Nakamichi High-Com II User manual

Nakamichi
Service Manual
Nakamichi
High-Com II
Noise Reduction System

CONTENTS
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1. GENERAL
1.1. Control Functions
The Nakamichi High-Com II control functions are shown below:
Tr Wasiaraknd sigtt~Coni 1 poles Reduction Syareon
Fig. 1.1 Front View
Fig. 1.2 Rear View
1.2, Voltage Selactor
1.
Peak-Level
Meters
9.
2.
Mode
Switch
40.
3.
Filter
Switch
+1.
4,
Output
Contro!
12,
5.
Inout
Level
Control
13,
6.
Master
Input
Level
Control
14.
7.
GND
Jack
15.
8.
Line
Jacks
7
Line In Jacks
Rec, Out Cal, Controls
Rec. Out Jacks
Play In Jacks
Play In Cal. Controts
Line Out Jacks
Power Cord
Voltage selector is installed on the rear panel for other versions of the Nakamichi High- -Com Il.
This voltage selector can select either 120 V or 220-240 V at customer's disposal.

2. PRINCIPLE OF OPERATION
2.1. High-Com Hi Noise Reduction System
The Nakamichi High-Com Ij is a-noise reduction system
furnished with peak level meters, a 19 kHz MPX filter, a
10 Hz subsonic filter and a built-in 400 Hz tone oscillator.
Following are the outline of the High-Com II:
{a)_ Compression (encoding) is made at recording and
expansion (decoding) at playing back. The ratio of
input to output is 2:1 in compressor, vice versa in
expander.
{b} Frequency is divided into two bands of area for the
reduction of noise and the noise reduction as a whole
is about 20 to 25 dB.
(c)
priate attack-time or release time.
Further, special measures are taken.to keep: off iesits
ing disturbance.
Fig. 2.1 shows the block diagram of the High-Com lt.
Operation of the High-Com this done by. the Mode Switch
and the Filter Switch.
Line input signal is fed through independently separated
input level control for L or R channel and simultaneously
variable master control for L and R channels and reaches
L.P.F.
With this L.P.F.,higher frequency over 22 kHz, i.e., the -
frequency over audible frequency is eliminated.
Next to L.P.F. are a MPX filter and a subsonic filter which
permit the selection of 4 different stages of Subsonic, Off,
MPX, or MPX/Subsonic with the use of Filter Switch.
At the further stage, it has a compressor or expander com-
posed of U401B ICs and peripheral circuits, that permit’
* [aoa Hz
Osc.
Input
Level Master
Coloration of sound is eliminated by means of saad :
the selection of Cat. 400 Hz, Rec. (Encode), Pass, or Play
{Decode) with the use of Mode Switch. ;
At recording, select Rec. with the Mode Switch.
Compressed line input signal will appear at Rec. Out
Jacks, while the original signal wilt appear at peak level
meters and at Line Out Jacks.
For playback, select Play with the Mode Switch. Then,
the expanded play input signal will appear on both Line
Out Jacks and peak level meters.
Preceding input signal will reach L.P.F. and H.P.F. via
amp, Frequency is divided into two bands, higher band
and tower band, by H.P.F. and L.P.F. respectively, and
these two bands are either compressed or expencen by the
compressor or the expander,
Compression or expansion is performed by the changes of
amplifying rate of amplifier through the VCR (Voltage
_ Controt Resistor). The VCR is varied by the level sensor
output in correspondence with the input signal level and
the frequency.
Following shows the block diagram of the iC U401B.
U401B __ 18, 18 20 22 24
Output
ia
Filter Switch:
Mode Switch:
A. Cae. 400H2
con
+ .
1,
Subsonic
:
H
2. Ott
3. MPX
A. Subsonic / PX
8. Record Ag : io
C. Poss Bo - Rec.
OC, Playbock c Output
Fig. 2.1 High-Com I! Biock Diagram

(1) Compressor (Encoding)
Fig, 2.2 is the basic circuit of the compressor.
V1 sand V2 are variable gain amplifiers, and they are so
designed that each of them keeps the same gains, The
gains of V1 and V2 are so made that P2 is always kept
at a constant level by the control of the feedback signal
sent through the level sensor.
This function is shown in Fig. 2.3.
The gain of Vi and V2, SP1 and AP2, remains always the
same, therefore as seen in the figure, output Pt, the thick
Jine, comes out to be 1/2 of the gradient of PO.
For instance, ~40 dB input at input PO will be com-
pressed to —20 dB at output P1.
This compression principle is very simple. if V1 and V2
are equat, then the output P1 will always be 1/2 of the
input PO.
Fig. 2.2
input (dB)
“40 -30 -20 “19
Pa are
wo o
Output (dB)
-40
Po:
Fig, 2.3
(2} Expander (Decoding)
Fig. 2.4 is the basic circuit of the expander.
_ Compressed signal is fed to V1‘ and V2’ from P1’.
V2' is similar to V1 and V2 amplifiers of the compressor.
Level sensar and V2’ operate so as to keep output signal
level P2’ at a constant level.
Level sensor output is given to V1‘ as well. Gain of V1‘ is
1/V and therefore the compressed input P1’ will be ex-
panded by V1’, and subsequently the original signal is
obtained at output PO’.
‘This is shown in Fig. 2.5. Output PO’, the thick line, is
two times the gradient of Pt’. For instance, —-20 dB input
at input PT’ is expanded to —40 dB at output PO’.
PY Po
1“¥
ve
y > PZ
Le eee
: Level
Sensor
Fig. 2.4
Input (dB)
-40 -30 -20 -10
j re)
Output (dB)
Pr’ Po’
Fig, 2.5
The basic circuit of V1 and V2 of Fig. 2.2 and V2’ of Fig.
2.4 is shown in Fig. 2.6.1. On the other hand, the basic
circuit of V1' is connected as shown in Fig. 2.6.2 in order
to obtain the reciprocal of V2’ gain,
Zt
Vq=~—~—— V3
21+ Ze
Fig. 2.6.2
ities aR TE , Soh dele ak
pre ee
er
oe eee

Input signal is divided into two bands, each of which is expansion, with the parameter for example 50 Hz, 400 Hz,
independently separated in performing compression or 3 kHz and 10 kHz. '
expansion, and subsequently the modification in noise at :
the lower band, so called breathing noise, is intercepted. vi],
This division of two bands results in noise reduction ;
approximately by 20 dB in both bands, not only in the si Pee ae
higher band but also in the lower. band as well.
Further, in each of these two bands of frequency, differ-
ent time constants are used for controlling the amplifying
rate so as to reduce the distortion of the middle and lower
band and to improve the response at the higher band, pre-
venting the tone quality from deterioration.
Fig. 2.7 indicates the compression characteristics. From
this chart you can see the changes of compression fevel in
accordance with the frequency, for instance, less compres-
sion to lower frequency and more compression to higher
frequency. But you will see the limit at the higher fre-
quency band that no compression is made below —40 dB.
As the frequency goes up to a higher value, the operating
point of compression shifts to the lower level and lowers
the recording level at the higher frequency. This is to put
i
Output Level(dB)}
the saturation level into consideration at the higher band Sy See ee, ee Se ee on eG aN ey
Input Level(dB)
on the tape deck,
From the chart of the Fig. 2.8, you can read the input Fig. 2.8 Compression (Encode) and Expansion
level. vs output level characteristics of compression and (Decode) Characteristics
Rec, Out Level (dB)
20 50 © 100 200 500 1K 2k OK 10K 20K
Frequency (Hz)
Fig, 2.7 Compression {Encode} Characteristics

2.2. Subsonic Filter -
The frequency response of ordinary hi-fi phono cartridges
covers the subsonic range. The resonance point is near 10
Hz with a peak of 5 to 15 dB. These factors are
determined by the mass, compliances and damping resist-
ance of the cartrige and tone arm. Further, near the —
resonance frequency, the disc record is likely to be
eccentric or warped, or the turntable vibrates abnormally.
In extreme cases, the resonance frequency increases to the
level of disc record playback signals (the worst condition
occurs when the vibration caused by the speaker is fed
back to the cartridge via air or floor vibration). Usually,
the subsonic effect thus produced is not found. Because |
of inter-modulation distortion, the subsonic effect causes
unclean sound from amplifier, speakers or tape decks, It
especially affects such systems whose response curves
cover lower frequencies {note that, if the woofer moves
unsteadily during playback of disc records, the above-
mentioned adverse effect can be produced}.
The turntable, cartridge and tone arm must be improved
to completely eliminate subsonics. However, even im-
proved turntable, etc. could not completely eliminate
subsonics. One solution of this problem so far achieved is
using commercially available preamplifiers that incorpora-
te subsonic fitters. But most of them shows poor
attenuation curves of 6 dB/Oct. or 12 dB/Oct., and they
can not sufficiently eliminate subsonics. And they have -
fault to attenuate the low frequency band {near 30 to 40
Hz}. The subsonic filter used in the High-Com II, based on
the new active filteration technology, can realize an ideal
filter characteristic,
Fig. 2.9 shows the subsonic filter of the High-Com II. The
portion represented by FT is generatly known as a twin T
filter, Its characteristics are illustrated in Fig. 2, 10 (1). As
shown, the curve of the twin T filter rapidly drops at 20
to 50 Hz, and attenuation at below 5 Hz is rather small.
These demerits have been smartly eliminated by the
High-Com II as foflows:
Input
C107
680OP{N }
Improvement 1: Improved Twin T Filter with Boot Strap’
As shown in Fig. 2.9, the output from the twin T filter is
amplified by OP-amp. |C302 and taken from its output
terminal. This output provides positive feedback to the
non-inverting input of 1C302 through C107 and R109.
‘This greatly reduces the level down in the range 20 to 50
Hz. For greater attenuation in the range below 5 Hz,
R110 is added to lower the load impedance of the filter
and to change the impedance of each element so that the
asymmetric curve as shown by Fig. 2.10 (2) can be
achieved, .
Improvement 2: Addition of CR Filter
The curve shown in Fig. 2.10 (2) is satisfactory for the
subsonic filter except insufficient attenuation at below 5
Hz,
Besides the High-Com 1I uses a CR filter consisting of
C108 and R112 to achieve a more ideal subsonic filter
curve as shown by Fig. 2.10 {4}. (In Fig. 2.10 (3) shows
the CR filter curve and {4) is a combination of curves (2)
and (3).}
“104
1
Ny oO
ri
-304
ATTENUATION (dB)
~404
-504
- : +
3 5 10 20 70 100
FREQUENCY iHz}
Fig. 2.10
Posbtaree aac Foe
ctrog
O.OSbut)
Output
Rit2
120K
a
Fig. 2.9 Subsonic Filter Circuit
pea

2,3. Mute Signal
Output signals are muted for a certain time to prevent
transient noise when power is ON or OFF, Fig, 2.11 shows
the mute circuit and Fig. 2.12 shows a timing chart of the
mute signals,
2.3.1. Power ON
Transformer output is rectified through diode D402 and
smoothed by capacitor C401. Therefore, positive poten- —
tiat appears at C401 (transistor O401 base}, That is, 0401
is in the cutoff state, while C402 (47 uF) is charged with
negative potential through R404 (2.2 MQ). At the level
where the voltage across C402 exceeds Vbe {base-emitter ©
voltage} of 402, O402 turns from OFF to ON, As a
result, Q403 turns ON and the mute signal is changed
from + V to —12 V, releasing the mute state. This means
the mute time depends on C402 and R404 after power is
ON,
2.3.2, Power OFF
Transformer output becomes zero, as a result of which
C401 is charged with negative potential through R403. At
the level where the voltage across C401 exceeds Vbe of
0401, 0401 turns from OFF to ON and C402 is quickly
discharged. Thus, 0402 is cut off and Q403 is also cut off.
+V¥
D403
i : C403
Output
Mute )4-7K
0403
280945
2
2208 25V 47 1OVILN)
The mute signal voltage becomes positive to mute the
output signal. D403 acts to prevent + V from being
discharged easily when power is OFF..
Power ON
Switch :
OFF
c4o1 pas Oe \ t
. Q401 Vbe\,
Q402 Vbe 1
C402 we 1 =
0402 me =
Coliecfor
-12V
+V
Mute
OV
-12V
Open SS ———S
Output ' !
ov——— Los
Fig. 2.12 Timing Chart
0402
R40)
270K
Fig. 2.11 Mute Circuit

3.5. Main P.C.B. Ass’y
Refer to Fig, 3.2.
(1) Refer-to Fig, 3.1. Remove Front Pane! Ass’y referring
to item 3.4.
{2) Remove the connector and wires connected by wrap-
ping from FQ2 (Main P.C.B. Ass’y}.
(3) Remove FO1, then disassemble FO2 (Main P.C.B.
Ass‘y). ,
3.6. Power Transformer
Refer to Fig. 3.2,
(1) Refer to Fig. 3.1, Remove Top Cover Ass‘y referring
to item 3.1.
(2) Remove F03 and F04, then disassemble FOS (Power
Transformer). ,
3.7. Meter Ass’y, Meter and Lamp P.C.B. Ass‘y
Refer to Fig. 3.2.
(1) Refer to Fig. 3.1. Remove Front Panel Ass’y referring
to item 3.4. ;
{2} Remove FQ6, then disassemb!e FO7 (Meter Ass’y}.
(3) Remove FO8 (Meter Band), then disassemble FO9
{Meter).
Remove F10, then disassemble F114 (Lamp P.C.B.
Ass’y).
—
(4
3.8. Power Switch
Refer to Fig. 3.2.
{1} Refer to Fig. 3.1. Remove Front Panel Ass’y referring
to item 3.4,
{2) Remove F12, F13 (Power Switch Flange} and Fi4,
then disassemble F15 (Power Switch),
3.9. Rear Panet Ass’y
Refer to Fig. 3.3.
{1) Refer to Fig. 3.1. Remove Top Cover Ass‘y and
Bottom Cover Ass’y referring to items 3.1 and 3.2,
{2) Remove FO1, then disassemble FO2 (Rear Panel
Ass'y).
3.10. 2P Pin Jack and Voluma
Refer to Fig. 3.3.
(1} Refer to Fig. 3.1. Remove Top Cover Ass‘y and
Bottom Cover Ass’y referring ta items 3,1 and 3,2.
(2) Pull out FO3 (Volume Knob), Remove F04 and FO5,
then disassemble FO6 (Volume 10 KQ{B)) and FO7
(Volume 50 KQ(B)).
(3) Remove FO8, then disassemble FOS (2P Pin Jack}.
FO2

3. REMOVAL PROCEDURES
3.1. Top Cover Ass'y
Refer to Fig. 3.1. Remove F01 and F02, then disassemble
FO3 (Top Cover Ass’y).
3.2, Bottom Cover Ass’y
Refer to Fig. 3.1. Remove F04, then disassemble FO5
(Bottom Cover Ass’y).
3.3. Side Panel Ass’y
Refer to Fig. 3.1, Remove FO6 and F07, then disassemble
FO8 (Side Panel Ass‘y).
3.4. Front Panel Ass’y and Meter Escutcheon
(1) Refer to Fig. 3.1, Remove Top Cover Ass’y, Bottom
Cover Ass’y and Side Panel Ass’y referring to items
3.1 through 3.3.
(2) Pull out FO9 {Volume Knob A Ass’y) and Ft0
(Volume Knob B Ass‘y}, Remove F11, then disassem-
ble F12 (Front Panel Ass’y}.
{3) Remove F13 (Meter Escutcheon).
FOS
FO4
Fig. 3.1