Nakamichi ST-7, ST-7E Service Manual

Service Information

Nakamichi ST·7/ST·7E

Model (AM/FM Stereo Tuner) Nakamichi

Serial No. from -

No. OOO·Sl-3094 (1/12)

Subject

Principle of Operation

Date 15 April 1986

Contents

1.Power Supply Circuit

2.	~-~=	1
2.	Line Mute Circuit and Power Off Signal Circuit ................................ .	1
3.	Synthesizer IC and Peripheral Circuits ...................................... .	3
4.	FM/AM Mode Switching Circuit .......................................... .	6
5.	Seek Tuning and Manual Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
6.	Memory Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
7.	FL Display Control Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
8.	Schatz Noise Reduction (Schatz NR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11

1.Power Supply Circuit

When the Power switch

is turned ON, the AC power is

(2)

Output from 30 V AC Winding

applied to the primary side of power transformer T1.

The output from

the 30 V AC winding is full-wave recti-

The secondary side of power transformer T1 consists of a

fied by diode bridge D8402, smoothed by C403, and sent

15 V AC winding, 30 V AC winding, and 3.3 V AC x 2

into the voltage stabilizer circuit consisting of 0401,

winding.

20401, etc. The voltage stabilizer circuit produces a

(1)

Output from 15 V AC Winding

stabilized output

of +15 V DC which is used as the bias

The output from the 15 V AC winding follows two paths.

voltage of the charge pump circuit.

One goes to the line mute circuit via 0401 and 0402. The

other is full-wave rectified by diode bridge D8401,

(3)

Outputs from 3.3 V AC x 2 Winding

smoothed by C401, and sent into the +15 V 3-terminal

The outputs from the 3.3 V AC x 2 winding are rectified

regulator IC401

to provide a stabilized +15 V DC output.

by D403 and D404, and smoothed by C408 and C407.

This

+15 V DC

output

provides the +5 V DC output

The result is a nonstabilized power supply of approx. -4

which is obtained through a voltage stabilizer circuit com-

V DC. The -4 V

DC serves as the bias voltage of the line

posed of 0405, ZD402, etc. The +5 V DC is supplied to

mute circuit. The outputs from the 3.3 V AC x 2 winding

the digital ICs, synthesizer IC (IC310) and FL (fluorescent)

are also applied across pins 1 and 25 of the FL display

display control circuit.

tube so that they serve as the filament power supply.

Line Mute Circuit and Power Off Signal Circuit

Rll3 76.SKlF)

Cl07

!Lchl

Rll4

4.7µ 25V

2.2K

lKSl

Line 01.1tp1.1t<Lchl

+ 15V

- ,

0406

OTAU4ES

R414

IOK IOK

J lOOK

QIOI

2SC2878

F403

I A

0402

(T\A)

!SSl32

0401

155132

< --- to Q31q roisplny P.C.8. An'y>

I'N"H

Power

Off

(Main P.C.8. Ass'y)

Clo IC3I0-40l ----+f------=~

0338

Fig. 1 Line Mute Circuit/Power Off Signal Circuit

Printed in Japan

ST-7/7E OOD-Sl-3094 (2/12)

The J ine outputs are muted when the power is turned ON

or OFF or when synthesizer IC310-28 (Mute) becomes H.

When the power is turned OFF, the Power Off signal

instantly becomes L, muting the line outputs and also extinguishing the FL display tube. At the same time, a L

signal is sent to synthesizer IC310-40(1NH) to cause IC310 to execute the power-off sequence.

(1) Power ON

When the Power switch is set to ON, the output of the 15

V AC winding is full-wave rectified by 0401 and 0402 and, without smoothing, applied to the base of 0402.

Accordingly, 0402 is ON and C409 is discharged. Thus,

0403 cannot turn ON but it stays OFF after the power is

turned ON.

On the other hand, when the power is turned ON, a

differentiated base current flows into 0404 through

C41 O. As a result, 0404 turns ON in the form of a pulse

over a certain period of time, which turns ON 0103,

0203, 0101 and 0201, muting the line outputs for this period. (Since 0406 is ON at the power ON time, R415 (10 kQ) is parallel-connected with R410 (330 kill. For

this reason, 0404 has adequate base current when the

power is turned ON. As 0404 becomes ON, 0406 becomes OFF.)

(2) Power OFF

When the Power switch is set to OFF, the output of 15 V

AC winding instantly ceases, setting 0402 to OFF. After

C409 is charged, 0403 turns ON and the Power Off signal

becomes L. This turns 0404 ON and mutes the line

outputs. At the same time, the non·inverting inputs of the

buffer amp. IC309 in the last stage is grounded through 0405, disabling the outputs.

The Power Off = L signal is also applied to the base of

0319 on the Display P.C.B. Ass'y.So 0319 turns ON and

0320 turns OFF, extinguishing the FL display tube

without flickering.

Furthermore, the "P_o_w_e-r'O~f~f - L signal is also applied to

synthesizer IC310·40 (INH). Consequently, when INH =

L, IC310 enters the data storing status to store the data.

The data (broadcasting station frequencies, etc.) stored in

IC310 is held for about a week due to backup by C381 (47,000µF).

(3) Mute of inter-station noise

To remove the inter-station noise, synthesizer IC310-28

(MUTE) outputs H. This turns ON 0326 and then 0404,

thus muting the line outputs. The line outputs are also muted when the FM Mute switch is ON, because IC310-28 (MUTE) becomes H.

			Power		Switch ON		Power Switch OFF
			+15V	T _______T
	+l5V						I
		0					I
			ON				J.:r
	Q403
			OFF
			H				I
	Power	Off	L	h 111			:L
			L				:L
	Q404 :;.						!r
MUTE			H	i	\	"-------f
MUTE				I			I
IIC310-281				I
			L	I
			H	I
			H
	Line	Mute

Fig. 2 Line Muting Timing Chart

ST-7/7E OOD-Sl-3094 (3/12)

3.Synthesizer IC and Peripheral Circuits

(1)1/0 signals and functions of synthesizer IC310 (TC9147BP)

Table 1

Pin No.

Symbol

Description

In/Out

Function

GND

GND terminal

Crystal oscillator terminals

Crystal for 7.2 MHz reference frequency is

connected.

FM band select

MW band select

Provides FM/MW/(LW) band selection by resetting

currently effective band. Active H.

LW band select (Unused)

Selects manual or auto tuning mode. While Manual

MANUAL

Manual tuning mode select

Tuning knob (incremental rotary encoder) is

rotating, manual tuning mode is selected. When

AUTO

Auto seek tuning mode select

Seek button (UP or DOWN) is pressed, auto tuning

mode is selected.

Active H.

Up Seek button input

Selects a station by increasing (UP) or decreasing

(DOWN) the frequency.

DOWN

Down Seek button input

Active H.

STO

Memory store command

In/Out

Sets preset memory in write state. Active H.

M1 to MB are used in conjunction with MC1 and

Preset Station button inputs

MC2 to read and write 16 stations to preset

Memory.

Active H.

MC1

Memory control input {non·shift)

MC2

Memory control input (shift)

OSC2

Oscillator terminal for AM

Capacitor and resistor that determine scan speed

in AM seek are connected.

OSC1

Oscillator terminal for FM

Capacitor and resistor that determine scan speed

in FM seek are connected.

50 kHz

50 kHz step select (FM)

Out

Causes 50 kHz step display for FM band in the

European area. Becomes H level when 50 kHz

display is ON.

CK2

Clock output

Transmits serial data and timing clocks to the

received·frequency display driver.

CK1

Clock output

Out

DATA

Received·frequency serial data

output

MUTE

Muting output

Out

Becomes H when muting. Active H.

ST-7/7E OOD-Sl-3094 (4/12)

Pin No.

Symbol

Description

In/Out

Function

AREAO

Area select

Selects area of U.S.A. and Canada, Europe or Japan.

AREA 1

STOP3

AM-IF signal input

Unused.

STOP2

Auto-search stop signal input

With H level given to STOP1 input, stops auto seek

upon input of H level to STOP2. Active H.

STOP1

Scan speed slow input

Unused. Connected to pin 32.

D0-2

Sent from tri·state buffers. Pin 35 is unused.

Phase comparator outputs

Out

D0-1

TEST

Test terminal

Unused. Grounded.

FM IN

FM program counter input

Output of prescaler TD6104 isconnected.

PSC

Prescaler control output

Out

Controls switching between 1/30 and 1/32 dividing

of prescaler TD6104. (1/30 for Japan only)

AMIN

AM program counter input

Receives AM local oscillator signal.

INH

Inhibit input

Causes normal operation at H level, and inhibit

mode at L level. Active L.

INT

Initialization input

Causes normal operation at H level, and initiali-

zation of internal status at L level. Active L.

VDD

Power supply terminal

Connected to +5 V. Backup is available for above

approx. 2 V.

(2) FM Prescaler

IC311 (TD6104) is a 5-bit digital counter. The frequency from the FM local oscillator circuit is input to IC311, which performs 1/32 division to produce a 25 kHz reference frequency. The reference frequency is input to FM program counter input, pin 37 of synthesizer IC301 (TC9147BP).

The functions of IC311 are described in Table 2.

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