Onkyo TX 28 Service Manual

Page 1

SERIAL N0, 3224

ONKYO SERVICE MANUAL

QUARTZ SYNTHESIZED
TUNER AMPLIFIER
MODEL TX-28

Black and Silver models

BUD» BUDN,UD 120V AC, 60Hz
BUG, UG 220V AC, SOHZ
BUQA 240V AC, SUI-[z
BUW, UW 120 or 220V AC. 50/60Hz

SAFETY-RELATED COMPONENT WARNING
COMPONENTS IDENTIFIED BY MARK ,5: ON

TABLE OF CONTENTS

THE SCHEMATIC DIAGRAM AND IN THE PARTS Specifications -------------------- 2
LIST ARE CRITICAL FOR RISK OF FIRE AND Service procedures ----------------- 3
ELECTRIC SHOCK. REPLACE THESE COMPOV Exploded View --------------------- 4
NENTS WITH ONKYO PARTS WHOSE PART Parts list --------------------- 5
NUMBERS APPEAR AS SHOWN IN THIS Block diagram *Tuner section? ------------ 6
MANUAL. Block diagram *Amplifier section """""""" 8
Circuit description ----------------- 9
MAKE LEAKAGE CURRENT 0R RESISTANCE Adjustment procedures ---------------- 15

MEASUREMENTS TO DETERMINE THAT EXPOSED Pcb view/parts list
PARTS ARE ACCEPTABLY INSULATED FROM FM/AM tuner ------------------ 17
THE SUPPLY CIRCUIT BEFORE RETURNING Video circuit """""""""""" 18
THE APPLIANCE TO THE CUSTOMER. Digital circuit -~" ' . - ~_ iifi 21
Selector switch ------------------ 22
Equalizer amplifier --------------- 25
Tone/Volume/Headphone ~~~~~~~~~~~~~~ 26
Power amplifier ~~~~~~~~~~~~~~~~~ 29

Schematic diagram
Tuner section *IZOV modeli ~~~~~~~~~ 19
*ZZOV modelr ---------- 23
Amp. section *IZOV model' ---------- Z7
\ -22(}V model --------- 31
[ Block diagram of IC """"""""" 33
ONKYO ..................... 34

I numn comma

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CIRCUIT DESCRIPTIONS

1. Dynamic bass expander

INPUT (7

other channel

Lc1561

MIX
1, BUFFER

OUTPUT
POWER AMP. I;
R565
LEVEL
Q661 D561 (3552 -L-
% #FILTER D562 # FILTER
AMPLI. RECTIFIER CONTROL

other channel

In earlier super base systems. only the frequencies about 70Hz were boosted by about 4dB to expand the playback frequ*
ency response to enable playback of the super low region. l-lowever,when there was no input signal, the above frequency
response resulted in deterioration in the S/N ratio in the 70Hz region. This problem has been overcome by the dynamic
bass expander where the 70Hz boosted level is varied according to the input signal level.That isthe frequency response
remains flat when no input signal is appliedbut is boosted at the 70Hz region to the specified level when the input

signal exceeds a certain level.The left and right channel input signals from the INPUT terminals are mixed by QSGI and
pass through the level volume and filter amplifier.lhe signal is rectified by D561 and D56Z.and the resultant DC componemt
control signal is applied to the gate of Q562.When the input signal is at a adequate level,Q562 is turned on and the super
base circuit of power amplifier is controlled by the input signal.

2. Operation of analog switch

V551

IHN

05C

ALL'OFF

MUTE

SlGrAl

816* A2

SlCr A3

816* A4

SIGAF)

COMrA

VssZ

1
L,

ml

-'1

24

23

V dd

SELl

SELZ

SELS

SEL4

SEL5

SIG'Bl

810* 82

SIGBB

SIGB4

SIGBS

COM-B

LVssl - . - Connect to the ground.

2.1NH - - - Inhibit input terminal.0perates normally at the high level.
Inhibit status at the low level.

3.05C ~ - ' CR connection terminal for the oscillator that determines the

muting time and selector timing of analog switch.

4.ALL-OFF~ Off designation input terminal of analog switches.All analog
switches turn Off at the high level.

5.MUTE ' Muting signal output terminal.
If an H" level input is received at this selection input terminal
(SEL-l NSELS), this terminal becomes "H" level for a fixed period
only. Within that period the analog switch changes.

6.SIG*A1 - - Signal input terminal.

18.81081 If SELI is selected, analog switch 1 goes ON. and conductivity
exists between this terminal and the COM terminalr

7*10.SlG-A2 ~SlGrA5 ------ Same as above.

17*14.SlG~BZ~SIG>B5

11.COM*A - Analog switch common terminal.

13ICOM~B

23.8EL-1 ' Selector input terminals for the various analog switches.

22.81172 If an "H" level is set for the SELl ~SEL-5 terminalscach analog

ZIISEL73 switch that is selected goes ON. SELI,SEL2.SEL*3.SEL*4.and

20.5ELr4 SELS are of the matually reset type. If there is other than

19.SEL*5 select input. they go OFF.

24.Vdd - - - Terminal of the power supply voltage