Telefunken M 204 E - Recording Level Adjustments; VU-Meter Calibration and Recording Levels; Stage and Oscillator Alignment; Output Stage and Oscillator Frequency Alignment

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analogously

for

the

second

channel

too.

(Adjustment

of

the

bias

trimmer

C

22,

recording

input

at

phono

jack,

pins

2

and

5,

and

'

playback

output

at

radio

jack,

pins

2

and

5).

It

is

advisable

to

carry

out

a

check

measurement

of

the

frequency

response

at

19

cm/s

tape

drive

speed.

Here

the

level

for

1

kHz

may

show

a

maximum

discrepancy

of

+3dB

with

respect

to

the

level

for

15

kHz,

corresponding

to

a

voltage

ratio

factor

of

0.7

to

1.4

at

1

kHz.

Adjusting

the

Recording

Level

The

recording

drive

manual

controls

must

be

set

to

maximum

for

the

following

adjustments.

Adjusting

the

VU-Meters

in

the

Stereo

Setting

Connect

the

VTVM

to

the

stereo

earphones

jack

alternately

to

pins

2—3

and

2—5.

From

the

AF

signal

generator,

feed

a

signal

at

1

kHz

into

the

phono

jack,

pins

2—3

and

2—5,

at

such

a

level

(about

225

mV)

that

the

VTVM

indicates

2

V

on

channel

1.

Check

by

transferring

the

VTVM

to

channel

2;

the

reading

here

should

also

be

2

V.

With

this

input

voltage,

adjust

the

preset

controls

R

151

and

R

251

such

that

the

pointers

of

the

VU

meters

are

on

the

dividing

lines

between

the

red

and

black

sectors.

Adjusting

the

Recording

Level

for

Stereo

Without

changing

the

input

voltage,

make

a

stereo

recording

on

the

free

section

of

the

test

tape.

Upon

playback,

channel

1

as

well

as

channel

2

should

both

give

the

noted

test

tape

level

+

3dB.

In

case

of

discrepancy,

repeat

the

test

recordings

after

having

appropriately

readjusted

the

preset

potentiometer

R

102

for

channel

1

or

the.preset

potentiometer

R

203

for

channel

2.

Adjusting

the

Recording

Level

for

Mono

2

Without

changing

the

input

voltage,

make

a

recording

on

the

free

section

of

the

test

tape.

Playback

of

this

recording

should

again

produce

the

test

tape

level

+

3dB.

If

this

is

not

the

case,

repeat

the

test

recording

after

appropriate

readjustment

of

the

preset

potentiometer

R

103.

Alignment

of

the

Output

Stages

Measure

in

the

playback

seting

without

signal.

The

collector-emitter

voltages

should

be

equal

(center-point

voltage)

in

both

of

the

two

output

stages.

Alignment

of

the

Oscillator

Frequency

In

the

setting

“Stereo”

recording,

tune

the

oscillator

coil

L

103

such

that

the

oscillator

runs

at

85

kHz

(+

4

kHz).

Circuit

Description

Connection

of

the

amplifiers

to

the

heads

and

output

stages:

Parallel:

Both

head

systems

are

connected

to

the

parallel-con-

nected

amplifiers.

Both

output

stages

are

driven

from

the

amplifier

1.

Each

head

feeds

one

respective

amplifier

chain.

The

output

stages

can

be

used

to

monitor

the

signals

ahead

of

the

tape

during

recordings.

The

particular

selected

head

system

(1

or

2)

is

con-

nected

to

amplifier

1

and

both

output

stages

are

driven

from

amplifier

1.

For

recording,

only

amplifier

1

switches

to

recording

and

the

output

stage

1

serves

for

monitoring

during

recording,

whilst

the

output

stage

2

is

then

switched

off.

The

other

(nonselected)

head

system

is

connected

to

amplifier

2.

According

to

the

direction

of

transcription,

the

signal

pick-up

(playback

function)

head

system

is

connected

to

amplifier

2,

which

remains

in

the

playback

setting,

whilst

amplifier

1

switches

to

recording

when

the

recording

button

is

pressed.

The

mixed

signal

can

be

monitored

via

the

output

stage

1.

The

output

stage

2

is

switched

off.

Stereo:

Mono:

Multi-Play:

Earphones

Outputs

The

signals

being

handled

by

amplifier

1

can

be

monitored

at

the

mono

and

stereo

earphones

output,

whereas

the

signals

being

handled

by

amplifier

2

can

be

monitored

only

at

the

stereo

earphones

output.

Amplifiers

The

combined

recording

and

playback

amplifier

is

a

five-stage

circuit

for

each

channel.

The

input

stage

(T

101

/

T

201)

is

driven

directly

via

a

coupling

capacitor

(C

102

/

C

202)

for

radio

or

microphone

recordings.

For

recordings

from

a

record

player

or

tape

source,

the

input

voltage

is

reduced

via

a

2.2

MQ

series

resistor

(R

1

/R

2).

Current

negative

feedback

is

applied

to

this

stage

via

an

unbypassed

emitter

resistor,

and

also

AF

negative

feedback

is

applied

from

the

output

to

the

input.

This

establishes

the

dynamic

input

impedance.

The

stage

will

accept

signals

40

dB

greater

than

normal

without

overloading,

and

it

possesses

a

favourable

signal/noise

ratio.

The

drive

level

control

(separately

R11

and

R

21)

lies

beyond

the

input

stage.

R

11

is

used

to

adjust

the

drive

level

for

the

monaural

recording

functions.

R

11

also

serves

as

drive

level

control

for

the

new

recording

in

the

multi-

playback

setings,

irrespective

of

whether

this

recording

is

taking

place

on

channel

1

or

on

channel

2.

Similarly,

the

volume

of

the

channel

to

be

transcribed

(1

or

2)

is

always

adjusted

with

R

21.

A

buffer

stage

with

collector

output

(T

102/T

202)

lies

ahead

of

the

equaliser.

The

gain

of

this

stage

in

channel

2

can

be

ad-

justed

with

R219

for

playback,

in

order

to

compensate

once

and

for

all

for

differences

in

sensitivity

of

the

record/playback

heads

1

and

2.

Furthermore,

the

sensitivity

switching

between

playback

and

recording

is

effected

in

this

stage.

A

switched

RLC-network

provides

equalisation

for

recording/

playback

and

19/9.5

cm/s

tape

speeds.

This

network

is

connected

between

the

output

(C124

/C224)

and

the

emitter

resistor

(R

133

/

R

233)

of

the

first

equaliser

transistor

(T

103

/

T

203).

Among

other

functions,

this

equaliser

circuit

provides

strong

treble

lift

for

the

recording

process

and

it

also

ensures

a

level

overall

frequency

response.

The

next

stage

(T

104

/

T

204)

com-

pensates

for

the

gain

loss

produced

by

the

equaliser

circuit.

The

final

stage

is

an

emitter

follower

(T

106

/

T

206)

and

provides

a

low

output

impedance

of

the

amplifier,

so

that

the

driver

stage

for

the

output

stage

and

the

VU-meter

circuit

can

be

connected

here.

In

the

multi-play

settings

of

the

function

selector

switch

(S3/S4/86),

the

coil

in

the

equaliser

network

previously

connected

in

parallel

with

the

emitter

resistor

of

the

equaliser

transistor,

is

now

disconnected

from

chassis,

but

only

in

the

amplifier

serving

for

playback

(always

channel

2).

The

treble

lift

is

therewith

removed

in

this

amplifier

and

the

tendency

to

instability

reduced.

In

addition,

a

RC

combination

(RQ

/ C

23)

directly

at

the

playback

head

damps

out

any

parasitic

oscillation

otherwise

produced.

The

signal

to

be

transcribed

is

fed

via

R118/C111

to

the

base

of

the

equaliser

transistor

of

the

recording

amplifier

(always

channel

1).

The

recording

voltage

is

coupled

out

via

C

126

/

C

226

and

fed

via

a

tuned

trap

(L101

—

C

104

/

L201

—C

204)

and

an

adjustable

series

resistor

(R

102

/

R103

/

R203)

to

the

record/playback

head

1

or

2.

The

tuned

trap

prevents

the

bias

voltage,

which

is

also

applied

to

the

recording/playback

head,

from

sending

a

current

back

into

the

amplifier.

The

series

resistors

serve

for

setting

the

AF

voltages

in

the

recording

process

such

that

the

playback

levels

are

equal

for

the

various

operating

modes.

The

Output

Stages

The

output

stages

are

driven

from

the

recording/playback

amplifier,

via

respective

volume

controls

(R12

/

R22)

and

tone

controls

(R

31

/

R

41).

The

driver

stage

operates

into

an

interstage

transformer.

The

driver

stage

is

stabilised

with

an

emitter

resistor

(R

307

/

R

407).

The

pnp

power

transistors

(T

302,

303

/

T

402,

403)

are

driven

in

antiphase

from

the

respective

trans-

formers

(TR

301

/TR

401).

The

loudspeaker

is

coupled

to

the

center

point

of

each

output

stage,

via

a

large

electrolytic

capa-

citor

(C

306,

C

406).

The

output

transistors

are

connected

in

series

for

DC

considerations,

so

that

a

47

Ohm

NTC

resistor

is

required

in

each

base

circuit.

The

56

Ohm

resistor

in

parallel

therewith

gives

the

stabilisation

curve

the

required

form.

The

harmonic

distortion

factor

is

further

reduced

with

the

negative

feedback

loop

from

the

center

point

of

the

output

transistors

to

the

base

of

the

driver

transistor.

The

output

stage

is

not

damaged

by

an

open-circuited

load,

but the

output

transistors

are

destroyed

by

short

circuits

across

the

output

load,

or

by

load

impedances

smaller

than

3.5

Ohms.

Chassis

shorts

from

the

tran-

sistor

casings

also

lead

to

immediate

destruction

of

these

transistors.

It

is

therefore

quite

essential

to

disconnect

the

mains

plug

before

commencing

work

on

the

output

stages.

Caution:

Continued

on

Page

17

ne

ee

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