THORENS PC 650 - Electronics Performance and Calibration

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8.

10.

Record/Playback

Head

Replacement

Should

a

loss

of

high-frequency

signals

be

noted

in

the

reproduced

program,

the

calibration

and

test

pro-

cedures

of

Sections

9,

10

and

13

should

first

be

conducted.

!f

specified

performance

cannot

be

attained,

replace

the

record/playback

head

(3062335,

Fig.

9)

as

follows:

a)

Unsolder

the

shielded

cables

from

the

terminal

board

of

the

head.

b)

Remove

the

left-hand

mounting

screw

and

loosen

the

right-hand

screw.

c)

Remove

the

head

and

mount

a

replacement

head.

The

left-hand

screw

is

to

be

tightened

completely,

while

the

right-hand

screw

should

be

turned

until

the

head

is

paralle!

to

the

mounting

plate

(chassis).

d)

Solder

the

shielded

cabies

onto

the

terminal

board

of

the

head.

e)

Perform

the

adjustments

in

Sections

9

and

11

-

13.

Azimuth

Adjustment

The

azimuth

(head

alignment)

adjustment

can

be

performed

without

removing

the

front

panel

assembly

through

the

access

hole

above

the

cassette

compartment.

a)

Demagnetize

the

erase

and

record/playback

heads.

b)

Insert

an

azimuth-adjustment

cassette,

or

a

cassette

recorded

with

a

10

KHz

sinewave

signal,

and

connect

a

dual-channel

oscilloscope

(in

chopping

mode)

to

the

outputs

L

and

R.

c)

In

the

PLAY

mode,

turn

the

right-hand

mounting

screw

of

the

record/playback

head

(Fig.

9)

to

achieve

the

maximum

output

voltage

and

the

identical

phase

in

both

channels.

Avoid

secondary

volt-

age

maxima!

RECORD/PLAYBACK

ELECTRONICS

Before

checking

the

performance

of

the

record/playback

system,

verify

the

correct

head

bracket

height

according

to

Section

4.

Demagnetize

the

heads

and

clean

with

alcohol.

The

tests

and

calibration

procedures

are

to

be

carried

out

in

the

order

presented.

Measurement

points

MP

1

-

MP

4

are

located

on

circuit

board

VM

1.

The

location

of

other

measurement

points

and

electronic

components

is

given

by

the

first

or

the

first

two

identifying

numerals;

thus,

R

1513

is

located

on

board

VM

15,

etc.

All

voltage

specifications

are

referred

to

0

V.

Performance

Check

of

the

Record/Playback

Electronics

a)

Switch

in

the

ultrasonic

filter

(20

Hz

-

20

KHz:

linear)

of

the

audio

millivoltmeter

intended

for

the

measurements.

b)

Feed

in

a

333

Hz

sinewave

signal

and

set

the

voltage

level

at

MP

1

(L)

and

MP

2

(R)

to

-30

dBm

(app.

25

mV)

with

the

signal

generator

or

the

INPUT

level

controls.

—

The

voltage

amplification

of

the

Dolby

record

board

VM

6/A

is

approximately

29

dB,

so

that

a

signal

level

of

about

1

mV

should

be

measured

at

points

S

612

(L)

and

S

603

(R).

~

The

voltage

leveis

at

MP

1

and

MP

2

and

at

S

614

(L)

and

S

605

(R)

should

be

approximately

the

same.

c)

Check

the

BIAS/EQ

switching

at

board

VM

8:

Voltage

Level

at:

S

808

(L)

/

S

806

(R)

$

811

(L)

/S

802

(R)

for:

FeCr

app.

4.5

mV

50

mV

Fe

5.0

mV

55

mV

Cr

8.5

mV

95

mV

d)

Insert

an

unrecorded

cassette,

set

the

BIAS/EQ

selector

to

the

appropriate

position,

and

press

the

REC

and

PLAY

buttons

simultaneously.

Switch

off

the

filter

at

the

millivoltmeter

and

measure

the

100

KHz

voltage

at

VM

10:

S

1004

(Bias

Voltage)

S

1003

(Erase

Voltage)

for:

FeCr

47

Vrms

37

Vos

‘

Fe

33

Vams

28

Vrms

Cr

57

Vrams

45

Veams

Switch

in

the

filter

at

the

millivoltmeter

for

the

remaining

measurements

below.

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