Tektronix 494AP service manual O o o o o O o o o o o o, CRT READOUTDAGRAM30

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Theory of Operation-

494A/4g4Ap

Servlce,Vol. 1

CR4078and CR40Z at the input to U4ogil,protect

2. CharacterCount€r-

to acc€ss the RAM and

the input against excessivevoltage excursions.

controlthe scan.

high-voftage

-Oy

The

osciilator is protected

enfteft

(d

?/

and R4074in case the +i00 V suppi

RgOTg,

shouldfait. Nor_

-

mally,CR1037is back biased.ti tne +t00V is not

3. CharacterGenerator to unblankthe crt b€am.

present,CR1037conductsand clampsthe input nega_

-

tive; the output of U4093swings negativeancle1073

4, DIA Converters to deflectthe crt beam.

remainscut

off. This circuit ensurei that e1073 witl

5. InstrumentBus Interface -

to store characters

be.ginto oscillateonly after

the 100V supplyr€achesa

and control the display. A

more detailed block

voltagesufficient

to sustainoscillation. ChiOzZ (in

drawingis provided

the

regulatoroutput

adjacentto Diagram

30.

fromexcessive

circuit) protects the base ot OtOZg

negativevoltage.

Forty characterscan be displayedper line,with up

to sixteen lines selected. Normally,up to three lines

are displayedwhile simultan€oustydisplaying

Z-Axis Clipper

the spec-

trum. When over three lines are to be displayed,the

This circuitconsistsof

diodesCR1056and CR1046,

spectrum display is disabled to keep the readout

refresh rate above60 Hz.

plus^associated

components.The 22iyac trom pin i

of T2065is couptedthroughC1O5gand R104gto the

junctionof GR1046and CR.I0S6The. regutatorcircuit,

that consists of VR1041,

R2050, R2O4d,

Readout-On/OlfTimlng. Charactersare written one

holdsthe cathodeof

and e204g

CR1046at approximatety

at a time. This allowsa portionof the spectrumto b€

*100 V

to 143,V,dependingon the settingof R2040. CR1O46

drawn between each character. Th€ character duty

and CR1056ctip the incoming 2llyac.

R2040 is

cycle is between10% and 257" becauseit varieswith

adjustedto completely

at minimum.

cut-offthe crt with Z-AxisDRIVE

the characterdrawn.The time sharingbetweencharac-

The voltagethat passesthe clippercircuit

ter writing and spectrumdisplay is pseudorandom

is coupledthroughCl091 to the Z-Axisrectifier.

reduce

the effect of

gaps

to

in the spectrum

movingthemon the trace.

displayby

_ The clipped Z-AXS DRTVEsignal is rectified by

CR2044and CR2046, which are

1ne

principte

com-

The readout-off

ponentsof the second section of the Z-Axis circuit.

time is set to 140 ps by one-shot

multivibratorUl055

(Figure 7-18). Ftip-flopUtO41B

The rectifiedvoltageis then fed to the grid of th€ crt.

assertsGENRUNNINGafter ul055 tim€s out, altowing

Cl041 couplesthe fast changesof drive-voltage

a character to be drawn. After a characteris written.

to the

gl griOto .speedup the responseof the grid circuit.

ROW 0 COL 0 resetsthe flip-flop,whichclocksoff time

Th€ crt

^gri^d^is

protected from

high_voltage

one-shotU1055. The ON control bit must have been

arcs by

neonsDS20S2,

asserted by the microcomput€rto get readout (as

DS2054,anOD$2057. RlO43 protect;

CR2046and CR2044,respectively,

describedunder InstrumentBus Interfacelater in this

surgesif the crt shouldarc.

from high-vottage

section).

cRT READOUT(DAGRAM30)

lf BLANK(MSBof the characterdata)is not set,the

GEN RUNNTNGflip-flopunassertsR/o oFF throughoR

The Crt Readoutassemblystores readoutcharac-

gate U20448;this switchesthe readoutdeflectionsig-

nals for the deflectionamplifierinputs (Diagram271.

ters and generatesdeflection

and Z-Axis signals to

BLANK can be set by the microcomputer

to load a

displaythosecharacters.tt also handlesthe frequency

space into the characterRAM so the readoutdoesnot

dot markerdisplay. Both charactersand frequencydot

use time for the spectrumtrace to scana blankcharac-

displaysaretime-shared

ter.

withthe spectrumtrace.

GeneratingReadout

CharacterScan. Althoughthe 8678 characterg€n-

Crt readoutis handledby sequentiallogic, clocked

erator lC, U2048,is often used in rasterscans,in this

at 3.41MHz, supptiedby the piocessor Loard. The

applicationit is used to write completecharacters,as

readoutcircuitry(Figur€7-17)is composed

of the fol_

shown in Figure7-19. A characteris drawn as a pat-

lowing€lements.

tern of dots in an 8 x I matrix wherEthe top row and

first three columnsare blank. These blankdots allow

1. Readout On

Timing _

RAM for character

for beam retrace and spacing. The idle position

storage.

betweencharactersis indicatedon the figure.

7-51