Heathkit SG-6 - Using the RF Signal Generator

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The

AF "Output"

source

can

be

used

to

test

audio

amplifiers.

To

make

such

tests,

plug

the

test

lead

into

the

AF "OUT"

jack

and

throw

the

modulation

switch

to

internal

position.

Connect

the

alligator

clip

on

the

braided

lead

to

the

chassis

of

the

amplifier

under

test.

The

alligator

clip

on

the

center

conductor,

when

touched

to

the

grid

(through

a .05 MFD

condenser)

of

the

audio

stage

should

produce

an

audible

output

of

approximately

a 400

cycle

note

if

the

stage

is

ope

rat

-

ing

correctly

and

is

connected

to

a good

speaker.

OUTPUT

VOLTAGE

The

RF

signal

strength

going

into

the

output

control

depends

upon

the

strength

of

oscillation

of

the

6C4

oscillator.

In

all

variable

frequency

oscillators,

the

amplitude

will

vary

with

the

tuning

condenser

setting.

With

careful

design,

the

variation

may

be

minimized.

In

the

Heathkit

signal

generator,

the

variation

is

kept

down

to

a

ratio

of

about

2~

to

1

on

each

band,

except

on

band

E

where

the

L/C

(inductance

to

capacitance)

ratio

becomes

sufficiently

unfavorable

that

oscil-

lation

may

stop

with

the

tuning

condenser

nearly

fully

closed.

However,

sufficient

overlap

is

provided

to

insure

complete

frequency

coverage.

The

maximum

output

on

all

bands

is

usually

greater

than

100,000

microvolts.

CIRCUIT DESCRIPTION

Incorporated

in

the

design

are

the

following

features:

the

RF

oscillator

coils

are

prec1s10n .

wound

and

adjusted

to

calibration

before

shipment,

thereby

assuring

maximum

accuracy.

The

coils,

bandswitch,andthe

tuningcondenser

all

mount

as

a

turret

assembly

so

as

to

offer

the

ad-

vantage

of

short

wiring

leads

and

easy

mounting

of

parts.

To

prevent

tuning

past

a

desired

RF

output

signal

when

operating

the

generator,

the

tuning

con-

denser

has

a

vernier

drive

(3

revolutions

of

the

outside

knob

produce

~revolution

of

condenser

rotors).

This

condenser

varies

the

capacitance

in

the

Hartley

osciilator

thus

giving

the

band

cove-rage.

The

individual

bands

are

selected

by

means

of

the

bandswitch

which

connects

any

one

of

the

5

coils

acting

as

inductance

in

the

Hartley

oscillator.

The

audio

oscillator

is

a

Colpitts

oscillator

which

produces

approximately

a 400

cycle

note

of

good wave

form.

This

oscillator

has

the

advantage

of

being

a

source

of audio

output

by

merely

throwing

the

modulation

switch

to

"INT"

position

and

plugging into

the

AF

output

jack.

This

handy

source

of

AF

can

be

used

for

testing

of

audio

amplifier

stages,

etc.

When

the

modulation

switch

is

in

the

"INT"

position,

the

AF

oscillator

modulates

the

RF

output.

External

modulation

can

be

obtained

by plugging

the

external

source

into

the

AF "IN" phone

jack

on

the

panel

and

throwing

the

modulation

switch

to

"EXT"

position.

Design

is

such

that

external

modulation

of

small

signal

strength

from

a high

impedance

source

can

be

used,

and

thus

the

ver-

satility

of

the

instrument

is

increased.

The

6C4

tube

which

is

in

the

audio

oscillator

circuit

on

"INT"

modulation

position,

on

"EXT"

position

acts

as

an

audio

amplifier

to

the

external

modula-

tion.

The

RF

output

circuit

is

of low

impedance.

This

is

accomplished

by

the

use

of

cathode

coupling

to

the

output

jack.

The

level

of

RF

output

is

varied

by

means

of

the

RF

steps

switch

located

on

the

panel.

On

this

switch,

"HI"

indicates

high

RF

output

(therefore

low

attenuation).

The

middle

position

of

the

switch

produces

a

lower

RF

output

and,

the

maximum

C'ounterclockwise

position

gives

the

lowest

RF

output.

(Maximum

attenuation).

With low

attenuation,

the

smallest

amount

of

resistance

is

in

the

cathode

coupling

circuit

to

the

RF

output

jack.

In

the

middle

position

of

the

steps

switch,

a

single

pi

section

is

added and

for

highest

attenuation,

a

second

pi

network

is

thrown

into

the

circuit.

AC

line

input

is

connected

to a high

quality

varnish

impregnated

power

transformer.

A

ruby

red

pilot

light

on

the

front

panel

acts

as

a

convenient

indicator

as

_to

whether

power

is

on

or

off.

The

1

lOV

power

line

has

both

sides

bypassed

to

minimize

the

signal

feeding

back

through

the

power

lines.

The

power

supply

circuit

consists

of a

selenium

rectifier

connected

to

one winding of

the

power

transformer

and

thus

half

wave

rectification

is

produced,

the

filtering

being

accomplished

by

an

RC

filter

circuit.

A unique

design

feature

is

the

convenient

AF

"IN-OUT"

control

which

adjusts

audio input

if

ex-

ternal

modulation

is

employed

and

likewise

adjusts

the

AF output

level

when

using

the

generator

as

a

source

of audio

output.

'

Page

10

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