTROLS ON THE FE2O

The FE20 HI-LO Meter is as simple to operare as a srandard VOM. Thefront_panel controls include the RANGE swirch, FUNCTION switch, meterzERo ADJUST, OHMS ADJUST, oN-oFF swirch, and the battery restswitches. Thefollowing is a brief description of each control and how ir isUsed:

RANGE Switch: The RANGE Switch selects the desired range of voltage,current or resistance to be measured.

FUNCTION Switch: The FUNCTION Switch selects the type of measurement@ minus DC volts or current, AC vo1is, HI or LO powerohms.

ZERO ADJUST: The ZERO ADJUST is used to position the pointer of themeter over the zero at the extreme left of the meter for most measurements,or over ttre center zero for the zero center ranges.

OHMS ADJUST: The OHMS ADJUST is adjusted with the FE20 leads heldopen. The OHMS ADJUST control is adjusted so that the pointer of the mereris positioned directly over the infinity mark on the extreme right hand sideof the meter. The ohms zero is adjusted with the test leads shorted togetherand the Range Switch set to any position except RXI or RX10.

ON-OFF Switch: The ON-OFF Switch controls the power ro the meter cir-f f i p o s i t i o n , t h e b a t t e r i e s a r e d i s c o n n e c t e d f r o m t h e m e t e rcircuits. In the ONposition, batterypower is applied to the meter circuitryand the meter is ready t9 go. When the tid of the FE20 is shur, a speciaitab automatically shuts off the FE20 to be sure rhat the batteries do nof weardown needlessly.

BATTERY TEST Switches: The FE20 uses two battery resr swirches. Theft is the test for the main (power) battery, and

the switch to the right is the ohms battery test.

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Measuring DCV w:jtha zsro center scale: The zerocenter scales are usedto read plus or minus voltages without the necessity of changing polaritywith the leads or the Function switch. To use the zero center scales, setthe switch on ttre probe to the Normal position. Set the Function switch toplus DCV, andadjust theZERO ADJ until the meterreads at the zero markin the center of the scale. This gives ranges of plus or minus 0.5 volts(upper scale), or plus or minus 1. 5 volts (lower scale). The meter scalesare then multiplied by 0. l, 10, 100, or 1,000 depending on rhe setring ofthe Range switch. If the Range switch is on tfre 300 volt position, then the1.5 volt scale would be multiplied by 100 resulring in a range of plus orminus 150 volts. If the Range switch were on the 0. I volt range, the plusor minus . 5 volt range would be read as plus or minus .05 volts.

If the meter moves up scale to the right, the voltage is positive, and if thepointer moves to the left, the voltage is negative. With a calibrated zerocenter scale, you candetermine notonly the polarity of a voltage, but alsoits magnitude.

Measuring DC Current: Tomeasure DCcurrent, set theprobe switch to theNormalposition, the Function switch to the *DC mA position, and the Rangeswitch tothe desiredcurrent range. Remove power from the equipment tobe measured, and connect the FE20 in series with the current to be mea-sured. Connect the negative meter lead to the negative source of currentand the positive meter lead to the positive source of current. (If ttre currentto be measured is unknown, start with the 1A range and work toward the. I mA range). Reconnect the power to the equipment being measured, andread the currenton the appropriate meter scale. If the meter reads inthenegative direction, the currentpolarity is reversed from thatexpected andthe Function switch of the FE20 should be rurned ro rhe -DC mA posirion.

The zero center scales may also be used to measure DC current. With theFunction switch in the +DC mA position, and the Range switch set correctly,adjust the meter for zero center and apply current to be measured to FE20.

The100 uA to 300 mA currentranges are protected bythe frontpanel fuse.Onthe l amp range, theshuntresistor is a coilof resistance wire, andwillwithstand sustained heavyoverloads, however, it is possible todamage thisshunt resistor if the overload is not removed immediately.

Measuring AC Volts: To read AC Volts, sef the Function switch to the ACvolts position and the Range switch to the desired voltage range. The switchon the probe should be in the Normal position. To measure RMS volrage,the twoblack scales markedACV (rms) areused. Toineasure peakropeakvoltage, the rwo red scales marked ACV (P-P) are used. The scale readingsare multiplied by the same multiplier factors as were used in DCV measure-ments. On the 0 to 1 volt scale, each small division represents . 02 volts,and on the 0 to 3 volt scale, eachsmall scaledivision represents .l volts.Onthe 0 to2.5 vol t peakto peakscale, eachsmal lscale div is ionrepresents.1 volt, andon the 0 to 9 volt peak to peakscale, eachsmall scale divisionrepresents .2 volts.

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CAUTION: When making resistance measurements be sure that the powerlo-TFequlpment under test is disconnected. If voltage is applied to theohms ranges, the fuse Fl will have to be replaced.

MEASURING HIGH VOLTAGE WITH THE FE2O

CAUTION: High voltage isdangerous and caution should be observed whenmeasuring high voltages. Be sure that ttre ground lead from the FE20 isconnected to the appropriate ground in the equipment being measured. Ifrhe ground lead is not connected, you are merely extending the high voltagecloser ro you. If the ground lead is connected to the *tJng groind in t[e

Iequipment being measured, you run the riskof possible dam4ge to the equip-meni being tesled. Stand away from the equipment being measured and keep Ione hand in your pocket as.a safety measure. High voltage in color TV isespeciallydangerousbecause itcan go ashighas 30,000 volts andit is reg-ulated. If you should come into contact with it, it will not load down.

The 39A30 high voltage probe is used to extend the range of the FE20 to30,000 volts DC. This probe maybeusedwith any DCvoltage range on theFE20 to act as a times 100 multiplier and to increase the input impedanceof the FE20 to 1500 megohms. In other words, the 0. 1 volt range wouldbecome a 10 volt range with a high enough input impedance (1500 megohms)which would not affect the most critical circuit.

To use the 39430probe, remove the two partsof the probe from the bracketsin rhe cover of the FE20. Insert the end of the probe body with the bananaplug of rhe probe body seat in the banana jack inside the handle. Insert thenormal testprobe of the FE20 into the small openingof the 39430 probe, andplace the probe switch in the normal position.

To read the acrualvoltage, multiplythe readings onthe 3 voltscaleby 1000for the 3KV range, the I volt scale by 10, 000 for the 10KV range and the 3volt scale by 10,000 for the 30KV range.

To measure high voltage, set the Function switch to.either the plus or minusDCV position and the Range switch to one of the three high voltage positions,3KV, 10KV or 30KV. Connect the ground lead of the FE20 to the appropriateground point in the equipment being measured. Be sure to read the preced-ing paragraphs onthe caution tobe taken when measuringhigh voltage, _a!4on the op-eration of the 39A30 high voltage probe. The 3KV, 10KV and 30KVranges are to be used only with the 39A30 probe.

Measuring Decibels (db): The FE20 has a convenient db scale for easy dbmeasurements. The nine AC voltage ranges are in perfect 10 db steps, sothatstart ingfrom

"O" db onthe 1.0 vol t range, youaddl0 dbfor eachrangeabove 1 . 0 volts and subtract 10 db for each range below I . 0 volts. Db mea-surements are actually a measure of the ratio between the power in the cir-cuit under test, and some reference power level. The reference level usedfor the FE20 is a I milliwatt into 600 ohms. If the impedance being mea-sured across is other than 600 ohms, you will have to use the chart in FigureI to determine the actual power level or db. Example: The scale reading

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ToO..,, or an actual db measurementof 56 db.

The chart in Figure 2 can be used to determine the actual power being dis-sipared in a circuit if the circuit impedance along with either the appliedvolmge, or the db as measured on the FE20 meter is known. The chart inFigure 2 also shows the relationship of AC volts to db. If the AC voltageis increased 10 times, the db reading goes up by 20 db.

Nore: The db scales of the FE20 and the charts in Figure 1 and Figure 2only apply when the voltage being measured is a pure sine wave as from anaudio generator. The FE20 is a true reading peak to peak meter and thecomplex waveform of music material will not give a true RMS reading forthe db system to work on.

Measuring AC Current with the FE20: The FE20 can be used to measureAC current with the circuit shown below. The resistor or resistors areplaced in series with the AC source going to the device under test. TheFE20 is set to the AC Volts function and the AC volnge drop across theresistors is measuredandcanbe converteddirectlyinto AC amps. Itwouldbe most convenient to build this adaptor into a box with a switch to changeranges. Theresistors usedare all I ohm of at leastone watt. TheOhmite995 series of 1 L/2wattwirewound resistors is ideal for this. A standardone watt carbon resistor will also work, but if the unit is to be left on for anextended period of time, a two watt should be used.

Fig. 3 AC Current Adapter

When measuring from zero to one offip, the one ohm resistor is used, andthe FE20 Range switch is in the one voltposition. To measure three ampsof AC current, the switch is closed so that there are now three one ohmresistors in parallel. A currentof three amps will now produce a one voltdrop across the three resistors, so leave the FE20 on the one volt range,and read the 0 to 3.0 scale for three amps of AC current.

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CIRCUIT DESCRIPTIONGeneral

The Sencore FE20 Field Effect meter uses a balanced bridge impedanceconverter consisting of rwo Field Effectand twobi-polar ransistors. Thiscircuitry provides high inputimpedance andexcellent stability. Whenmea-suring AC volts an additionalamplifier stage which is compensated againstchanges in voltage, temperature and frequency is used todrive the peak topeak detector.

DC Voltage: The DC voltage at the inputpassesthroughthe ACrejectionfilter consisting of Rl and Cl to the voltage divider (R2 through R10). TheRange switch S3B selects the required input voltage to be applied to the gateof TRl. The FET TRI provides the necessary high input impedance, anddrives the base of TR2. The source output of TRI and the collector outputof TR2 are inparallel, with bothourput currents flowing through the commonload resistor R29. TR2 provides the necessary current gain required todrive the meter. The TR3 and TR4 section of the balanced impedance con-verter is held constant, because the gate of TR4 is tied to ground throughthe AC cal control R36. The meter, in series with the DC cal control, isconnected between the collector of TR2 and the collector of TR3, with thepositive meter lead connected to the collector of TR2.

This balanced impedance ccnverter is temperature stable because anychangein temperature will affect both sides of the balanced circuit equally, keepingthe circuit balanced. The circuitis compensatedagainstchanges in supplyvoltage, because the voltage at the source of TR4 remains nearly constantand is usedas a bias voltage for TRI gate. Itis apptied to the gate of TRIthroughresistor R40, the DC balance controlR39 and the zero adjust R50.

DC Current: When measuring DC current, one of the standard resistors,ffi-ffiugF R25, is selected by the Range switch to be put in series withthe circuit to be measured. The circuit described in the preceding para-graph (DC voltage) is then used to measure the voltage across the standardresistor. The fuse Fl protects R17 through R24, or up to the 300 mArange.The meter movement itself is protected on all ranges because it is isolatedthru the impedance converter and shunted with a diode.

Resistance (HI POWER): To measure the resistance of an unknownresistor,� n o w n r e s i s t a n c e , a 1 . 5 v o l t b a t t e r y , a n d a s t a n d a r dresistor in series. The FE20 then measures the voltage across the unknownresistance. The ohms adjustcontrol has the same effect as the DC cal con-trol, in that it is connectedin series withthe meterandis usedtoadjust thesensitivityof the meter. When there is an infinite unknownresistance (opencircuit) the FE20 will measure the full battery volrage. The ohms adjustcontrolis adjustedwith the test leads open, sothat thefull voltagewill readinfinity on the meter. When the FE20leads are shorted together, the voltagewill be zero, and the meter will read zdro. If an unknown resistance isexactly equal tothe standardresistance inthe FE20, thevoltage across the

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of the case at the same time. Take care to do this gently, as the batteryholder will remain inside the case. Fourth, lift the battery holder off thetwo posts that locate it in the bottom of the case.

For the location of the following adjustments, refer to the PC board layoutdiagram. Figure 5.

Calibration of the HI-LO Meter

DC Balance: The DC balance control is adj usted, so that with the front panelzeFo adj-ust control futly clockwise and the Function switch in the DC voltsor current position, the meter will indicate between . 6 and .7 on the 0 to 1volt scale. To adjust the DC balance control, set the Range switch to thelKVrange, set theFunction switch to eitherDC volts position, andturnthefront panel zero adjust control fullyclockwise. Adjust the DC balance con-trol until the meter indicates .7 on the 0 to I volt scale.

Zero Range adjustrnent: Set the zero adjust control to exactly I/2 rotation.Adjust zero range adjustment so that meter reads zero. Recheck DC bal-ance range and readjust if necessary.

DC Volts Calibration: First check ttre setting of the mechanical and elec-@s. Set the Range switch tothe .1 voltrange, and theFunction switch tothe plus DC volts position. Checkto see if probe switchis in normalposition. Apply exactly . I volts DC to the input terminals, andadjust R41 for exactly a 1.0 indication on the meter.

AC Volts Calibration: First check the setting of the mechanical and elec-@ s . S e t t h e R a n g e s w i t c h t o t h e . 1 v o l t p o s i t i o n , a n dthe Function switch to the AC volts position. Short test leads together andzero meter. Apply exactly . 1 volts AC sine wave (60 Hz to lKHz) to theinput terminals, and adjust R36 for exactly a 1.0 indication on the meter.

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FE2O TROUBLE CHART:

Symptom

Unit dead, meter doesnot read;

Probable cause

Defective batteries:

Poor battery contact;Broken wire frombattery bracket;

Corrective Measure

Check and replace;

Check battery contacts ;Check bamerywiring;

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1 amp rangeinoperative;

Burned out R25 Replace resistor(coil of resistancewire)

Ohms functionswill not operate;

Blown fuseDefective ohmsbattery;Poor contact onohms battery;Broken wire frombattery bracket;

Replace fusecheck and replace;

Check contact tension;

Check wires fromohms battery;

AC volts readingsInaccurate;

Probe switch inwrong position;Weak mainbatteries;

Defect in P/Pdetector;CR4, C16R26 changed value;

Set switch on probeto "NORM";

Operate main battery;check switch; replacebatteries if necessary;Check components andreplace when necessary;

Check resistance of R26:

Meter reads upscale in .1VDCposition only andon RXIM range only;

Defective TRl, orCR2;

Check components forleakage ard replacewhen necessary;

Meter reads upsca le in .1VACposi t ion;

AC Pickup (Normalcondition in strongelectrical f ield);

Short test leadstogether to removeAC pickup;

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SERVICE AND WARRANTY

You have just purchased one of the finest f ield effect voltmeters on the markettoday. The Sencore HI-LO Meter has been inspected and tested twice at thefactory to insure the best quality instrument to you. If so?nething shouldhappen, the HI-LO Meter is covered by a standard 90 dby warranty as ex-plained on the warranty policy enclosed with your instrument.

For thebest service on out of warranty work, send the HI-LO Field EffectMeter directly to the factory service department. Be sure to state the natureof your problem to insure faster service.

If you wish to repair your own HI-LO Field Effect Meter, wehave includeda schematic, parts l ist, and trouble chart. Any of these parts may be or-dered direcily from the factory service department.

We reserve the right to examine defective components before an in warrantyreplacement part is issued.

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