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Ask Me

Another

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Detector

for FM, AM,

CW

and

SSB

How

can 1 modify

a communications receiver

to

work

on FM as well

as AM, CW and SSB?

-R.

H., Seattle,

Wash.

You

can replace the AM detector and first

AF stage

with

a gated

-beam tube

(such

as

6BN6) using the circuit shown in the schematic

diagram. The existing IF transformer

feeds the

control

grid of the tube.

When SI is set to

AM

and S2

is set to F (for

fast

AVC response)

or

S (slow

AVC response), the

grid

and

cathode

function

as a grid

leak

or diode detector and

the

rest of

the

tube

functions

as

an AF amplifier.

AVC voltage

is developed across R2 -R3 and

is

tapped

off at their junction. Diode DI prevents

AVC action until the signal reaches a satisfac-

tory

level.

With S3

open,

the

positive

bias on

the

quadrature

grid

can

be varied

with

R1 for

minimum audio distortion.

When SI

is set

to

SSB /CW, the signal from

the

receiver's

BFO

(beat

frequency oscillator)

is fed to the

quadrature

grid

to form

a product

detector.

AVC attack time can be

selected by

setting S2 to

F

or S.

For FM narrow

band

reception, Si

is

set

to

FM and C1 -LI are connected

to the quadrature

grid to form a gated

-beam discriminator.

By

setting

S2

to the

None position, AVC

is

cut out

and the

tube

also

functions as a

limiter.

Closing

S3 makes it possible

to

vary

the cathode

bias

on both the control and quadrature grids

with

RI for

maximum sound recovery

and best

limiter action.

The quadrature

coil (LI) is

tuned to the

same

frequency

as

the receiver

IF

and trimmed

for maximum

FM

audio

recovery.

FROM

IF

AMP

High- Frequency Problem

I

have a (Brand X) 4 -tube

superhet shortwave

receiver

which

has

good

selectivity

and sensi-

tivity

up

to

about

14 mc. But, from

there up

to

30 mc, it lacks the ability

to

pull in all

but the

very strongest signals. Would

the addition of

a preselector solve this problem?

-C. L., Brantford, Ontario

Have

you had the

tubes

checked on a

critical

tube tester? Try new converter and

IF- ampli-

fier tubes. Also adjust

the highest frequency

band

RF

trimmer when tuned in to a CB or

10 -meter ham

station

and

again

when

receiving

the

weakest

signal

you can tune in. If

you

have

or can borrow a signal

generator, realign the

IF transformers.

Be sure to use a proper out-

door antenna. A preamplifier (preselector)

would help, but it doesn't sound as if

you

were

getting all

of

the

performance

that

was

designed

into your

receiver.

Sure

Mike!

Can you give me

the address of

a company

that produces a

transceiver -type mike

for

ham

transmitter use?

-L. Dec., Austin, Texas

Roanwell Corp.,

180 Varick Street, New

York, N. Y.

10014 makes mikes of

this type.

So

do many others including

Electro- Voice,

Turner and Sonotone.

Try your

local

elec-

tronics

-parts distributors

who should have them

in stock.

6BN6

500MMF

52

AVC

26

R3

2.7MEG

NONE _

RI

33K

TO.IMF

FUNCTION

S1

'

FM

01

100K

TO

AF

AMP

VOLUME

TROL

B+

500K

QUADRATURE

f -+

-1

COIL

SSB

/CW

I

j

AM

100

FROM

BFO

RADIO -TV

EXPERIMENTER

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