Sencore FE20 Service Manual

p Af\JC3£

*=UMCTlOM

FE

20

HI-L O

METER

SENCORE SERVICE

3200 SENCORE DRIVE, SIOUX FALLS, SOUTH DAKOTA 57107

M A N U A L

Safety Precautions

When testin g elec t ronic equipment, there is always a danger p rese n t . Un
expecte d high voltages can be present at unusual locatio ns in defective equip
ment. The te c h n ic ia n should become familiar with the de v i ce that he is
working on and ob s erve the following precau t io ns.

1. An is olatio n tr ans f ormer should always be used on equipment having the
chassi s tied to one si de of the AC power line.

2. When making te st lead con n ec tio n s to high voltage points, r e m o ve the
power, if this cannot be done, be su r e to avoid contact with other equip
ment or m e t a l objects. Place one hand in your pocket a s a safe t y p re
caution and stand on an insulated floor to reduce the poss i bilit y of shock.

3. Discha r g e f ilter ca pac i tors be f o re con nec ting test leads to them. Ca
pa cito r s ca n st ore a ch a r ge that could be dangerous to the techni cian .

4. Be su r e your equipment is in good ord e r. Broken or fra y ed tes t leads
can be e xtre mely dangerous and can expose the te c h ni c ia n to dangerous
poten t ia ls.

5. Remove the test lead s im mediate ly a f t er the test has been completed to
reduc e the possibil i t y of shock.

6. Do not work alone when working on hazardous circu its . Always have
another pe r son c l ose by in case of accident . Re member, even a minor
shock can be the c ause of a mo r e seri ous ac c id e n t, such as fallin g a
gains t the equipment, or coming in con ta c t with higher voltages.

TABLE OF CONTENTS PAGE

SAFE TY PRECAUTIONS..................................................................................... 2

DES CRIPTION ....................................................................................................... 4

SP ECIFIC ATION S ................................................................................................ 4

OPE RATION........................................................................................................... 6

Shielded T est Lead and Probe for the HI-LO M eter
Contro ls on the HI-LO Meter

Batter y Install at io n and Testing ........................................................... 7

Measuri ng DC Voltage.............................................................................. 7

Measurin g DCV with a Zero Center Scale......................................... 8

Measurin g DC Curren t............................................................................. 8

Measuri ng AC Voltage............................................................................. 8

Measuri ng R esis t ance (HI POWER O HMS )......................................... 9

Measuri ng Resi s tance (LO POWER OHMS)......................................... 9

Measurin g Res i s tance of Thermistors................................................ 9

Measur in g DC High Voltage................................................................... 10

Measurin g De c ibels (db) .......................................................................... 10

Measu r in g AC Curren t............................................................................. 12

Measuri n g sm all L eak ag e Current....................................................... 13

Overload Prote c t io n..................................................................................... 13

.................................................................. 6

......................

6

CIRCUIT DESCRIPTIO N ..................................................................

Gen eral........................................................................................................... 14

DC Vol tage.....................................................................................

DC Cur rent.................................................................................................... 14

Resis tan ce (HI POWER)

Resis tan ce (LO PO WE R) .......................................................................... 15

AC Vol tage .................................................................................................... 15

MAINT E N A N CE .................................................................................................... 15

Intro d u c ti o n .................................................................................................... 15

Dis asse mbly Ins t ructi ons.......................................................................... 15

Cali b r a t i o n of the HI-LO M ete r

Building a Me ter Cal ibrator................................................................... 17

Trou b le Chart ................................................................................................ 18

Schematic and P arts Li st (S e p a r a te Sheet)

ILLUSTRATIONS AND CHARTS

Fig. 1 db Corre c t i on Cha rt

Fig. 2 Voltage vs. Power C hart....................................................... 11

Fig. 3 AC Cur r e n t Adapt e r ................................................................... 12

Fig. 4 Measurin g s mall Leakage Curren t s..................................... 13

Fig . 5 P. C. Board Layout...................................................................... 16

Fig. 6 Calib r a t o r fo r HI-LO Met er.................................................... 17

.................................

...............

.................................................................. 11

............................................ 14

............................................ 16

..................

...

14

14

I ISTIOCTIM M1 IML Fll THE SEICII E FE20

H I-L I FIEL I EFFECI METEB

DESCRIPTION

As the st ate of the art in e lec tro nics p rogress es, m o r e tra nsi stors , F ETs,
and integrate d cir cu i ts are being used in rad io , telev i s i o n , and indu s tri a l
m a rket s. S e ncore engin e ers have kept up with todays pace and have de

veloped a new Field Effe ct m eter to mee t the more demanding requ i r e ment s
of sol id s ta t e and integrat ed cir cu it servi c i n g.

The F E20 HI-LO m e t er fea t ures new low AC and DC ranges of . 1 volts full
scal e and al o w power ohms circu it allowing for the f irst time a ccur ate re
sistance m e a s u r e ments in t ransisto r and integ ra ted circ uits.

Just co mpare these fe atur e s and see why the FE 20 is the most advanced
meter on the m a r ket today:

* High input impedance on all AC and DC ran g e s (15 megohm DC, 12 me g 

ohms AC).
* T r u e pe ak to peak AC r anges.
* Z ero ce nter ran g e s on DC for s olid s t a te servi cing.
* Db s cale fo r audio and lab work.
* DC cu rrent ranges for genera l tes tin g .
* Low power (low applied voltage) ohms ra n g e s for solid s t a t e s ervic ing.
* Mirrored sca l e m eter for g reater accu r acy in readings.
* Me t e r pro te ct e d fr om over loads.
* C i r cuits pr o te c te d from acc i d e n t a l ap pl icati on of wrong voltag es .
* All ste el carry i ng c ase with cover fo r grea ter durab il it y and portability.

* Long life "C" cell power.

DC VO LTS

SPECIFICATIONS

Ran g e s : 0 to .1 , .3, 1, 3, 10, 30, 100, 300, and 1000 full s cale

- .05 to. 05, - .15 to. 15, -.5 to. 5, -1.5 to 1 .5 , -5 to 5,

-15 to 15, -50 to 50, - 150 to 150, and -5 00 to 500 Z ero
Cent e r Scale Ran ges .

Input Re s istance: 15 megohms shunted by 90 pf in probe "NORM"

positio n ; or shunted by 10 pf in probe " 120K ISOLATION’
position.

Acc uracy : +1. 5%
AC Rejec t i on: 30 db Minimum

AC VOLTS
Ran g e s : (RMS)

0 to .1, . 3, 1, 3, 10, 30, 100, 300, and 1000 Full s cale

Ranges: (Peak to Peak)

0 to . 28, . 84, 2. 8, 8. 4, 28, 84, 280, 840, and 2800 F u l l scale

Input Resistance: 12 Megohms shunted by 9 0 pf

Freq u e n cy Res ponse : 1 db 10 Hz - 150 KHz

3 db 5 Hz - 500 KHz
OHMMETER
Ran g e s : HI & LO POWER

0 to 10 00 ohm s, 10K, 100K, 1 Meg, 10 Meg, 100 Meg, and

1000 Meg with 12 ohms ce nter scale
Accurac y: + 2 degree s ARC
Applied Voltage: Hi Power 1.5 Volts

Lo Power . 08 Vo lts

DC CURRE N T MEASUR E M E N TS :
Ran g e s : 0 to 100 mi c r oamps, 300 microamp s, 1 mA, 3 mA, 10 mA,

30 mA, 100 mA, 300 mA, and 1 amp.

Ac c u r acy: + 3% full s cale
Interna l voltage drop: 80 mv on 100 mic r o a m p to 10 mA r ange s ,

100 to 900 mv on 30 mA to 1 amp ra n ges.

Dec ib e l (db)
Ran g e s : -20, -10 , 0, 10, 20, 30, 40, 50, and 60 db

Refer enc e: 1 mv into 600 ohms

GENERAL SPECIFICAT IO NS:

M eter : 6 inch, 200 mic r o a m p , 2%
Mul tip lier resistors: 1/2% deposited ca r b o n type

Semiconductor com ple m en t :

T ransis to rs: 2 - 2N5457 FE Ts, 2 - 2N4248, 1 - 2N5172
Diodes: 2 - 1N456, 3 - 1N695, 1 - 16S10

Power supply: 6 e a c h 1. 5v " C" ce lls Evere a d y #935 or equivalent

(not supplied)

Ohms Batte r y : one 1. 5V "C" ce l l Eve re a dy #1035 or equivalent

(not supplied)
Weight: ( less b atteries) 8 lbs.
Dime n sions: 9 1/2” High X 7 3/4" Wide X 3 3/4" Deep.

The FE20 HI-LO Meter is equipped with a shielded tes t lead to allow ac
cu r ate meas u r e ments on AC in the presen ce of strong e lectr ost ati c and
ele ctromag netic fields. The special pro be has a 120K isola t io n res isto r
to iso l a t e the cab le capacity when making DC voltage m e a s urem e n t s in crit
ic al cir cuits such a s RF, IF or sweep cir cui ts. For a ll normal read in gs

including DC voltage, the pr o be switch is le f t in the n ormal position. The
only time that the switch is placed in the isola t e p o sitio n is when it i s nec
ess ar y to is olate cable cap a ci t y from the ci rcu it being m e a sured. When the

switch is in the isola t e po si tio n, the DC voltage rea di ng on the FE20wil l be
onlyO. 6% low. The isola t e position should only be used with the DC voltage
and AC voltage at 60 Hz functions as much greater e rro r will re sult on other

functions.
CONTROLS ON THE FE 20
The FE20 HI-LO Met e r i s a s simple to operate as a standard VOM. The

front panel c ontrols include the RANGE switch, FUNCTION switch, met e r

ZERO ADJUST, OHMS ADJUST, ON-OFF switch, and the batter y test

swit c h e s . The following i s a brief desc r i p t i o n of each c o n t r o l and how it is

Used:

RANGE Switch: The RANGE Switch sele cts the d esire d range of voltage,
cu rrent or resistan ce to be measu r e d .

FUNCTION Switch: The FUNCTION Switch sele cts the type of measu r e ment

to be made, plus or minus DC volts or curre nt, AC vo lts , HI or LO power
ohms.

ZERO ADJUST: The ZER O ADJUST is used to po sitio n the pointer of the
m eter ov e r the ze r o at the extreme left of the me t er for m o s t mea s u r e ment s ,
or ove r the cen t er z ero for the z ero ce nter ranges.

OHMS ADJUST: The OHMS ADJUST i s adjusted with the FE 20 le a ds held
open. The OHMS ADJUST co ntrol is adjusted so that the po in ter of the me t e r
is positioned di rect l y o ver the infinity ma r k on the extreme right hand side
of the meter. Th e ohms zero is adjusted with the test leads shorted togeth er
and the Range Switch s et to any posi t io n ex c ep t RX1 or RX10 .

ON - OFF Switch: The O N-OF F Switch controls the power to the m e t er cir
cuit . In the OF F position, the b atteries ar e disco n nect ed f r o m the me t e r
circ uits. In the ON position, b a t t e r y power is applied to the m eter circui try
and the mete r is re a d y to go. When the lid of the FE20 is shut, a s pecia l
tab autom a t ic a ll y shuts o fft heFE20 to be su r e that the ba tteries do not wear

down nee d le s sly.
BAT T E RY T EST Swit ch es : The FE20 use s two bat t e r y test sw it c he s . The

bat t e r y t est switch on the l e f t is the t est fo r the main (power) b a t te r y , and
the switch to the rig h t is the ohms ba t t ery t e s t.

The S e n c o r e FE20 is s im p le and easy to opera t e . The long li f e "C " cell

power supply allow s the FE20 to ope ra te anytime, anywhere.

Batt e r y Inst al la tio n and Test ing: To gain ac cess to the bat tery compartmen t,
remove the two s crews in the lead compa rt m en t . Lift out the lead c om

partm e n t, exposing the batte r y ho l d ers.

Th ere a re two power supp lies used in your F E20. Six "C" ce lls are con 
nected in series to power the amplif ier and bridge circuit, and one "C" cell
is used to power the ohms function. To install the s ix "C " c ells for the
bridge power supply, plac e 3 "C" cells in each of the two tubes provided.
Inst a ll the tubes containing the th re e batte ries each , between the spring
bat tery conta cts. Be s ure to o bserve pola r i t y . The ohms ba t t e r y is in s ta l le d
in the single bat t e r y hold er that is mounted at an angle. It i s recomm ended
that a "p r e mium " cell be used in this app lication as the c urre nt drain can
be quite high when using the RX1 HI power ohms range.

The two s prin g r e t urn slid e switche s on the front panel of the FE20 provide
a batte ry c heck function. Push the Ohms switch to the righ t to test the con
dition of the o h m s bat t e ry, and push the Main switch to the l eft to t est the
condition of the bridge power supply b a t t e r y .

Operatio n : To o p e ra t e the F E20, mer ely select the desire d test with the

Function switch, the app ro p ri ate rang e with the Range switch, connect the
test lead s to the points being measu r e d , and obs e r v e the indication on the

m eter. H er e is a bri ef desc r i p t ion of how each test is p e rf o r m e d .

Meas u r in g DC V o lt s : Set the switch on the pro be to the NORM po s it io n ; set

the Function switch to eit her the plus or minus DCV positio n; se t the Range
switch for a mete r indication a s c l ose to full sc ale as possible. (I f the volt

age to be me a s u r e d is unknown, start with the 1000 volt range and work

toward the 0.1 volt range).
The DC voltage is rea d on the two blac k scal es j ust above the mir ro r on the

m eter. The top DC sc ale i s calibr ated from Oto 1.0 volts, with each smal l

sc ale division r epre s enting . 02 volts, and is used with the . 1, 1, 10, 100,

and 1,000 volt ran ges. The second DC scal e i s c a l i brated fr o m 0 to 3.1

volts with each s mall s cal e division repr esen t i ng 0.1 volts, and is used for
the . 3, 3, 30, and 300 voltr ang e s . l i ieO to 1.0 v olt scale i s readdi rec tly

for the 1.0 volt range, and is multiplied by . 1, 10, 100, and 1,00 0 for the

.1, 10, 100, and 1,000 volt ran g es respectively. The 0 to 3 .0 volt scal e
is rea d di r ectl y for the 3. 0 volt range and is multiplied by . 1, 10, and 100

for the . 3, 30, and 300 volt ranges re spectiv ely. For Exam p le: If the sca l e

indication i s . 42 on the 0-1 vo l tsc ale and the Range switch is set to the 10
volt range, you would multiply . 42 by 10 r esult i n g in an actu al reading o r

4.5 volts. If the scal e indication is 1. 6 on the 0 to 3 volt scal e and the

Range switch is se t to the 0. 3 ra n g e , the actu a l reading would be . 16 volts.

Meas u r in g DCV with a z ero ce nter sc ale: The zero ce nter scal es are used

to rea d plus or minus volta ge s without the ne cessity of changing po l a r i t y

with the lead s or the Function switch. To use the zero center s cal es, se t

the switch on the probe to the Normal po si tio n. Set the Function sw itch to

plus DCV, and adjust the ZERO ADJ until the meter r ead s at the zero ma r k

in the cen t er of the sca le. Thi s gives rang es of plus or minus 0. 5 volts
(upper sc ale ) , or plus or minus 1. 5 vo lts (lower sca l e). The meter scale s
are then multipli ed by 0. 1, 10, 100, or 1,00 0 depending on the se tt in g of

the Range switch. If the Range switch is on the 300 volt position, then the

1. 5 volt sca l e would be multiplied by 100 r e s u l t i n g in a range of plus or
minus 150 v o lt s. If the Range switch were on the 0.1 volt rang e , the plus
or minus . 5 volt range would be read a s plus or minus . 05 vo lts.

If the me t er moves up scale to the righ t , the voltage is p o si t i v e , and if the
pointe r moves to the left, the voltage is negative. With a cali brated z ero
ce nter scale, you can determine not only the p o l a r i t y o f a voltage, but a ls o
its magnitude.

Meas u r in g DC Curren t: To me asur e DC curre nt, set the pro b e switch to the
Normal position, the Function switch to the +DC mA p osi tio n, and the Range
switch to the des i r ed curren t ra nge. Remove power f r o m the equipment to
be measured, and co n n ec t the FE20 in s er ies with the curre nt to be mea
sured. Connect the negative me t er lead to the negative s o urce of current
and the posit iv e met er lead to the p o s it iv e s ource of current. (If the curr e nt
to be measur e d is unknown, star t with the 1A ra n ge and work toward the
. 1 mA ra n g e) . Rec o n n e ct the power to the equipment being measure d , and
rea d the cur r ent on the approp ri ate mete r sca l e. If the met er r eads in the
negative dire c t i o n , the cu rrent pola r i t y is reve rsed from that expected and
the Function switch o f the F E20 should be turned to the -DC mA position.

The zer o ce nter sc ales may als o be used to meas ure DC cu r rent. With the
Function switch in the +DC mA position, and the Range switc h s et cor rectly,

adjus t the met er for zero center and apply c urrent to be measu r e d to FE20.

The 100 uA to 300 mA c urrent ran g e s a re p rotecte d by the frontpanel fuse.
On the 1 amp range, the shunt res isto r is a coil of r esist an ce wire, and will
withstand sustained heavy ov e rloads, however, it is poss i ble to damage this
shunt resistor if the overlo a d is not rem o ved imm ediately.

Meas u r in g AC Volt s : To read AC Volts, se t the Function switch to the AC
volts position and the Range switch to the de s i r e d voltage range . The switch
on the pr obe should be in the Norm al posi tio n. To meas ure RMS voltage,
the two black scal es marked AC V (rms) a re used. To measur e peak to peak
voltage, the two red scales mark ed ACV (P-P) a re used. The sc ale readin g s
are multiplied by the sam e m u lt iplier fact ors as wer e used in DCV m easure
ments . On the 0 to 1 volt scale, each smal l div is io n r epres ent s . 02 volts ,
and on the 0 to 3 volt sc ale , each sma l l s cal e division re prese nts . 1 volts .

On the 0 to 2. 5 volt peak to peak sc ale, each small s cal e division re presen ts

. 1 volt, and on the 0 to 9 volt peak to peak sc ale, ea c h s mal l sca l e division

repr ese nts . 2 volts .

Exa mple: If the scale indication is . 6 on the 0 to 1 V AC (RMS) sca l e, and
the Range switch i s set to the 0.1 volt range, the actu a l voltage reading
would be 0. 6 multiplied by 0 .1 , or . 06 volts AC RMS.

Measu ri n g Resis tan c e (HI POWER OHMS): T o me asure r esi sta nce, set the
Function switch to the HI POWER OHMS position, and the Range switch to
the ra n ge that will put the resi stance to be measured a s c l ose to the center
of the me t er scal e as po s s i b l e. When you switch the Function switch to the
ohms functions, the m eter po in t er wi ll move f r o m the lef t hand edge of the
m e ter (Z ero) to the infinity ma r k on the rig ht hand side of the meter. U se
the ohms adjust con t ro l to adj u st the mete r pointer to exa c t l y the infinity
mark on the rig h t hand side of the m eter.

When the m eter le a d s ar e shorted together, the mete r will indicate zero
ohms on the le f t hand side of the me t er. Be sure that the Probe switch is
in the Normal positio n. Any resistance on the Rxl range is the resi sta nce
of the switch cont a c t s and inte r n a l wiring in the meter c ircuit . Thi s sma l l
value of res istan ce (ap p rox imately 1.4 ohm) should be sub t ra c t ed f rom the
mea sured read ing.

When using the HI POWER OHMS function in sol id st a t e c irc uit s, the applied
voltage from the FE20 will be high enough to caus e the diodes or t ransisto rs
to conduct. Thi s can be used to an advantage because the front to b a ck ratio
of the tran sistor or diode may be mea su r e d by r eversing the m eter le a d s.
The probe lead i s + and the blac k lead - in the ohms functions.

Measu rin g Resistance (LO POWER OHMS): The LO POWER OHMS function
is used to make r esi sta nc e me asure ments in solid stat e c irc uit s when the
resistance of the diode or t ran sistor junctio n s c a u s e an o b jectiona b l e erro r.
The r esist an ces m ea s u r e d with the FE20 in the LO POWER OHMS function
will be the r esi sta nce of the ci rcuit components surrounding the trans i sto r
or diode, and wi ll not be affect ed by either. The LO POWER OHMS function
applies only . 08 volts to the ci rcu it being m ea s u r ed , and even a germanium
diode re qui res a higher voltage to start conducting. The opera tio n of the
LO POWER OHMS is the sa m e as t hatof HIPOWER OHMS. Note: To c o n
se rve OHMS batter y power, DO NOT LEAVE THE FE20 IN THE LO POWER
OHMS POSITION WHEN NOT MAKING RESISTANCE MEASUREMENTS.
Th ere i s a continuous drai n of appro xi m ate ly 7 mA from the ohms ba t te r y
in the LO POWER OHMS position. This 7 mA ohms b a t t e r y d rai n can be
elimin a t ed by switching to anoth er function.

Measu rin g Resistance of The rmis tors: The LO POWER OHMS function is a
must for making accurate measur e ment s o f resistanc es of T her misto r s of
low r esi stance value. If this m e a s ureme n t is made in the Hi Power Ohms
function, a changing resistance readin g will res ult due to heating caused by
the higher curr ent flow- through the Ther mistor. The T her mistor should
not be held in the hand and should be kept away fr o m any other heat to obtain
the m o st accurat e me a sureme n t .

CAUTION: When making resis tance m e asure ments be sure that the power
to the equipment under te s t is disco n ne cted. If voltage is applied to the
ohms ra n ges, the fuse F I will have to be r e placed.

MEASURING HIGH VOLTAGE WITH THE FE20
CAUTION: High voltage is dangerous and caution should be ob s e rv e d when

mea s u r i n g high voltages. Be su r e that the ground lead fro m the FE20 is
connected to the app ro pri at e ground in the equipment being measur e d . If

the ground lead is not connected, you a re m e rely extending the high voltage
close r to you. If the ground lead is connected to the wrong ground in the
equipment being measured, you run the risk of pos s i b le damage to the equip
ment being tested. Stand away from the equipment being measu re d and keep
one hand in your pocket a s a saf e ty mea s ure. High voltage in co lor TV i s

es peci ally dangerous because it can go as high as 30, 0 0 0 volts and it is reg

ulated. If you should come into conta c t with it, it will not load down.

The 39A30 high voltage probe is used to extend the rang e of the FE 20 to

30,000 volts DC. Thi s probe m ay be used with any DC voltage ran ge on the

FE20 to act a s a tim e s 100 mult i p l i e r and to increas e the input impedance
of the FE20 to 1500 megohm s. In other words, the 0.1 volt range would
becom e a 10 volt ra n ge with a high enough input impedance (1500 megohms)
which would not aff ect the mo s t criti cal c ircuit.

To use the 39 A30 pro be, r e move the two part s of the pr o be fro m the bracke t s

in the cover of the F E20 . I nse r t the end of the probe body with the banana
plug of the probe body se at in the banana jack inside the handle. In sert the

norma l t est probe of the FE20 into the sm all opening of the 39 A30 probe, and
pla c e the pr ob e switch in the nor m al positio n.

To re a d the a c t u al voltage, multiply the readin g s on the 3 volt sc ale by 1000

for the 3KV range, the 1 vo ltscal e by 10,0 00 for the 10KV rang e and the 3

volt scal e by 10 , 00 0 for the 30KV r a n g e .

To meas ure high voltage, set the Function switch to eith e r the plus or minus
DCV positio n and the Range switch to one of the thre e high voltage positions,
3KV, 10KV or 30KV. Connect the ground lead of the FE20 to the appropri ate

ground point in the equipment being m e a su r e d . Be su r e to read the preced

ing parag r a p h s on the caution to be taken when m e a suring high voltage, and
on the o p er at io n of the 39A30 high voltage probe. The 3 KV, 10KV and 30 K V
ranges are to be used only with the 39A30 pr ob e.

Measu r ing D e ci b e ls (db): The FE20 has a convenient db s cale for easy db

me a s urem ents. The nine AC voltage ranges a re in per f ect 10 db st e p s , so

that sta r ting f r o m "Ο" db on the 1.0 voltran ge, youaddlO d bfor each range

above 1. 0 volts and su btract 10 db fo r each range below 1. 0 volts. Db mea

sure ments are ac tu a ll y a meas ure of the r atio between the power in the ci r
cuit under test, and som e refe rence power leve l. The ref ere nce leve l used
for the F E20 is a 1 milliwat t into 6 0 0 ohms. If the impedance being me a 

sured across is oth er than 600 ohms, you will have to use the ch art in Figure

1 to deter m ine the actu a l power le vel or db. Ex a mple: The s cal e reading

Fig. 1 db Corre c t i on C ha rt

Fig. 2 Voltage vs. Power Cha r t (sin e wave conditions only)

is -4db, the Range switch i s on the 100 volt sca le, and the impedance is
6 ohms. The 100 volt range i s 4 ra nges higher than the 1. 0 volt r a n g e , so
adding 40 db to the meter reading results in 36 db. Using the cha rt in F ig u r e

1, the cor r ect i on fac t or is plus -3Φ db, or an ac tual db measure ment of 56 db.
The cha r t in Fig ure 2 can be used to dete rmine the actu a l power being M is-

sipated in a circ uit if the circ uit impedance along with either the applied
voltage, or the db a s m e a s u r e d on the FE20 m eter i s known. The char t in
Figure 2 also shows the r e lationship of AC volts to db. If the AC voltage
is i ncre a sed 10 t i mes, the db rea di ng goe s up by 20 db.

Note: The db sca les of the FE20 and the ch arts in Figur e 1 and Figure 2
only apply when the voltage being mea sured is a pure sine wave as from an
audio ge nera t o r. The FE20 is a true readin g peak to peak m eter and the
complex waveform of mus i c materia l will not give a tru e RMS reading for

the db syst e m to work on.
Measuri n g AC Cur r e nt with the FE 20: The FE20 can be used to me a s u r e

AC curre nt with the c irc uit shown below. The r esistor or resistor s are
placed in series with the AC source going to the devic e under test. The

FE20 is se t to the AC Volts function and the AC voltage drop acros s the
resistor s is m ea s u r ed and can be c o nve rt ed directly into AC amps. It would
be mo s t convenient to build this adaptor into a box with a switch to change
ranges . The resistor s used are a l l 1 ohm of at l eas t one watt. The O h mite

995 series of 1 1/2 watt wirewound r esistor s is id e a l for t h is. A standard
one watt carbo n re sistor will also work, but if the unit is to be left on f or an
extended perio d of time, a two watt should be used.

t >

Fig . 3 AC Curren t Adapter

When me a s u r i n g f r om z ero to one amp, the one ohm r esis tor is used, and

the FE20 Range switch is in the one voltp os it io n . To meas ure three amps
of AC cu r rent , the switch is clo s ed so that ther e a re now thre e one ohm
resistors in par allel. A c urrent of three amps will now produce a one volt
drop acros s the thre e r esistor s, so leav e the FE20 on the one volt range,
and rea d the 0 to 3.0 scal e fo r th r e e amps of AC curre nt.

Measuring Small Leakag e Curren t: The FE 20 can be used to meas u r e the
leakag e cu rrent of any low leakage dev ice . All that is nec essary is a power

supply r a t e d at the tes t voltage of the device being measure d , 2-15 meg ohm
resistors and your FE20. Connect the 2 - 15 meg ohm re si stors in series
acros s the input of the FE20 , and then con ne ct the power supply, the device
under test, and the F E20 in s er ie s. (See F ig. 4).

HI-LO METER

(0.1 VDC)

PO W E R

SUP P L Y

+ -

III

DEVICE

UN DE R

TEST

I

LEAKA GE

| CURRENT

I5mtg

I5meg

t

Fig . 4 Measu ri n g S mall Leaka ge C urren t s

Set the Range switch to .1 V, and the Function switch to + DCV position, the
Probe switch to the norm al po sitio n. The FE20 will now read leakag e from
0 to .0 1 ua (lOna) on the 0-1 s cale.

Overload Pr o t e c tion: Your FE 20 is pro t e c te d again s t acciden t a l overlo a d s
on all ranges and func tio ns. The voltage ran ges are prote c t e d beca u s e the
high input impedance of the F E20 li mits the amount o f cur r ent that can flow
into the m eter to damage it. On the resistance and cur r ent functions, the
fuse FI on the fr ontpan elof theFE 20 pr otec ts both the m eter movement and
the circ uitry of theFE 20. This fuse should be re p l a c e d with one of the same
type so that the p r o t e c t io n feat u r e or ac curacy of the F E20 are not affecte d .
Input switching and components are p rotected again s t accide ntal overlo ad
by a spark g a p . Met e r prot e c ti o n is provided by a germanium diode across
its terminals.

FUSE TYPE: 3AG . 6AMP. DO NOT use a slow blow type fuse. In a c curacy
and p o s s i b l e damage to the HI-LO met e r may re sult .

CIRCUIT DESCRIPTION

Gene ra l

The Sencor e FE 20 Field Effect mete r use s a balanced bridge impedance
co nver t er co nsisti n g of two Fiel d Ef fect and t wobi -polar tra nsi stors . Th is
ci rcuitry pro vi de s high input impedance and exce l l ent stabilit y . Whenm e a-
suring AC volts an additional ampl i f i e r s ta g e which is compen sat ed again s t
changes in voltage, temperat u re and fr e q ue n c y i s used to drive the peak to

peak detector .

DC Volta g e: The DC voltage at the inpu t p asses through the AC r eje ction
filter consistin g of R1 and Cl to the voltage div ider (R2 through RIO). The
Range switch S3B s elect s the r e q uired input voltage to be applied to the gate
of TRl . The FET T Rl provides the n ece ssa ry high input impedance, and
dri v es the base of TR 2 . The source output of T Rl and the collector output
of TR2 are in parall el, with both output cu rrents flowing through the common
load re sistor R29. TR 2 pr ovi de s the nec ess ary cur r ent gain requ ir e d to
drive the meter . The T R 3 and T R4 sec t i o n of the balanced impedance c on 
verter is held constant, be c a u s e the gate of TR4 is tied to ground through
the AC c al c o ntrol R 3 6 . The meter, in se ries with the DC cal con t ro l , is
connected between the coll ector of TR 2 and the co llecto r of TR3, with the
posi tive m eter lead con nected to the collector of TR 2 .

Th is balan ced impedance converter is tempe r a tu r e s t a ble becaus e any change
in tempera t u r e will affe ct both sides of the balanced circuit equally, keeping
the c irc uit b ala nced. The cir cui t is compen sat ed aga inst changes in supply
voltage, becau se the voltage at the so u r c e of TR4 remai ns n e arly constan t
and i s used as a b i a s voltage for TR l gate. It is applied to the gate of T Rl
through re sistor R40 , the DC bala n ce co n t r o l R39 and the zero a d ju s t R5 0.

DC Cu r r e nt: When measur ing DC cu rren t , one o f the standard r es isto rs ,
R17 through R25, i s s elect ed by the Range switch to be put in s er ies with
the ci rcuit to be meas u re d . The circuit d e scrib e d in the pr ec e din g par a
graph (DC voltage) i s then used to me asure the voltage acros s the standard
res istor. The fuse FI protect s R17 through R24, or up to the 300 mAr a n g e .
The meter movement it sel f is prot e c t e d on a ll ranges b e c a u s e it is isolated
thru the impedance con verter and shunted with a diode.

Resis t anc e (HI POWER): To mea s ure the resistance of an unknown resistor,
the FE 20 conne c ts the unknown re sis t an ce, a 1. 5 volt battery , and a standard
res isto r in se ries . The FE20 then mea s ure s the voltage across the unknown
resi sta nc e. The ohms a djus t cont r ol has the s am e ef fec t as the DC cal con

trol , in that it is connected in s er ies with the meter and is used to adjust the

sensitivity of the m eter. When there is an infinite unknown resi sta nce (open

ci rcu i t) the FE20 will measure the full b a tt e r y voltage. The ohms adju st
con tr ol is adjusted with the test leads open, so that the full voltage will re a d
infinity on the meter . When the FE 20 leads a re sho rted toge t her , the voltage

will be z ero, and the me ter will r e a d zero. If an unknown re sis tance is

exactly equal to the sta ndard re sis tance in the F E20, the voltage acr os s the

unknown resistance will be one half of the total, or the met er will r e a d one
half sca le. With a standard re sistor of 1200 ohms, and the unknown re

sis tance of 12 00 ohms, one half of the voltage would be acros s the unknown
res isto r, and the F E20 would rea d one half scale. A rea din g of 1/2 sc ale
would be equal to 1200 ohm s. The S3A s e ct i o n of the Range switch i s used

to select dif fe r e nt standard r es isto rs for the differe n t resistance ra n g es.
All resista nce standard resistor s, e x c e p t the R XlO and R X100 r a nges , a re
prot ec t ed from overlo a d s by the fuse FI.

Resis t anc e (LO POWER): The LO POWER ohms function on the FE20 is to
be used when working on solid st a te equipment in c ases where the res istan ce

of the res isto rs alone, and not the diode junction resi sta nce of the trans i stors

is impo rtant. The LO POWER ohms function applies only . 08 volts (8 0 mv)
to the circ uit being m e a su r e d , so that the diode jun ct io n s of any t ran sist or

will not conduct.

The LO POWER ohms function oper a t e s the same as the HI POWER ohms

function, with the ex ce p t io n that a voltage divider c o mpris e d of R14 and
R15 is used to s et the test voltage to .08 volts. The DC voltage m ea s u r in g
ci rcuit of the FE20is stil l connected across the unknown res istan ce, but the

sen sitivi t y of the voltage measurin g circ uit is increased so that . 08 volts

will read full s cale. The S3A sec t ion of the Range switch i s used to sele ct
diff e r e n t standard r esistor s for the ohms ra nges.

AC Vo ltage : The AC voltage at the input is coupled through Cl to the top of
the voltage divider co mprise d of R2 through R10. The S3B sec t ion of the

Range switch se l ects the voltage to be applied to the input of T R l. TRl acts
as a so urce follow er . The output o f TRl and T R 2 i s coupled through Cl2
and R38 to the input of T R 5. T R 5 acts as a common emitt er amplifi e r to
provide a voltage gain of approxi m ate ly 10 times. T R5 dr ives the pe ak to
peak dete c t o r co mprise d of C13, C16, CR3, and CR4. A negative output
from the peak to peak dete c t or is applied through R33 and the AC ca l con tr o l
(R36) to the gate of T R4. The DC voltage applied to the gate of T R 4 caused
the brid ge to be unbalanced and the meter to read the AC voltage.

MAINTENANC E

Introduction: The only norm al maintenance r e q uired by the FE20 is re

plac e m e n t of the b atteries. The c alibr ation and bala n c e adjustments have

been made c aref ully and a c curatel y at the fact o r y , and should not need r e
adjusting, un le s s a component should change value.

If you wish to ser vic e the FE20 yourself, r ef er to the following p ro c e dures

for i n s t r u c t i o n s on how to disa s s emb l e , cali bra t e and trouble sho ot.

Di s a ssembly In st r uctions: Ther e a re six screws that must be remove d to
remo v e the FE20 f ro m i t s c ase. F irst , remov e the four scre ws from the
ba ckof the FE20. Thi s lo o s e n s the main as sem bly from the c ase. Second,
rem o v e the two s crews inside the lead co m p art m e nt . Th i s loosens the sub

panel from the case. Third, l i f t the main as sem b l y and the sub panel out

of the case at the sam e time. Take car e to do this gently, as the ba tt e r y
holder will re main inside the case. Fourth , l i f t the ba t t ery holder off the
two p o s ts that lo c ate it in the bottom of the c ase.

For the locatio n of the following ad ju s t m e n ts , refer to the PC board layout

diagr a m . F i gure 5.

Cali b r ation of the HI-LO Meter
DC Bala n c e : The DC balan c e co n t r o l is adjusted, so that with the front panel

ze r o adju s t control fully cloc k wise and the Fun ction switch in the DC volts
or current position, the m eter will indicate between . 6 and . 7 on the 0 to 1
volt scal e. To adjus t the DC ba la n ce co ntrol, set the Range switch to the
1K Vrange, s et the Function switch to eith e r DC volts po si tio n, and turn the
front panel zer o adjust cont r o l fully clockwise. Adjust the DC balan ce con 
tro l until the meter ind ic a t e s . 7 on the 0 to 1 volt scale.

Ze ro Range a d ju st m e nt : Set the zero adjust con t r o l to exactly 1/2 rotat io n .
Adjust zero range adjustm en t so that me t er reads z ero. Rec heck DC bal
ance ran ge and r e adjust if necessary .

DC Volts Ca l ibrati o n : First che ck the setting of the mech a n i c a l and e le c
tric al zer o adju s tments. Set the Range switch to the . 1 volt range, and the
Function switch to the plus DC volts p osi tio n. Check to see if probe switch
is in normal position. Apply ex actl y . 1 volts DC to the input term i nals , and
adjus t R41 fo r exact l y a 1. 0 indication on the m e ter.

AC Volts C a l ibratio n: Firs t ch eck the set ti ng of the me c hanic a l and ele c
tric al z ero adjus t m e n ts . Set the Range switch to the . 1 volt position, and
the Function switch to the AC volts position. Short test le a d s together and

ze ro met er. Apply exact l y . 1 volts AC sine wave (6 0 Hz to lKHz) to the

input t e r min a l s , and a d just R36 f o r ex actly a 1. 0 indication on the m eter .

Fig . 5 P. C. Board Layout

If an accurate sour c e of . 1 volt AC and DC is not availab l e , you may wish
to build an inexpen sive meter c alibra tor such as the one shown below: You

may ord er a l l the electron i c components fr o m Sen core, with the exce ptio n
of the batte r y , fo r $ 5.00. Or, you may purch a se pa r ts locall y . The ac
cu racy of the ca l i brati on with this fixt u re will be the ac cur acy of the battery,
which on a new cell is on the order o f 1 . 5 %. The 2. 8 V ba t t e r y is two 1.4

volt mer cury bat t eries connected in se ries.

TRl

Rl

-VW

WHTa RE D

BL U

Fig . 6 Cali b rato r for HI- LO Met e r

Tl - 28A35 T ran sfo r me r - Any low c urrent trans former

T Rl - 19G29 Tra nsi sto r 2N5172
R1 - Res istor 5. 6K 1/2W 10%
R2 - Resis tor 10K 1/2W 10%

I

with seco n d ary voltage between
6 volts and 12 volts.

With the AC signal from the transform er connected to the base of the tr an
si stor, the f i x t ure will produce a squa re wave of the same peak to peak value
as the bat t ery voltage, (2. 8V). Due to the slight e rr or of reading peak to

peak voltages of a s q u a re wave with any true rea din g peak to peak meter,

the 2. 8V peak to peak signa l will indicat e ex actl y 3. 0V peak to peak on the
FE20 . Use the 2. 8 volt peak to peak sc ale, and adjust R36 the AC cal co n 
tro l fo r 3.0 volts peak to peak on the m eter. With the signa l removed from
the base of the t ran s istor, the output will be the bat t e r y voltage or 2. 8 volts

DC. This 2. 8 volts DC may be used to ca librate the 3 volt DC range . Set

the F E20 to the 3 vol t r a nge, a ndadjustR41 the DC cal contr o l f o r 2. 8 volts

on the m eter .

FE20 TROUBLE CHART:
Symptom

Unit dead, met er does

not r e a d;

1 amp range
inope ra t iv e ;

Ohms functions
will not opera te ;

AC volts re ad ings
Inac c urate ;

Probable cau se
Defect iv e b atte ries;
Poor ba t t e r y co n ta c t ;

Broken wire fro m
batt e r y bracke t ;

Burned out R 25

Blown fuse
Defect iv e ohms
bat tery;
Poor co n t a c t on
ohms battery ;
Broken wire from
bat t e r y br a c ket;

Probe switch in
wrong po si tio n ;
Weak main
batt eries;

Defe c t in P/P
det ector ;
CR4, C16
R26 changed value;

Corre ctiv e Me a s u r e
Check and replace;
Check ba tt e r y c o ntact s;

Check bat t e r y
wiring;

Rep l a c e r es isto r
(co i l o f resi sta nce
wire )

Rep l a c e fuse
che c k and repl ace;

Check cont a c t tension;
Check wi r e s fro m

ohms ba t t e r y;
Set switch on probe

to "NO RM ";
Operate main b a t t e r y ;
chec k switch; replace
batterie s if necess ary;
Check components and
re pla c e when necessa r y;

Check resistance of R26;

Me te r r eads up

scal e in . 1VDC
positio n only and
on RX 1M ra n ge only;

Meter re a ds up AC Pickup (Normal Short te st leads

sc ale in . 1 VAC condition in st ro n g together to remo v e
posit io n ; electrical field ) ; AC pickup;

Defecti v e TRl , or
CR2;

Check components for
leakag e a nd repla c e
when nec essary ;

Symptom

Probable cause

Corre ctiv e Me a su r e

Met e r pins in
ohms function;

Met e r r e a ds
sligh tl y up scale in
RX1 and R X10 ohms

positions with t e s t

leads shorted;

Range switch in
correctl y s et;

Indication is that
of test lea d s ,
switch con t a c t s
and fuse
resistance. Th is
is a no rm a l
condition;

Set Range switch
to act i v e ran g e ;

Ze r o Adj ohms in
RX1 0 0 or above
ranges;

κ . χκ

SERV IC E AND WARRANTY
You have ju st purchased one of the fin est field effect vol t meter s on the ma r k et

today. The Sen co r e HI-LO Me te r h a s been insp ect ed and tested twice at the
fac t ory to ins u r e the bes t quality instrum e n t to you. If soTnething should
happen, the HI-LO M e t e r is cov e re d by a standard 90 day w ar ra nty as ex
plained on the warranty po li cy en c lo s ed with your in strument.

For the best service on out of warra n t y work, send the HI-L O Field Effect
Met e r di rect l y to the factor y service department. Be sur e to state the nature
of your proble m to insu r e faster service.

If you wish to re pai r your own HI- LO Field Effec t Me t e r , we have included
a sch e matic , p arts l ist, and trouble chart . Any of these part s may be or
dered directly fro m the fac t ory servic e department.

We r eserve the right to exa m ine defe c t iv e components before an in war ra nty
rep l acem ent pa r t is issu ed .

IK . — .U PC

Form 545

SENCORE

NO. 1 MANUFACTUR ER OF ELE CTRONIC MAINTENANCE EQUI PME NT

3200 SENCORE DRIVE, SIOUX FALLS, SOUTH DAKOTA 57107

ίν s e n c o r e

\ NO. 1 M A NU FA C TU R ER OF EL ECT RO NIC MAI N T E N A N C E EQ U I P M E N T

J 3200 SENCORE DRIVE, SIOUX FALLS, SOUTH DAKOTA 57107

SCHEMAT IC AND PART S LIST

F OR

FE20 HI-LO MET E R

CAP A CIT OR S
SCH E MAT I C
REFERENC E
C2
C3
C4
C6 24 G1 8 2
C7
C8
C9

P AR T NO. DESCR IPTI ON PRICE

24 G2 2 0
24 G1 8 7
24 G217

24 G1 8 3
24 G1 8 4
24 G1 8 5

39PF @7 00 V D i sc
91P F @6 3 0V 2. 5 % Po l y
300 PF @ 6 3 V 5 % P o l y
330 0PF @3 3V 5% Po l y
. 01 MF @33 V 5% Po l y
. 03 3MF@6 3V 5% MY LA R .25
. 07 1MF@3 3V 5% P o ly .2 5

.2 5
.2 5
.25
.25
.25

R ES IS TOR S

R1
R 2
R 3
R4
R 5

14C 2 9= 2 8 76
1 4C31 -8 20 6
1 4C28 -2 60 6
1 4C2 8- 82 05
1 4C2 8- 26 05 260 K l / 2 w 1 /2 %

2 .8 7M EG l/2 w 1%
8 .2 M EG l w 1% .75
2 .6 M E G l / 2 w 1 /2 % 1. 25
820 K l / 2 w 1 /2 % 1. 2 5

.7 5

1. 25

R ESISTORS
R 6
R 7

R8
R9
R 10 & 12
R ll
R 13
R14
R 1 6
R 1 7
R 1 8
R 1 9
R 20
R2 1
R 22

R 23

R 2 4

R 25

Hi V P r o be

SEM I CO ND U CT OR S

14C 28=8 2 0 4
1 4C28 - 260 4
1 4C2 8- 820 3
1 4C28 - 260 3
1 4A4 1- 11 93
1 4C29 - 108 2
1 4 B6 5- 7
1 4C 32 - 12 01

1 4A4 1- 80 02
1 4 A 4 1-253 2
14A4 1-80 0 1

14A4 1-25 3 1
1 4 A 4 1-800 0
1 4 A 4 - 253 0
1 4B 65 - 5
14B6 5 - 8
1 4 B65 - 6
1 4A6 3- 3 1485 ME G, 30 KV

82 K l /2w 1/ 2%
26K l/ 2w 1 /2 %

8. 2K l /2w 1/2 %

2. 6K l/ 2w 1/ 2 %
1 . 19K l /2w 1%
10 8 o h ms l/ 2 w 1%
. 7 o hm w i r e
12 o h m lw 1%

80 0 o hm 1 /2w 1%
25 3 o hm 1 /2w 1%
80 o h m 1 /2 w 1%
25 . 3 o h m 1 /2w 1%
8 oh m 1 /2w 1%

2. 53 o h m l/ 2w 1%
. 1 oh m wi r e
. 2 5 oh m wi r e
. 06 7 oh m wi r e

1.2 5

1. 25

1. 25

1. 25
.7 5
.75
.7 5
.7 5

.7 5
.75
.7 5
.7 5
.75
.7 5
.7 5
.7 5

. 75

T R 1, 4

T R 2, 3
T R 5
CR 1 , 2
CR 3, 4, 5
CR 6

CONT RO LS

R 3 6
R 39 , 4 2
R4 1
R 44 , 5 0

M I SC EL LA NEO US

51

52

19C1 1-1

19C1 4-1

19C4 - 1
50C5 - 1
5 0C3 -2
16S 1 0

1 5C7 - 6
1 5C7 - 2
1 5C7 - 15
15 B8 -1

23 C3 5
25 A135
25 A13 4
25 A1 3 2

3 9 A 2 8 -2

39 A 3 0

2N54 5 7 FE T
(sp ec i f y c ol or )
2N 4 2 4 8 P NP S i l i c on
2N 517 2 NPN Si l i c on
1N 4 5 6 D I OD E
1N 6 9 5 DIO D E

IN 40 0 4 REC T IFI E R

5 00K VERT P .C . M T.

10K V E R T P. C. M T.

20 0 OH M VERT P. C . M T.

2K PAN EL MT

6 " 2 00 uA M ETER
RA NG E SW I T C H
FUN C T I O N SWIT CH
MIN I SL I DE SW I T C H
FOR T E ST PRO B E

TE S T P R O B E C OM PLE TE
WIT H CA BL E F OR FE 2 0

HIGH VOL TAGE P ROB E

1. 2 5

.5 0

.5 0
. 5 0
.5 0

1. 0 0

.7 5
.7 5
.7 5
.7 5

25. 00

9. 9 5

8. 95
. 5 0

12.00

FORM 5 4 6

RED

LA ST R
LAST C

-VOLTAGE DEPENDS ON

FET SELECTION;

FE 20 SCHEMATIC

ISSUE DATE: 6-17-70

SENCORE INC. ~ FE20