RCA BA-13A Manuel utilisateur

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BROADCAST

AUDIO

EQUIPMENT

IB-24712

Type

BA-13A

Program

Amplifier

RADIO

CORPORATIOn

OF

AfACRICA

cnGinecRinG

products

DCPnRimcnT

cnmDcn,

n.

j

.

BROADCAST

AUDIO

EQUIPMENT

INSTRUCTIONS

Type

BA-13A

Program

Amplifier

MI-11233,

-A

RADIO

CORPORATION

OF

AMERICA

ENGINEERING

PRODUCTS

DEPARTMENT,

CAMDEN,

N.

J.

Printed

in

U.S.A,

ui

18-530

Figure

1

—

Front

View

of

Type

BA-13A

Progr m

Amplifier.

1

DESCRIPTION

TECHNICAL

DATA

Power

Suppl

Required

100

to

130

volts,

50

to

60

cps,

5

5

atts

Fuse

Rating

1

ampere,

Type

MDL

Source

Impedance

600

or

1

50

ohms

Input

Impedance

Matching

600

or

150

ohms

(approximate)

Bridging

20,000

ohms

(approximate)

Rated

Output

Level

+

30

dbm

ith

0.5%

total

rms

distortion

from

50

cps

to

15,000

cps

+

30

dbm

ith

less

than

1.0%

total

rms

distortion

from

30

to

15,000

cps

Connections

T o

10-prong

plugs,

mounted

on

the

rear

panel

of

the

chassis,

hich

engage

ith

t o

receptacles

provided

ith

the

amplifier

Dimensions

and

Weight

Length

—

14

inches

Width

—

8

inches

Height

—

7%

inches

Weight

—

17

pounds

Noise

Level

Less

than

—

52

dbm

ith

the

gain

control

in

the

maximum

position

and

the

input

and

output

circuits

terminated

in

600

ohms

Mounting

Shelf

mounting

in

the

Type

BR-2A

Panel

and

Shelf

Finish

Light

umber

gray

except

for

output

and

po er

transformers

hich

are

satin

black

Isolation

90

db

ith

matching

input

100

db

ith

bridging

input

Gain

Measured

at

1000

cps

ith

gain

control

in

maxi*

mum

position

Matching

Input:

65

db

rt

1

db,

600

or

150

ohm

source

to

a

600

or

1

50

ohm

load

Bridging

Input:

28

db

from

a

600

ohm

terminated

line

to

a

600

or

150

ohm

load

(2

atts

output).

The

amplifier

has

a

plug-in

type

chassis

hich

allo s

easy

removal

from

shelves

or

racks

for

servicing

or

interchanging

units.

The

Type

BA-13A

amplifier

consists

of

three

stages

of

amplification

ith

a

self-contained

po er

supply.

A

po er

s itch

and

an

a-c

line

fuse

are

mounted

on

the

chassis.

Provision

is

made

for

controlling

the

gain

of

the

amplifier,

for

adjusting

the

hum

to

a

minimum,

and

for

checking

the

operation

of

the

individual

tubes.

The

gain

control

The

Type

BA-13A

Program

Amplifier,

MI-

11233,-A,

is

a

high-fidelity,

high-gain,

lo -distor



tion

amplifier

designed

to

raise

the

output

of

pre



amplifiers

or

mixers

to

a

level

suitable

for

feeding

broadcast

telephone

lines

or

radio

transmitter

inputs.

It

may

be

used

as

a

bridging

amplifier

for

isolation

or

other

applications

here

a

20,000-ohm

amplifier

input

is

desirable.

The

Type

BA-13A

may

also

be

used

as

an

emergency

monitoring

amplifier

Frequenc

Response

±1

db

(referred

to

1,000

cps)

from

30

to

15,000

cps

Measured

from

either

a

600-ohm

or

a

1

50-ohm

source

to

a

600-ohm

load

ith

either

bridging

or

matching

inputs

Tube

Complement

(MI-11266)

2

RCA

1620

1

RCA

1622

1

RCA

5Y3-GT

When

increased

hum

distortion

and

microphonics

can

be

tolerated,

the

follo ing

tube

complement

can

be

substituted:

2

RCA

6J7

1

RCA

6L6

1

RCA

5Y3-GT

Maximum

Input

Level

Matching

Input:

+

11

dbm

for

1.0%

total

rms

distortion

from

30

to

15,000

cps

(output

+30

dbm)

+

10

dbm

for

0.5%

total

rms

distortion

from

30

to

15,000

cps

(output

+18,

dbm)

Bridging

Input:

+

30

dbm

for

0.5%

total

rms

distortion

(output

+

18

dbm

)

Load

Impedance

(tapped

transformer)

600

ohms

balanced,

taps

for

150,

15,

7.5

and

5

ohms

2

VDOV

--

2

yosr

u

zz

I

41

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ww

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Figure

2

—

Schem tic

Di gr m

of

Type

BA-1

3

A

Progr m

Amplifier.

I

fi

0

0

oj>

0

0,

OQ

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3

INSTALLATION

INPUT

RECEPTACLE

J-1

Terminal

No.

Connection

Receptacle

Connections

The

connections

to

the

t o

10-prong

receptacles

hich

mate

ith

the

amplifier

plugs

are

sho n

in

the

follo ing

tabulations:

No

connection

No

connection

Bridging

input

Bridging

input

Output

of

bridging

pad

Test

meter

(4-

)

Output

of

bridging

pad

Amplifier

ground

Matching

input

Matching

input

3

4

5

6

7

8

9

1

11

12

Power

Transformer

Connections

The

po er

transformer

T-3

is

connected

for

operation

from

all

5-volt

a-c

po er

line.

If

the

line

voltage

is

normally

120-130

volts,

disconnect

the

red-and-black

ire

from

terminal

number

11

on

T-3

and

connect

it

to

terminal

number

12.

If

the

line

voltage

is

normally

100-110

volts,

disconnect

this

same

red-and-black

ire

and

connect

it

to

terminal

number

10

on

T-3.

Refer

to

figures

2

and

8.

ith

the

shelf

assembly

(MI-

11599,

-A)

by

means

of

four

of

the

No.

6-32

fillister-head

machine

scre s

furnished

ith

the

shelf.

Mount

the

receptacles

and

brackets

in

the

correct

spaces

on

the

shelf

ith

six

of

the

No.

8-32

round-head

machine

scre s

and

lock ashers

supplied

ith

the

shelf.

Install

the

receptacle

cover

or

shield,

according

to

instructions

for

the

Type

BR-2A

Panel

and

Shelf,

contained

in

IB-24411

after

the

receptacles

have

been

ired

according

to

the

follo ing

information:

Attach

one

of

the

guide

bars,

furnished

ith

the

shelf,

to

the

shelf

on

each

side

of

the

space

hich

is

to

be

occupied

by

the

amplifier.

Use

t o

No.

4

self-tapping

scre s

also

supplied

ith

the

shelf,

to

mount

each

guide

bar.

Holes

are

provided

in

the

guide

bars

and

in

the

shelf

for

this

purpose.

The

square

ends

of

the

bars

should

be

to ards

the

front.

Slide

the

amplifier

chassis

onto

the

shelf

so

that

the

plugs

fit

into

the

receptacles

and

the

hooks

of

the

ejector

handles

fit

into

the

square

holes

in

the

shelf.

Push

back

on

the

ejector

handles

to

obtain

good

contact

in

the

receptacles.

Install

the

panel

by

slipping

it

over

the

control

shafts

of

the

amplifier

and

attaching

it

to

the

shelf,

by

engaging

the

panel

latches

on

the

rear

at

each

side

of

the

panel.

Adjust

the

spring

catches

on

the

top

section

of

the

panel,

for

minimum

and

equal

snap

action.

This

adjustment

may

be

made

by

loosening

the

No.

6-32

hex

nuts

holding

the

catches

and

moving

the

catches

up

or

do n.

Close

the

top

section

of

the

panel

and

place

the

knobs

on

the

control

shafts.

Should

it

become

necessary

at

any

time

to

remove

the

amplifier

for

servicing,

first

remove

the

knobs

and

panel.

Then,

pull

for ard

on

the

handles

and

slide

the

amplifier

out.

Electrol tic

Capacitors

Plug

the

four

electrolytic

capacitors

into

the

red

sockets

provided

for

them

on

the

amplifier

chassis.

Carefully

check

each

capacitor

as

it

is

installed

to

ascertain

that

the

ratings

marked

on

the

capacitor

cover

correspond

to

the

values

stenciled

on

the

chassis,

adjacent

to

the

sockets.

Tubes

With

the

po er

s itch

in

the

OFF

position,

install

the

tubes

in

the

correct

sockets.

The

tube

type

for

each

socket

is

stenciled

on

the

chassis

near

the

socket.

Mount

the

t o

grid

caps

and

t o

grid-cap

shields

on

the

1620

tubes.

Each

shield

must

make

a

good

electrical

connection

to

the

shell

of

the

tube

to

insure

noise-free

operation.

is

mounted

on

the

left

side

of

the

amplifier

front

panel

and

the

meter

s itch

on

the

right,

see

figure

1.

The

hum-adjustment

potentiometer

is

mounted

on

top

of

the

chassis,

in

front

of

the

output

trans



former,

see

figure

3.

The

MI-11233

and

MI-

1

1233-A

units

are

similar,

except

that

the

former

uses

a

dual

step-type

attenuator

gain

control,

and

the

latter

uses

a

dual

continuous-type

attenuator.

Mounting

The

Type

BA-

13

A

Program

Amplifier

is

designed

for

mounting

on

a

Type

BR-2A

(MI-1

1598,

-A

and

MI-

11

599,

-A)

Panel

and

Shelf.

The

gain-control

shaft

is

on

the

left

side

of

the

amplifier

(front

vie )

and

the

meter-s itch

shaft

is

on

the

right

side.

Mount

the

dial

plates

furnished

ith

the

amplifier

on

the

panel,

in

positions

cor



responding

to

the

respective

shaft

positions

(GAIN

dial

to

the

left

of

the

METER

dial),

securing

them

in

place

ith

the

bushings,

ashers

and

nuts

also

supplied.

Assemble

the

parts

ith

the

heads

of

the

bushings

on

the

inside

of

the

panel

to

insure

clearance.

Assemble

the

receptacles

furnished

ith

the

amplifier

to

t o

of

the

U-shaped

brackets

supplied

4

OUTPUT

RECEPTACLE

J-2

Terminal

No.

Connection

AMPLIFIER

OUTPUT

CONNECTIONS

Load

Impedance

Receptacle

Terminals

center

tap

(terminals

number

4,

■i

Matching

Input

Connections

Connect

the

incoming

line

to

terminals

number

11

and

12

of

the

input

receptacle

for

input

plug

J-

1.

Refer

to

figures

3

and

8.

Make

impedance

matching

connections

as

follo s:

Output

transformer

—

5-ohm

tap

Output

transformer

—

center

tap

A-C

input

to

po er

transformer

A-C

input

to

po er

transformer

Output

transformer

—

7.

5-ohm

tap

Output

transformer

—

center

tap

Output

transformer

—

1

5-ohm

tap

Output

transformer

—

Center

tap

Output

transformer

—

600

ohms

Output

transformer

—

600

ohms

A-C

Power

Supply

Connect

the

a-c

line

for

the

amplifier

to

terminals

number

5

and

6

of

the

output

receptacle

(for

output

3

4

5

6

7

8

9

10

11

12

*600

ohms

1

50

ohms

1

5

ohms

7.5

ohms

5

ohms

11

and

12

10

and

11

9

and

10

7

and

8

3

and

4

i

i

*600-ohm

inding

has

a

8

and

10

on

J-2

).

on

input

transformer

T-l.

At

terminal

number

2,

disconnect

the

ire

to

terminal

number

11

and

connect

it

to

terminal

number

5.

Connect

terminals

number

1

and

2

together,

and

con



nect

terminals

number

3

and

4

together.

Refer

to

figures

2

and

8.

d.

1

50-OHM

UNBALANCED

INPUT.

—

Remove

the

ire

connecting

terminals

number

2

and

3

on

T-l.

At

terminal

number

2,

disconnect

the

ire

to

terminal

number

11

and

connect

it

to

terminal

number

1.

Connect

terminals

number

1

and

2

together

and

connect

terminals

number

3

and

4

together.

Connect

the

grounded

side

of

the

incoming

line

to

terminal

11

on

the

receptacle

for

plug

J-l.

Output

Connections

The

amplifier

is

ired

for

operation

into

a

variety

of

load

impedances.

The

secondary

inding

of

the

output

transformer

T-2

is

provided

ith

a

number

of

taps

hich

are

brought

out

to

the

terminals

of

the

connection

plug

J-2

mounted

on

the

rear

of

the

amplifier

chassis

(directly

behind

the

po er

transformer).

The

center

tap

of

the

output



transformer-secondary

inding

(terminal

number

8

on

the

output

transformer)

is

connected

to

terminals

number

4,

8

and

10

of

the

connection

plug

J-2.

Refer

to

figures

2

and

8.

Connection

to

the

plug

J-2

is

made

by

means

of

one

of

the

receptacles

supplied

ith

each

amplifier.

The

follo ing

table

sho s

line

connections

for

loads

of

various

impedance:

Balanced

Bridging

Input

Connections

Connect

the

incoming

line

to

terminals

number

5

and

6

on

the

input

receptacle

for

plug

J-l,

figures

3

and

8.

Connect

terminals

number

7

and

12

together,

and

connect

terminals

number

9

and

11

together,

on

the

same

receptacle.

The

input

trans



former

is

connected

as

in

(a)

under

Matching

Input

Connections.

Input

Shielding

All

audio

leads

should

be

shielded

t isted

pair

copper

ire,

insulated

for

200

volts,

and

need

not

be

larger

than

No.

19

A.W.G.

All

joints

should

be

securely

soldered,

and

of

lo

resistance.

If

the

circuits

run

in

conduit

or

duct

hich

may

be

subject

to

moisture,

a

type

of

iring

having

a

natural

or

synthetic

rubber

covering

over

the

shield

or

lead

should

be

used.

To

minimize

un



desirable

noise

pickup

and

cross

talk

on

long

input

circuits,

the

shielding

of

the

lines

to

the

input

terminals

of

the

amplifier

or

rack

should

be

covered

ith

a

cotton

braid

or

other

suitable

insulation

and

the

shielding

should

be

grounded

only

at

the

amplifier

(point

of

lo est

level).

Make

sure

that

the

shields

are

electrically

continuous

and

that

the

ground

connections

are

perfect.

Do

not

run

the

audio-input

leads

adjacent

to,

or

laced

in

ith,

a-c

or

high-level

audio

lines.

a.

600-OHM

BALANCED

INPUT.

—

The

ampli



fier

is

ired

at

the

factory

for

this

type

of

operation.

Terminals

number

2

and

3

on

input

transformer

T-l

are

connected

together,

terminals

number

2

and

11

are

connected

together.

Refer

to

figures

2

and

3.

b.

600-OHM

UNBALANCED

INPUT.

—

Discon



nect

from

terminal

number

2

on

T-l

the

ire

hich

is

connected

to

terminal

number

11,

and

connect

it

to

terminal

number

1.

Connect

the

grounded

side

of

the

incoming

line

to

terminal

number

11

on

the

receptacle

for

plug

J-l.

c.

1

50-OHM

BALANCED

INPUT.

—

Remove

the

ire

connecting

terminals

number

2

and

3

5

Figure

3

—

Re r

View

of

Type

BA-1

3

A

Progr m

Amplifier.

CIRCUIT

MODIFICATIONS

Gain

Controls

In

applications

here

extreme

quietness

or

ex



tended

periods

of

operation

ithout

servicing

are

prime

considerations

the

dual

continuous-type

attenuator

or

gain

control

furnished

in

MI-11223-A

Hum

Adjustment

A

hum-adjustment

potentiometer

marked

HUM

is

located

on

top

of

the

chassis,

in

front

of

the

output

transformer

and

in

line

ith

the

meter

s itch,

see

figure

3.

A

scre driver

slot

is

provided

in

the

potentiometer

shaft,

hich

is

accessible

from

the

top

of

the

chassis,

for

adjusting

the

hum

level

to

a

minimum.

plug

J-2,

figures

3

and

8).

To

insure

a

lo

hum

level

the

a-c

supply

circuit

should

be

shielded

and

the

shield

grounded.

Metering

The

four-position

rotary

s itch

mounted

on

the

front

panel

of

the

amplifier

chassis

(right

side)

is

designed

to

be

used

in

conjunction

ith

the

Type

BI-1A

(MI-4388-B

or

MI-4388-C)

or

the

Type

BI-2A

(MI-1

1275

or

MI-1127

5-

A)

Metering

Panel

to

conveniently

check

the

operation

of

the

individual

tubes.

The

numbers

on

the

METER

dial

plate

indicate

the

tube

hich

is

being

checked.

Circuit

arrangement

is

such

that,

ith

the

tubes

operating

normally,

the

meter

ill

read

approximately

1

volt.

When

a

Type

BI

metering

panel

is

not

avail



able

any

suitable

voltmeter

having

an

internal

resistance

of

20,000

ohms-per-volt

or

greater

can

be

used.

Connect

the

positive

terminal

of

the

meter

to

terminal

number

8

and

the

negative

terminal

of

the

meter

to

terminal

number

10

on

the

input

receptacle

(for

input

plug

J-l,

figures

3

and

8).

6

CAPACITOR

VALUES

k

\

I

MAINTENANCE

CAPACITOR

VALUES

Response

Curve

Num er

1

2

3

5,600

8,200

10,000

Response

Curve

Num er

1

2

3

0.03

0.05

0.1

C-5

{200

vdc)

Capacity

in

mmjd

C-4

(400

vdc)

Capacity

in

mjd

amplifier

can

be

replaced

ith

the

dual

step-type

attenuator

furnished

in

the

MI-11233

amplifier.

Order

from

RCA

Replacement

Parts

Department,

Camden,

New

Jersey,

specifying

stock

number

57121.

This

attenuator

is

similar

electrically

to

the

continuous

type

of

attenuator

and

must

be

ired

into

the

circuit

as

sho n

on

the

iring

diagram,

figure

8.

capacitor

having

the

curve

number

in

the

Increasing

High-Frequenc

Response

Three

curves,

numbered

1,

2

and

3,

are

sho n

in

figure

6.

These

curves

illustrate

the

frequency

characteristics

of

the

amplifier

after

changes

are

made,

in

the

circuit

constants,

to

increase

the

high-

frequency

response

of

the

amplifier,

by

approxi



mately

one,

t o

and

three

decibels

at

10,000

cps.

To

obtain

a

frequency

response

corresponding

to

one

of

the

three

curves,

replace

the

3900

mmfd

capacitor

designated

C-5

in

figures

2

and

4

ith

a

capacitor

having

the

value

given,

opposite

the

curve

number,

in

the

follo ing

table.

ts

and

NOTE:

Check

the

high-frequency

response.

If

it

is

found

to

be

too

high,

substitute

for

C-5

a

capacitor

of

the

next

lo er

standard

value

as

required.

Increasing

Low-Frequenc

Response

The

curves

numbered

1,

2

and

3

in

figure

7

sho

the

frequency

response

of

the

amplifier,

after

alterations

have

been

made

in

the

circuit

to

obtain

greater

lo -frequency

response.

An

increase

of

2,

3

and

4

decibels

at

30

cps

can

be

obtained

by

making

suitable

modifications

in

the

circuit.

To

Frequenc

Compensation

The

normal

frequency

response

of

the

Type

BA-13A

Program

Amplifier,

sho n

by

the

curve

in

figure

5,

is

flat

throughout

the

audio

range.

Under

certain

conditions

it

may

be

desirable

to

increase

the

response

at

either

the

high-frequency

or

the

lo -frequency

end

of

the

range.

The

curves

in

figures

6

and

7

sho

the

increase

in

response

hich

may

be

obtained

by

making

compensation

adjustments

ithin

the

amplifier,

according

to

the

instructions

in

the

follo ing

paragraphs.

If

desired,

both

high

and

lo

frequency

compensation

may

be

applied.

NOTE:

Because

of

slight

differences

in

iring

and

tolerances

of

component

parts,

it

may

be

necessary

to

use

capacitors

differing

in

Value

from

those

given

in

the

follo ing

tables

in

order

to

secure

the

desired

response.

Cleaning

The

care

of

the

Type

BA-13A

Program

Amplifier

should

include

the

usual

precautions

observed

in

the

maintenance

of

high-quality

audio

components.

Do

not

allo

dust

or

dirt

to

accumulate

on

the

amplifier.

Perform

the

follo ing

operations

at

regular

intervals:

a.

Clean

the

tube

prongs

and

the

contacts

in

the

tube

sockets.

b.

Clean

the

prongs

of

the

plug-in

capacito

the

contacts

of

the

capacitor

sockets.

c.

Keep

the

prongs

of

the

amplifier

connection

plugs

clean

using

clean

dry

cloth.

Do

not

use

abrasive.

Clean

the

connection-plug

recep



tacles

approximately

every

t o

months

by

obtain

a

frequency

response

corresponding

to

one

of

the

three

curves

make

the

follo ing

changes:

a.

Disconnect

the

yello

lead

from

R-15

(on

terminal

No.

1

of

terminal

board

1)

and

connect

this

lead

to

a

compensator

composed

of

a

5600-ohm

resistor

and

a

1-mfd

capacitor

(in

parallel).

Connect

the

other

side

of

the

compensator

to

R-15

on

terminal

No.

1

on

terminal

board

1.

The

com



pensator

may

be

supported

by

a

small

terminal

board

mounted

on

the

side

all

of

the

chassis.

The

position

in

the

circuit

of

the

resistor

and

capacitor,

identified

as

R-27A

and

C-15A

is

sho n

in

figure

4

(they

should

have

ratings

of

1

att

and

200

vdc

respectively).

b.

Replace

the

0.25

mfd

capacitor,

designated

C-4

in

figures

2

and

4,

ith

a

value

given

opposite

the

follo ing

table.

In

applications

here

system

losses

or

other

deficiencies

require

an

increase

in

the

response

at

both

ends

of

the

audio

spectrum,

some

suitable

combination

of

high-frequency

and

lo

’

-frequency

compensation

can

be

selected

and

applied

as

described

in

the

t o

preceding

paragraphs.

7

VOLTAGE

AND

CURRENT

VALUES

360

volts

T-2

PLATE

3

5.

->»

TO

R-IO

3

7

2

V

TO

S-2

BUS

GROUND

M-I45306-I

Figure

4

—

P rti l

Schem tic

Di gr m

Showing

Ch nges

for

Compens tion.

Fuse

Replacement

When

replacing

i

o

65

volts

de

72

volts

de

1.8

volts

de

90

volts

de

48

volts

de

1.5

volts

de

Plate

to

ground

Screen

to

ground

Cathode

to

ground

Filaments

to

ground

Meter

Indication

Filaments

Plate

Current

Screen

Current

Second

stage

RCA

1620

o

y

B

>

ci

§

g

o

Q

5

O

<n

K>

Third

stage

RCA

1622

<D

or

R-27

A

AWW-

5.600

JI

r-

a:

RCA-1620

V-

2

cc

ct

8

<O

d

o

o_

o

C-15

A

Hb

IMFD

—

11

—

0.2

3

MFD

Tubes

and

Plug-in

Capacitors

Check

the

tubes

for

normal

characteristics.

Label

each

tube,

indicating

condition

and

length

of

serv



ice.

Check

the

plug-in

capacitors

for

leakage

and

capacity.

C-8

1200

MMFD

nd

nr

T0

Cl3

I800JL

I800IL

Voltage

and

Current

Values

Voltage

and

current

values

for

a

operating

under

normal

conditions

follo ing

table:

A-C

supply

to

amplifier

—

1

17

volts,

60

cps

(A-C

supply

connected

to

the

1

1

5-volt

tap

on

traTsformer

)

a

fuse,

make

certain

that

the

replacement

fuse

is

of

the

same

type

and

rating

Plate

Supply

Voltag<

Total

rectified

current

—

56

milliamperes

310

volts

de

270

volts

de

2

1

volts

de

18

volts

de

1

volt

de

6.3

volts

ac

45

ma

2.2

ma

18

volts

de

1

volt

de

6.3

volts

ac

0.84

ma

0.22

ma

18

volts

de

1

volt

de

6.3

volts

ac

1.7

ma

0.5

ma

First

stage

RCA

1620

(1

ampere,

Type

MDL,

stock

number

53447)

as

the

one

furnished

ith

the

amplifier.

In

emergencies

hen

this

fuse

is

not

available,

a

1

ampere,

Type

3-AG

fuse

can

be

used

as

a

temporary

substitute.

Do

not

use

a

fuse

of

higher

rating

for

replacement

purposes

since

this

ill

needlessly

endanger

the

indings

of

the

po er

transformer.

the

po er

T

m

I

■

S.

<0

”

cc

u.

typical

amplifier

are

given

in

the

removing

the

amplifier

from

the

receptacles.

This

action

automatically

cleans

the

receptacle

contacts

by

the

iping

action

of

the

springs.

Plug

the

amplifier

in

and

pull

it

out

several

times

to

secure

sufficient

cleaning

action.

d.

Clean

the

step

type

attenuator

in

the

MI-1

1233

amplifier.

Press

the

catch

nearest

the

rear

end

of

the

attenuator,

turn

the

rear-end

cover

sufficiently

to

release

it,

and

pull

the

cover

off

of

the

attenuator.

Apply

Davenoil

to

the

ex



posed

rear-section

contacts,

rotate

the

shaft,

and

if

any

dark

streaks

appear

ipe

off

the

contacts.

Repeat

this

procedure

until

the

con



tacts

are

clean,

then

apply

a

thin

film

of

Daven



oil.

Press

the

catch

nearest

the

front

of

the

attenuator

and

remove

the

rear

section

of

the

attenuator,

exposing

the

front-section

contacts.

Clean

these

contacts

in

the

same

manner.

Replace

the

rear

section

and

the

end

cover

and

lock

each

in

place.