Epson L90P - TM Two-color Thermal Line Printer Uživatelský manuál stáhnout pdf (Strana 67)

propriate

hardware

at each

corner. Drill

a

hole in

one end of

the chassis

to accept

a small rubber

grommet.

After

tying

them in

a

knot

to provide

a strain relief,

pass

the three

test

leads

from

the smaller

board through

this

grommet.

Attach a

small

alligator clip

to the

end of each

lead.

Use a black lead

for ground,

red

for +

and

another

color

for

trigger.

Assemble

the cover on

the metal

chas-

sis.

Using

some type of marker,

identify

each

spring

clip on the metal

lip adja-

cent

to

it,

as

shown

in

the photograph.

Note

that

pins

4

and 11

are missing

since

they

are connected internally.

In-

Circuit

Tests. To

check IC's

on

a

finished

board, apply

the

required

d.c.

power

to the board

( usually

+

3.6

volts)

and introduce

a trigger

signal. If you

have

no trigger

source available,

connect

the

black lead

of the test fixture

to

the

PC

board

ground

and the

red

lead

to

+3.6

volts.

Connect the test fixture

out-

put lead

to the PC board's input

terminal.

Switch Si

can be in either

the

LOW or

HIGH

frequency position.

Connect

the black lead from

the probe

to the PC

board ground

and the red

lead

to

+3.6

volts. The

probe lamp

should

be on. Check for the presence

of

+3.6

volts

at

the IC

(usually

pin

11

of

the in -line

type

and

pin 8 of

the TO -5

can). When

the

probe

makes contact

with

+3.6

volts, the lamp

should go out.

If it doesn't, check

back along the foil

pattern

and

locate

any break.

Note

that,

Interior of a completed

test

set.

The

three

leads

(one

for

positive, one

for ground,

and

one

for trigger output) are

knotted to provide

a strain

relief, before

being

passed

through the

rubber grommet.

April, 1969

SI

HOW IT

WORKS -PROBE

Transistors Q1

and 02 form

a high -gain

cur-

rent

amplifier using R1

to limit the input

base

current to

Q1 and prevent loading

of the IC

being

tested. When Q1 is cut

off, with the input either

grounded

or

left floating,

current

through R2

saturates Q2. Resistor

R3 reduces

the voltage

supplied to lamp 11 when Q2

saturates.

When the

input to Q1 exceeds

about

+0.6

volt.

Q1

conducts

and removes the base

drive

from Q2.

cutting

off this stage and

extinguishing 11.

Since most RTL (resistor -transistor

-logic)

IC's

require

more than

0.8

volt to

guarantee turn on

and less

than 0.46 volt to turn off, the

0.6

-volt

threshold of the IC Telltale falls

in the correct

place

to indicate the state of the input

or output.

when using the probe, it isn't necessary

to

watch the

lamp directly

as it is in

your line of vision

when

your

attention

is

on the probe

tip. Since the lamp

stays

on

when the probe tip is grounded,

it is

also possible

to check the ground

pat-

tern of the foil.

Once

it has

been

determined that

the

positive

d.c. and ground are correct,

place

the probe tip

on

the

signal input

terminal and observe that

the lamp

blinks on and

off

in step

with

the ap-

plied trigger signal.

It

is

easier to

see

the

lamp

blinking if Si is in

the LOW fre-

quency position. You can now

trace

the

trigger

signal

directly

to

the

IC

terminal.

When checking flip -flops,

observe

that

the signal

at

the output

(1 or Q,

0 or

Q) is

usually at a slower rate

than the

applied trigger. Using the probe

and a

schematic of the circuit

board, it is pos-

sible to trace

the path

of

the signal

and

SUPPORT

LEADS

(3)

QI

03

02

6

ON

EACH SIDE)

73

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