Epson L90P - TM Two-color Thermal Line Printer Manual de usuario descargar pdf (Pagina 61)

Construction. Referring

to the photo.

mount BNC connectors J2

and J3 on the

top of an appropriate

-size

metal utility

box. Then

mount

any location that

convenient but will

not interfere

with

the components in the circuit. Parts

placement

is not

too

critical, but keep

component leads

short

as possible.

Mount

a chassis solder

lug

shown.

and

wire

the components together.

careful

observe

the proper

polarity

when

connecting D1 into the

circuit.

You can

use a

larger

utility

box

than

that shown

in the

photo

if you

want to

mount

the

meter in the same box

with

the

bridge circuit. In this case you could

eliminate the extra utility box and J1.

When the Wheatstone

Bridge

circuit

is fully assembled, place an arrow

the

top

the utility

box, pointing

from J2

Spare alternate

value resistor

is kept

handy

with

strip of electrical tape

(upper left

chassis).

toward J3 to

indicate

which

direction

the

r.f. is supposed to

flow.

(This arrow

shows

clearly

in the photo at the begin-

ning of

this

article.)

How

Use.

The

bridge

easiest

work

with

you mount it, the

GDO,

and

test

meter on a board

(see

photo on page

66).

After

mounting the

instruments,

in-

terconnect them with appropriate

r.f.

cable

and connectors, and place

the

GDO

and a pickup

loop

close enough

together

to obtain a full -scale

deflection

of the

pointer on

M1 (no

connection

to J3)

Temporarily

connect a 68 -ohm

carbon

resistor (a

52- or

-ohm resistor

ei-

ther of

these

values

was selected for R4 )

to antenna

jack J3. The full -scale

deflec-

tion should

drop to zero

indicate

the

null. And varying

the frequency

control

on the

GDO

should

not

disturb the null.

Now, remove

the

resistor

and

plug

your

antenna lead -in.

(This

must

be sin-

gle -ended coax

;

your

lead

-in is

twin

lead

cable, however, install

a Balun or

other transformer

arrangement to

con-

vert from

balanced to single -ended line.)

Vary

the

frequency

control of the

GDO;

null indication

should appear on MZ

in one

and

only

one position of the

con-

trol.

There

are two signs

trouble you

may

encounter

the null frequency

-an

off -frequency null requiring

the

retuning

of the antenna

system,

and

null

that is

neither

sharp

nor

complete, an indica-

tion that the antenna

is reactive

to all

frequencies.

the

null

doesn't appear at the ex-

pected frequency,

tune

the GDO's sig-

nal on your receiver. This

will

give

you

closer approximation of the actual output

frequency of the GDO

than

indicated

the GDO

dial.

Then,

from

the

receiv-

er's dial,

you

will be able to determine

whether the antenna system nulls

higher or lower frequency

and, conse-

quently, which

way to

tune the antenna.

For a

first

approximation,

increase

or de-

crease the antenna

length

by the same

percentage

that the

frequency is high

low, respectively.

The shallow null may be a more diffi-

cult problem to deal with.

this case,

first examine the antenna system for

poor workmanship, corroded contacts

and joints,

out -of- parallel open -wire lead -

in, and

large

wire

loops

that

might

affect

transmission

line characteristics. Make

certain that

neither of the antenna

ele-

ments

is nearer

large

physical ob-

ject than

the

other

is.

The capacitive or

inductive

some

nearby object might make

it neces-

sary to unbalance

the

antenna physically

to obtain an

electrical balance. It's

all

right

if one

element is shorter

than the

other when

you're

finished

-just

so the

antenna

system works properly.

Finally,

when

your

antenna system

provides you

with

good readings,

take

notes on the

way

you performed

your

tests and

how

you set up the test condi-

tions.

Then, periodically recheck your

an-

tenna system. You'll be

surprised

how

often you

discover deterioration. -3@-

April,

1969

1 2 ... 56 57 58 59 60 61 62 63 64 65 66 ... 115 116

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