TRIGGEREDOSCILLOSCOPE

This easy-to-fo llow des ign lets you keep cost low by using a CRT of yourchoice. Its ope rat ional feature is a continuous zero basel ine.

DANIEL METZGER and DENNIS PERRY

....CDcoa

SOURCEFOLLOWER

03 0 5

RETRIGGERHOLDOFF1002

RETRACESUPPRESSOR

SWEEPGENERATOR

0 304IC301

triggering . A Schmitt trigger producessquarewaves in sync with the input sig­nal, and a differentiator produces sharpspikes from the edges of the square­waves. The negative spikes initiate a lin­ear ramp that always starts at the sameselected poin t on the input AC wave-

-...., r - BASELINE: GEN - - -',I'I ' BASELINE UJT I

TRIGGER I FLIP-FLOP BASELINE 'SYNCAMPLIFIER 'I 0 307 OSCI LLATOR J4r"--=.

020 1 , I 0 308 03 06 I0 20 2 LL_-_-_-_-_-_-_-_--=-.=~ _

SWEEP SYSTEM.------,

r-- - - -- --- - - - - - - - - - - - - - - - - - - ---,I VERTICAL AMPLI FIER I

: .<:::tr VERTICAL BASELINE SOURCE LOW-LEVEL I DgI P T SWITCH FOLLOWER DIFFERENTIAL .c:

I N U 0203 AMPLIFIER IV--I ATTENUATOR 0204 0 205 0206 , 0207 0208, 0209 ,L ---- -- J

HORIZONTAL AMPLIFIERIExTHOR - - - - - - - - - - :I ,--- --,I IN HIGH-LEVEL ,I @-o SOURCE DIFFERENTIAL II FOLLOWER AMPLIFIER I, 02 10 0211, 02 12 IL ...J

r--­II,,IIII SOURCE, EXT @--o FOLLOWER, TRIG 0 301L -,,...-__P..,OWER SUPPLY I

-900 VOLT ISUPPLY

0101 - 0 113 III NOTE: SERIES 10 0 COMPONENTS ON POWERI SUPPLY BOARD; SERIES 200 ON THEI AMPLIFIER BOARD AND SERIES 300 PARTS I~~ ~N.-:'~ E!. ~ND_B~E.:.I~ BOA'::D__ ...J

FIG. 1- BLOCK DIAGRAM of the zero-baseline scope. Operation is somewhat similar to a dual-tracescope with the baseline considered as the second tr ace.

proximately 3 ms each 15 ms, thus pro­viding a 1/5 duty-cycle baseline displayat a rate of abo ut 60 Hz-too fast for theeye to perceive the flicker.

A separate trigger amplifier is fed froma point ahead of the electronic baselineswitch to preserve continuity of sweep

ONCE A TECHNICIAN HAS EXPERIENCED

tro ubleshooting with a calibrated DClab scope, he'll probably want to keepthat scope probe close at hand most ofthe time he's at the service bench . Tran­sistor base-emitter voltages , collectorsat uration voltages, and IC logic levelscan be checked as easily as power-supplylines while the operating signals arepresent. No other instrument providesthat simultaneous readout of bias andsignal conditions. .

Two factors have conspired to keepthat scope probe out of the hands ofmost experimenters . The first is cost,which approaches $200--even for a kit.That problem is easily solved by sim­plified design . The scope described herecan be built from standard part s for$100, and considerably less if the junkbox is well stocked. Yet it boasts a 2­MHz bandwidth and 10mV-per-divisionvertical sensitivity.

The second factor is the annoyinglyfrequent need to lay down the probe,reach over to the scope, throw the inputswitch from DC to GRO UND, check theposition of the zero-volt baseline , andthrow the switch back to DC. That prob­lem is handled by incorporating a circuitthat prov ides a continuous display of theDC gro und level at a brightness levellower than that of the signal display.

How it worksThe operation of the scope as a whole

is be st understood from the block dia­gram, Fig . I. The vertical attenuator andamplifier provide a replica of the inputsigna l, both AC and DC, at the approxi­mately lOO-volt level needed at the de­flection plates of the CRT. The electron­ic baseline switch interrupts the signaland grounds the amplifier input for ap-

39

TO INPUTOF VERTICALAMPLIFIER

(Fig .31

TO INPUT OF.. HORIZONTAL

AMPLIFIER

(Fig.31

S402- b

.0 4

o .10.4

o I4

010

IMEGR404

HORIZONTALV/DIV

10

r ------TNPUT 1I RA CA~ FREO II (R405 COMP II TO 6·60 (C405I R4091 pF

cJ891I

I I

I RB(C410

I

I(R410 CB TO I

TO C4141 II R4141 II IL_____-:______ .J

ONE OF FIVE

o

o C404o 10pF

o

3 It RG-5B/U

S402-0

R402162K1%

R401B06K1%

R40340.2K

1%

C402100pF

EXTERNAL J2 r-- --<:--+-- SWEEPOUT'''!T----.--....-----.. IN~ FROM R325HORIZONTAL IN \0

J3 ~~~6PF.4" LINE EXT

~ r--+--4----;1;>2 S~3-b ~

V/DIV RA RB CB.0 4 IMEG

.1 604K 402K 62pF.4 909K lOOK 250pFI 953K 40.2K 625pF4 IMEG 10K .0025"F10 I MEG 4.02K .00621'F

RA AND RB - 1/2 WATTI % RESISTORS

C401.0 5" F

t-E60~AC

JIVERTICAL 0 DCINPUT -: S401-0

DCGND

OPTIONAL XIO PROBE

C41510pF

form . That ramp is applied to the hori­zont al amplifier to produce the cali­brated time sweep . A source-followerprovides a low output impedance for theramp, an d an op-amp comparator holdsoff further triggering signals until theramp voltage returns to zero.

An au to-trigger circuit senses whenthe Schmitt trigger is not switching andimmediately applies a voltage to theramp genrator commanding continuousramps, thu s providing sweeps for thedisplay of DC voltages.

A UJT baseline osc illator running atapproximately 60 Hz is synchronized tothe swee p generator to insure that theswitching from signal to baseline willalways occ ur during a retrace of thesweep. The baseline flip-flop drives thebaseline-switch ing FET' s at the input ofthe vertical amplifier.

The CRT cathode is operated at -900volts to accelera te the electron beam to­ward the CRT face. Deflection sensitiv­ity and hence calibration depend uponthat voltage, so it is regulated by a stringof l80-volt Zener diodes. Vertical andhorizontal pos ition and sweep time de­pend upon the 9-volt supplies, so theyare transistor-regulated. The + ISO-voltsupply serves only differential amplifi­ers, and thei r inherent common-moderejection makes regulation of that sup­ply unnecessary. We shall now proceedto a detailed description of each func­tional bloc k.

Vertical atte nuator: Voltage dividersRA and RB (Fig. 2) reduce the inputsignal to a maximum of 0.32 volt (8 divi-

FIG. 2.-THE ATTENUATORS. Component s for the verti cal attenuator are mounted on a specia lcircuit board. Also shown in the diagram of the option al mult iplier probe.

(/)ozoa:t;w...JW

6o«a:

40

PARTS LIST (Attenuators, Fig. 2)Resistors 1% to lerance or better, 1f2 wattR401-806,OOOohmsR402-162,000 ohmsR403, R412-40,200 ohmsR404, R408, R40&-1 megohmR405--604,OOOohmsR406-909,OOOohmsR407-953,OOOohmsR410-402,000 ohmsR411-1 00,OOOohmsR413-10,OOOohmsR414-4,020 ohm sR415*-9 mego hmsCapacitorsC401- .05)JF, 600 volts, ceramicC402-100 pF, MylarC403, C405-C414, C416*-6-60 pF

cera mic trimmerC404, C415*-10 pF, ceramicC410-62 pF, micaC411- 250 pF, micaC412---620 pF, micaC413-.0022)JF, MylarC414-.0062 jJF, MylarS401-miniature double-pole 3-position

togg le switch (Alco MST205T)S402-3-po le, 6-position rotary wafer

sw itchR403-2-po le, 6-posit ion rotary wafer

switc hMiscellaneous: printed circuit board*Note: Components required for optional

x 10 probe

sions at 0.04 volt-per-division) or a mini­mum of 0.0 I volt (I division at 0.01 volt­per-division) . Capacitors CA and CBswamp out stray capacitances to keepthe reactive division ratio exactly equa lto the resistive division ratio at high fre­quencies . AI--4-10 step-sequence per­mits coverage of the 10-mV to 10-volt­per-division range with two poles of astandard six-position switch .

Vertical amplifier: The overall gain ofthe vertical amplifier (Fig. 3) is about2000 in the full-gain (-;-4) position , andabout 500 in the calibrated (x I) positionof the vertical variable control. ResistorR20 I and D20I provide input protectionin the event of accidental overload.Source-follower Q201 and common-baseamplifier Q202 form a trigger amplifierwith a non-inverting AC gain ofabout 40and a high input impedance.

Transistors Q203and Q204are switchedon alternately by the zero-baseline flip­flop (Q307 and Q308, Fig. 4), con­necting the base of source-follower Q205alternately to the signal input and toground. The stray capacitance of theseFET's amounts to about 10pF, and pro­duces switching transients of about 10~S duration on the I-megohm input line.

TOP VIEW ·of the sco pe. The ampli fier board isbeneath the CRT. The power-sup ply board is atthe rear near the transformers mounted on theback pane l. The sweep board is up front near theco ntrols. The att enu ato r board, with its five trim­mers, is on a bracket held by the vertical-sensi­t ivity control. Astigmatism control is on rearpanel near base of the CRT.

The switching frequency must thereforebe held below a few hundred hertz toprevent those trans ients from being fre­quent enough to be seen on the CRTdisp lay .

Transistors Q206 and Q207 are wiredas a variable-gain differential amplifier.Potentiometer R213 is the IO-mV cali­brator and sets the gain to four times theindicated vertical sensitivity with R214

+9V +9V +9V +150V0201

0MPF4393 C202 0202R204

10O"F3.3K TO

G S 2N4400 S403-0 INT+1 TRIG R219

R202 10K2 .7K

R2112W

C201270n

R2121200pF 560n

VERTICALINPUTFROMATTENUATOR R201(FIG. 2) 330K

R221 R2220201 33n IK

IN914

R20533K

R207 -9V C203B 27K -9V B20

FROM R206 R217 pFSWEEP 33K R215 470nBOARD 470n(FIG. 4)

-9V +150VR20BL 27K

+9V R227 R228 -9V R224C204 47K 47K 6BOn330p IW IW +9V

C2050210 I.I"F -9V

HORIZONTAL R225 MPF4393330K 0 R231INPUT G 2.2K(FROM JFIG.2) S HOR DENOTES OFF

0202 R226 POS R232 PC BOARDIN914 2K 5K

HOR

C20G CAL R233

I·I"F 4.7K-9V

-9V

PARTS LIST (Amplifiers, Fig. 3)

FIG. 3-SCHEMATIC DIAGRAMS of the verti cal and horizontal deflection amplifiers. The latt er iscomparatively simple because its response is limi ted to the sweep frequencies.

at minimum resistance. Pot R216 is the40-mV calibrator. It adjusts the indi­cated sensitivity with R214 at maximumresi stance. Pot R218 is the DC balancecontrol; it sets zero voltage between thetwo emitters at zero input in order thatthe gain control will not shift the verticalposition.

Transistors Q208 and Q209 provide a

Resistors Y2 watt, 10%, carboncomposition, unless otherwise noted

R201. R225-330.000 ohmsR202-2700 ohmsR203--6800 ohmsR204, R209--3300 ohmsR205, R206-33,000 ohmsR207, R208--27,000ohmsR210, R211-270 ohmsR212-560 ohmsR213-20 ohms , trimmer, vertical mountR214-200 ohms , potentiometerR215, R217-470 ohmsR216--500 ohms, trimmer, vertical mountR218-100 ohms . trimmer. vertical mountR219, R22G--10,000 ohms , 2 wattsR221-33 ohmsR222-1000ohms, trimmer, verticalmountR223-1 000 ohms , potentiometerR224-680 ohmsR226, R23G--2000 ohms , trimmer,vertical

mountR227, R228--47,OOO ohms, 1watt

second stage of amplification, produc­ing a maximum differential output ofabout 180volts P-P. Capacitor C203 low­ers the impedance between the emittersto track the decrease in impedance be­tween the collectors caused by CRTplate capacitance at high frequencies.Since gain is essentially the ratio ofthose impedances, C203 tends to preserve

R229--3600 ohmsR231-2200 ohmsR232-5000 ohms . potentiometerR233-4700 ohmsCapacitorsC201-1200 pF, MylarC202-100~F, 15 volts, radial-lead

electrolyticC203-820 pF. micaC204-330 pF, micaC205, C206, C208-0.1 ~F, ceramic discC207, 1800 pF, MylarSemiconductors0201 , 0202-1 N914or similar silicon

diode0201,0203,0204,0205,021G--MPF4393

or similar N-channel FET (Motorola)0202-2N4402 or similar0206, 0207-2N4400 or similar0208,0209,021 1, 0212-2N3440 or

similar 'Miscellaneous: PC or perforated circuit

boa rd, hookup wire, mountinghardware , trans istor sockets, etc.

ZERO-BASELINE DISPLAY permits reading theDC component of this waveform. Scale factor is1 VIdiv and the sinewave is 3 volts peak-to -peakrid ing on a 4-volt DC level.

a constant gain as frequency increases.Because an 820-pF trimmer would belarge and unstable , we adjust the associ­ated resistor (R202) to suit the capacitor,instead of vice-versa . Capacitor C203thus determines the stage gain , andshould be altered if necessary to pro­duce a stage gain of about 50.

Horizontal amplifier: This amplifier(Fig. 3) is similar to the vertical amplifierexcept that the low-voltage differentialstage is omitted and the entire gain(about 70) is achieved in the high-voltagestage . The differential output voltage re­quired is about 250 volts P-P becausethe second (less sensitive) set of CRTdeflection plates is used. Bandwidth isabout 500 kHz.

...CDCDo

41

6

R313I K

0302 0303

\ I-e- lN914

3

S301-b

.9V

R312IK

03032N4400

R31 11.8K

TRIG •PICKOFF ~L---'__;;'.--_ .

( TO S302-0)

R310 10K

C304 4.7pF

SCHMITTTRIG

R30922.0.

R33627K

2

R326 R327 LM3 181.8K 3.3K RETRIG

HOLDOFF - 9V

D306 ·9V

IN9 14

R337 I C318 ®2,2K '=' .II'F

TO AMPR335 BOARD27K

0308 ( FIG.3)2N4400 ©

R3044.7K

R33110 K

.9V

JDENOTESOFF PC BOARD

R2b9l .9V-9V" 'W> ...9V

TRIG LEVELR306 R3084.7K IK

0302C303 10 0pF 2N4400

D307IN914

DIC30 1 6 1--+----------+'~H. 0305555 S MPF4393 lOOK

SWEEP LENGTH 1%~ r ,

II I I

i

- 9V

.9\,1

D304 · 9 V

IN9 14

C315680pF

R330220n

42

S301-c

D308 R333IN914 I ~

40104 1.4 50000 00

.Ims

- 9V

C30247pF

.9V

R329220n

E

C30 1 R301051'F 220K

OSC

R332270n

EXTHOR AMP~o 0

FROM TRIGPICKOFF .9V

- ~gC~~CT~R)AUTO

- .+ I '''·-f- I

I ' V I

5302-0 L _S~Q.5-,'

AC

R3 15I K

0305IN914

R31627K

(0303 •COLLECTOR)

RETRACE nSUPPRESSION-------- ---<...-.......- - -;{-----.-- --- --,( TO FIG. 5)

.9V

R32856K

FROM0202COLLECTOR

INT TRIG&

LINE TRIGFROM ---<>RI08, FIG. 5 5403-0

EXT@JTRIG

FIG. 4- THE SWEEP and zero -baseline generator circ uits comprise the mos t complex secti ons of thein strument.

PARTS LIST (Sweep and zero-baseline generators, Fig. 4)

(J)oZoc:::I­oW....JW

6o-cc:::

Resistors 1/2 watt, 10% unless otherwisenoted

R301-220,OOO ohmsR302-1 megohmR303, R304, R306-4700 ohmsR305, R311-1800 ohmsR307. R317-20,OOO ohms, potentiometerR308,R312,R313,R315,R333--1000

ohmsR309-22 ohmsR310, R318, R331-10,OOO ohmsR314-15,000 ohmsR316, R335, R336-27,OOO ohmsR319-1 0.000 ohms, trimmer, vertical

mountR32Q-100.000ohms,1%R321-25,OOO ohms. 1%R322-10,OOOohms, 1%R323. R327-3300 ohmsR324-1 00.000 ohmsR325--5000 ohms, trimmer, vertical

mou ntR326-18,OOO ohmsR328-56,000 ohms

R329. R33Q-220 ohmsR332-270 ohmsR334, R337-2000 ohmsCapacitorsC301-.05 ~F, 600 vol ts. ceramic discC302. C304-47 pF, ceramic discC303, C307-100 pF, ceram ic discC305, C306-1 0 jJF.25 volts. axia l-lead

elect rolyticC308-4.7 jJF,25 volts. axial-lead

electrolyticC309-.01 jJF, ceramic discC310*-1IJF. MylarC311*-D.1 jJF. MylarC312*-,01 ~F , MylarC313*-.001 jJF. MylarC314**-100 pF, ceramic trimmerC315--680 pF, ceramic discC316-21JF, 25 volts . axial-lead

electrolyticC317, C318-{).1 ~F, ceamic discC319-51JF, 25 volts , axial-lead

electrolytic*Note: selectto keep ratios within ± 1%

**Note: In prototype, C314 was made byconnecting a 47-pF disc in parallel with a6-60-pF ceramic trimmersemiconductorsD301-D308-1N914 or similar silicon

diodeIC301-555 timerIC302-LM318 op-arnp (National)0301 , 0305--MPF4393 or similar

N-channel FET (Motorola)0302,0303,0307, 0308-2N4400 or

similar0304-2N4402 or similar0306-2N4871 or similar uni junction

transistorS401. S403--see attenuator parts listS301-3-pole. t t-posltlon rotary wafer

switch (Centralab PA-10009 or equal)S302-2-pole. 4-position rotary switchMiscellaneous: PC or perforated circuit

boa rd, shielded cable, transistor and ICsockets, mounting hardwa re, knobs.etc .

42

010204001

L-__"'-:::'-- YERT'------ -'-- HOR

l-+-_-,I.=.O- HOR

C104 •

.I"F Rill *I KY 47K

( ~.

t-IMr-~_lINETRIG

RI09 (TO S403 -0)=15K

R102.10 MEG

ASTIG

Dill180YDII2180YDII3180Y

CI03

I .I " FIOOOY

DI08-e- 39V

IN5259

DI09180Y

DIIO180Y

CI02

I ·I" FGOOY

-e- (G)

IN5280

DII4IN4007 RI05

680il

+ClOG40l'F I20 0 Y

DIOI-IOGIN4007

L-*"_r-'W¥-_--_- +150 Y

RIOID106· DI05 470K

DI02DIOI

6.3 YIA

250YCT25MA

DII8IN4001

0119IN4 0 01

.---I!------~;)___1W___-_--< )......---.-~ + 9YTI0224 YCT

1/ ~MAL- --J

* MOUNTED ON TERMINAL STRIP NEARNECK OF CRT

J DENOTES OFF P C BOARD

j DENOTES PC BOARD TERMINAL

FIG. 5-THE POWER SUPPLY is simple and inexpensive to bu ild . The voltage-trip ler repla ces theexpensive and dangerous high-voltage pow er transfo rmer used in many scopes. The string of Zenerdiodes replaces a high-res istance voltage-divider string.

»\l:Dr....<0cea

tive pulses from C307. In the AUTO

mode , AC detectors D302, D303, andC306 furnish the positive supp ly as longas the Schmitt trigger is switching. How­ever, if the trigger remains inope rativefor longer than about 150rns, C306 dis­charges and R316 pulls the trigger inputlow, resulting in automatic triggeringwith no input signal.

Sweep circuit: Transi stor Q304 is a vari­able-current source that charges the se­lected timing capacitor (C31O throughC3 14) at a linear rate depending on thesweep variable control and the selectedtimin g resistor (R320 through R322). Pin7 of the NE555 automatically disc hargesthe cap acitor whenever pin 6 rises to +6volts. A so urce-followe r Q305 buffersthe ramp since any current drawn fromit would des troy its linearit y. Pot R325redu ces the ramp to 4.4 volts, thus pro­vidin g II divisions of sweep to the 0.4vo lt-pe r-division horizontal amplifier.

continued on page 80

SWEEP LINEAR ITY is evident in this photo of aSOO-kHz triangle waveform at t mY/div.

output (inverted or non-in verted) arecoupled through switch S302-a capaci­tor and C307 to the trigger input of theNE555, IC 301; that is where the nega­tive edges are used to initiate thesweep ramps.

In the DRI V EN mode , the trigger inputis held high by the + 9-volt supply throughR314 and triggering occurs only by nega-

PARTS LIST (Power supplies, Fig. 5)

010 7-1 N5242 Zener diode, 12 volts , 500mW

0108-1 N5259 Zener diode, 39 volts, 500mW

0109 -0113-1N5280 Zener diode , 180volts, 500 mW

0115 ,0116,0118,0119-1N400101 17, 0 12G-1N5240Zenerdiode, 10

volts , 500 mW0101-04101 (GE)or similar PNP silicon

transistor0102-04001 (GEl or similar NPN silicon

transistorF101- 1/8-amp fusePL101- neon pilot-light assem bly (NE-2

lamp with 68K resistor)S10 1-SPSTswitch (part of R103)T101-power transformer, 250 volts

center-tapped, 25 rnA; 6.3 volts, 1 amp.(Stancor PS-8416 or equ ivalent)

T102- power transformer, 24 volts center-tapped, 100 rnA (Stancor P-8395orequ ivalent )

Miscellaneous: Fuse holder , line cord , PCor perforated circu it board , hookupwire, te rminal strip , MuMetalsh ield forCRT, CRTsocket , transistor soc kets ,etc .

Resistors Y:z watt, 10% carboncomposition unless otherwise noted

R101-470,OOO ohmsR102-10 megohmsR103-500,000 ohms poten tiometer with

SPSTswitchR104-10 megohms, potent iomete rR105-680 ohmsR10~100,OOO oh ms, potentiometerRl07, R11G-2200 ohmsAl08-56,OOO ohmsR109-22,000 ohmsR1 11-47,OOOohmsCapacitorsC101. C102-Q.1 uF, 600 volts, tubularC103, C104-Q.1 uF, 1000volts, tubularC105-.22 uF, 1000 volts, tubu larC106, C107-40 uF, 200 volts , axial-lead

elect rolyticC108, C110-470 uF, 25 volts, radial-lead

e lect rolyticC109, C11l-Q.1 uF ce ramic discV1-CRT, 3RP2 was used in prototype.

3EP1 ,3RP1,3BP1 and3ACP11 can beused . See text.

semiconductors0 101-0106,0114-1N4007

Trigger circuits: Source-follower Q30 I(Fig . 4) provides the high input impe­dance required du ring ex ternal trigger­ing, and the low driving impedance neces­sa ry for good sensitivity of the Schmitttrigger, Q302 and Q303. The trigger willoperat e with 0.1 volt Pop input , and trig­ger up to 5 MH z with 0.3-volt Pop in­put. Th e edges of the Schmitt-trigger

43

lIMA ElECTRON ICS,DopU R">o W. WII>hi... Oklahom.C"" 0"'31" Box 735, Camp HIli, PA 17011·lJ.S.A. a..----------------------_..1 .- - - _ - - - - - - - --CIRCLE 44 ON FREE INFORMATION CARD CIRCLE 2 ON FREE INFORMATION CARD

~~/ Help light the ca>ld l, cf understand inq.

'I Conta ct your local chapt er of th eJ j Epilepsy Foundation of Ameri ca.~ Or write Epilepsy, Washington, D.C. 20036 .G( ! nro '1Hlrat" 19th,. lonn a Na tlUlla l Ep,I"1M!I l ..ragllt' . J

OSCILLOSCOPEcontinuedfrom page 43

The LM318 op-amp (lC302) is used asa voltage comparator, holding the trig­ger input of the 555 positive until thetiming capacitor has completely dis­charged. Premature triggering during re­trace is thus prevented. The 555 pro­vides a squarewave output at pin 3 thatgoes to +9 volts while the ramp is risingand drops to ground during retrace andhold-off. That line is capac itivelycoupled to the CRT grid to suppress thebeam except during the sweep.

Baseline generator: The retrace-sup­pression line is used via R330 to syn­chronize the zero-baseline oscillatorQ306, insuring that the switch frombaseline to signal display will always oc­cur at the start of a retrace when thebeam is suppressed. For the lower sweepspeeds, synchronization requires aslower oscillator, and for that C319 isswitched in.

Each time unijunction transistor Q306fires, C316 discharges through R332,setting flip-flop Q307-Q308 throughD308 and initiating a baseline sweep.After the baseline sweep (or severalsweeps if C316 is not discharged afterthe first one) pin 3 of the 555 goes low,bringing the base of Q307 low throughC315 and D307, thus resetting the flip­flop for a series of signal displays .

Power supplies: The power supp lies(Fig. 5) are entirely conventional exceptfor the - 900-volt tripler. Diodes D 105and D 106charge C 102 to the peak nega­tive voltage of the transformer secon ­dary on the negative half cycle. On thepositive half-cycle, C 102 and the sec­ondary appear in series to charge C 10Ito twice the peak secondary voltage(negative on top), through DI03 andD 104. On the next negative half-cycle ,C 101 and the secondary appear in seriesto charge C 103 to three timesthe peaksecondary voltage through D10I andD102. The drain on that supply is about200 uA , so the 0.1 /-IF Mylar filters arequite adequate. Some of those capaci­tors are used at 20% or so above theirrated voltage , but many have been test­ed at four times rated voltage with nobreakdowns. Any string of five to tenZener diodes adding up to about 900volts will do for DI09 through DI13 if180-volt Zener diodes are hard to find.Capacitor C 105 filters out the 6O-Hznoise picked up from the power trans ­former by the CRT heater winding.

We must breakoff our discussion ofthe oscilloscope's power supplies nowand will conclude it next month whenwe will also go into construction,checkout and calibration, R-E

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TRIGGEREDOSCILLOSCOPE

Part II-Construction details and calibration instructions for thelow-cost scope that features a continuously displayed zero baseline.

DANIEL METZGER and DENNIS PERRY

TOP VIEW of the scope. The amplifier board is beneath the CRT. The power-supply board is at therear near the transformers mounted on the back panel. The sweep board is up front near the controls.The atlenuator board, with its five trimmers, is on a bracket held by the vertical-sensitivity control.Astigmatism control is on rear panel near base of the CRT.

LAST MONTH WE DESCRIBED THE FEA­tures of this inexpensive DC scope andwent into detail on the operation of itsvarious circuits . We continue thismonth by resum ing our broken-off dis­cussion with suggestions on selectingthe CRT to meet your requirements.

ConstructionMan y types of CRT' s have been used

in this design , including 3BP l, 3EP I,3ACPII, 3FP7 , 3RPl, and 2AP I. Five­inch type s can be used , but whatever isgained in screen size will be lost insharpness of focu s. The 3RP IA and3WPI are especially nice because theyare flat-faced. The 3WPI has about twicethe deflection sensitivity of the others,and can be used to produ ce a scope with5-mV sensitivity. The CRT must be shield­ed with MuMetaI (nothing else willwork) unless the power transformerscan be located two feet from the CRT.Surplus houses that sell CRT's usuallyhave fully formed shields.

DISPLAY of a SOo-kHzsquarewave at 0.4 JlVIdlvshows a fast rlsetlme and clean squarewave re­sponse.

The vertical and horizontal outputwires must run straight to the CRT andbe kept away from eac h othe r and fromot her wiring and the chass is. The vert i­ca l and horizont al inputs should be keptshort and separa te from other wire bun­dles. The wires to and from the TRIGGERLEVEL switch carry fast squarewaves

and must be shielded to prevent cou­pling to other wires . The wires to theVERTICAL VARIABLE GAIN control shouldbe kept reasonably short. Other wiringshould be bundled and laced in the in- 5:terests of neatness. ?<

The input attenuator and sweep-tim- <0

ing resistors must be held to 1% if good ~

53

•

•

o

o

...J ...JIII III 0

TO B-L ONAMPL BOARD~

TO •ARMS OF S401-bAND S304-c

TOR317

TO ARM ,--..,S301 -b t

TO ARMS301-0

TO

........l..- .._ ..--..-----.. j;

o

o

•

•

FIG. 8-THE SWEEP-GENERATOR PC-board foil pattern. The pads along the top edge are forconnections to off-the-board components and leads to other circuit boards.

FIG. 9-HOW THE COMPONENTS ARE PLACED on the sweep-generator PC board. Note the posi­tions of the three jumpers.

FIG. 6-FOIL PATTERN for the attenuator usedin the vertical-sweep circuit.

FIVE 6-60pF TRIMMERS(C 405 - C409)

GND

TOS402b

r-1-1/81~

COPPERSIDE

S402-0

FIG. 7-COMPONENT PLACEMENT GUIDE forthe vertical-input attenuator. The precision re­sistors are on one side and the frequency-eom­pen sating capacitors are on the other.

COMP0NENTSIDE

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~------------ 4 - 3/8" -------------1~ TABLE 1 TROUBLESHOOTING CHART

NOTE: CONNECTION TO HORIZONTAL DEFLECTION PLATE SHOULD BE MADESO THAT BEAM SWEEPS FROM LEFT TO LIGHT ; VERTICAL DEFLECTION PLATESSHOULD BE CONNECTED SO THAT POSITIVE INPUT TO VERTICAL AMPLIFIERPRODUCES UPWARD DEFLECTION OF BEAM .

FIG. 11-THE DEFLECTION-AMPLIFIER board has three jumpers and six trimmers for circuit calibra­tion and adjustments. Leads to CRT deflection plates should be as short as practical to minimizestray capacitance.

s::>­-<coceo

55

Vertical: 50mV P-P, 1kHz sineware input ;R214 at min resistance, S401 at OC

Horizontal: 2V P-P, 1kHz sineware to EXTHOR, S403 at OAV/OIV

Sweep: 2V pep, 1kHz sinewave at EXTTRIG;+ SLOPE, AUTO, OAms/DIV, OC GNO

Power Supply - 100 Board

TEST VOLTAGE POSSIBLEPOINT DC AC CAUSE

pop

C103 - 1100 40 0101 thru 01060107-0113

0107A - 950 <1 C105, T101, CRTC107 + 145 <1 0114 , C106C108 - 19 0.6 0115,0116C110 + 18 1 0118,01190101E - 904 <5m 0117 ,01010102E +904 <5m 0120, 0102

Trig & Vert Amp - 200 Board

+ 1 to0201S + 2 =5m0202C + 5 -20205G 0 50m 0201,0205

+ 1 to0205S + 2 45m 02050206E Follows 0.6V below 0205S

+ 0.5 to0207E + 1.5 = 0 R2180206C0207C =4 1.0 0205, R215, R2170208C0209C 75 50 0208, 0209, R224

Horiz Amp s- 200 Board

+ 1 to0210S + 2 1.5 0210.02020211C0212C +75 60 0211 , 0212, R229

Sweep - 300 Board

0301G 0 2 0301 , D301+ 1 to

0301S + 2 1.8 030103028 1 to - 3 Var ied by R307

0302C0303C + 1 to +9 SOR 0302,0303C306 + 8 10 D302,030303048 +7.3 to+8.5 Varied by R317R320-R322 + 1.5 DC across RT AT CALIC301pin 6 o to '+6RAMP 03040305S . L 1 to +7 0305

RAMPIC302Din 6 + 9 to -9 SOR IC302, R327IC301nin 3 o to + 9 S0 R IC3010306E o to 7 RC Charge; 0306, R278030681 + 5 SPI,KE . 0306Q307C0308C o to + 9 S0 R 0307,0308

A=anode 8 =base C=collectorE=emitter G=gate S=source

o

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TOVERTDEFLPLATES

TO+150V

TOR223

TOS403-0

MD3VERT-HOR

AMPSD.PERRY

5/78

TOTO+'iN

+ 150

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FIG. 1o-PRINTED-CIRCUIT FOIL PATIERN forthe board for the horizontal and vertical deflection elrcults,

MD3POWER SUPPLIES

D.PERRY5/78

j ....J

..

0 ~\\\\\\FOC tN T tXfT CT +9

FIG. 12-THIS PRINTED-CIR­CUIT PATTERNsimplifies con­struction of the power supply.

FIG. 13-POWER-SUPPLY COMPONENT LAYOUT is simple. Be careful; some of its voltages aredangerous.

CalibrationVertical: First display a 2oo-kHz

squarewave and adjust high-frequencycompensation control R222 for sharpestcorners with no overshoot. With rangeS402 at 1 V/div and variable R214 atmaximum resistance, inject a 2.12 voltRMS (6.0 volts p oP) loo-Hz sinewave,and adjust R216for a six-divisiondisplay.Now change the range to 4 V/div and,with variable R214at minimumresistance(74), adjust R213 for a six-division dis­play. With the input grounded, adjustR218 so the trace remains stationary asR214 is rotated. The final step is to dis­playa I-kHz squarewave, and on eachof the ranges from 0.1 to 10V/div adjustthe corresponding trimmer capacitorfor the best squarewave with no round­ing or overshoot.

Horizontal: With the horizontal atten ­continued 01/ page 110

boards. The foil patterns for the attenu ­ator, sweep circuits, deflection amplifi­ers, and power-supply PC boards are inFigs. 6, 8, 10, and 12, respectively. Thecomponent layouts for those circuitboards are in Figs. 7,9, II,and 13.

Initial checkoutA spot can be focussed on the screen

with only the power-supply board andCRT circuitry wired in. The 9-volt sup­plies will each need a temporary 470­ohm load if they are to be tested at thispoint. Now disconnect the primary ofT 10I to disable the high-voltage sup­plies while the sweep and low-level am­plifiers are tested. The troubleshootingchart (Table I) shows the voltages to beexpected at various test points. Oncethe Schmitt trigger, sweep generator,baseline generator, and low-level ampsare determined to be functioning, thehigh voltage can be reconnected.

BI ,

E~B22N4871

( MOTOROLA)

3 TO 4CRT

E~B' .- "\+"" ,

C2N3440

( MOTOROLA)

0101 0102 ~ 0103~ ----.- T---CIOI--

---CI03 - - -

0105 0106--.+-- ----.I-

to be used, the fixed frequency-compen­sating capacitors (CB) must be held to5% tolerance.

The scope is constructed on four PC

IRI05

I

2N4400 a2N4402

( MOTOROLA a GE )

QIOI

~C-RI07-

+- - -CI07- -

QI02

C~- RIIO-

01201 10117:'CIIO- l f +-C I08-

CI06 - - - CI02- --CIII- -CI09- + I

~w 1Jt~t --R:~:8_ '''1-"oJ+~+\~+1~~

.J I Tl ·,~ 1 -AMPLIFIER S403-0 DEFL INTEN FOCUSAND SWEEP AMPS '::-::'M-......~:c-='

CIRCUITS~ -9V TO RI03 RI04

T I02 AMPLIFIERS~SECON DARY AND SWEEP TI0 2

CIRCUITS SECONDARY

TIOICENTERTAP

,Y :1C

LEADS CAN BE MPF4393FORMED TO A ( MOTOROLA)TO-5 PINCON FIGURATION

0400- 80410(GE l

PHYSICAL OUTLINES AND PINOUTSfor the discrete devices used In the scope as active circuits. Beespecially careful with the installation of the look-alike plastic devices.

calibration accuracy is expected. Thetiming capacitors must at least be in thesame ratio, so if one is 7% high, strive tomake them all 7% high. If a x 10probe is

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BUILD AMASTERPIECE OF SOUND

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OSCILLOSCOPEcontinued from page 56

WARC-'79continued fro m page 109

fixed and broadcast satellites that willbe able to operate in the 12 GHz por­tion of the spectrum in the westernhemisphere.

The 11.7- to 12.I-GHz band will beall ocated to the Fi xed Satellite Service(space-to-earth) shared w ith other ser­v ices; the 12.0- to 12.7-GHz band willbe allocated to broadcasting and broad­casing satellites, shared with other ser­vices . The specific frequencies to beassigned to the broadcasting satell iteservice will be allocated at a Conferencescheduled to be held in 1983. That willbe followed by a general satell i te con­ference as mentioned above.

The oveniding issue of the 80's willbe the movement on the part of de­veloping countries to plan the assign­ment and use of orbital slots and fre­quencies in such a way as to assure allcountries an equal "sli ce of the pie."WARC-79 did not address that issuecompletely, but only deferred it to sub­sequent space conferences which willbe held in th is decade. Major batt les onthe issue of equal right s may loomahead. R-E

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