Casio SK-8A Troubleshooting guide

CONTENTS
1. SCHEMATIC DIAGRAM
1-1. Main PCB M3123-MA1M. 1
1-2. Console PCB M3123-CN1M . 2
1-3. LED PCB M3123-LD1M . 3
1-4. Keyboard PCB M3129-KY1M . 4
2. PCB VIEW & MAJOR CHECKPOINTS . 5
3. MAJOR WAVEFORMS. 6
4. BLOCK DIAGRAM. 7
5. KEY & SWITCH MATRIX. 8
6. CPU (MSM6283-05GS) . 9
7. PERCUSSION GENERATOR (MSM6294-02). 11
8. MELODY CIRCUIT.12
9. SAMPLING CIRCUIT.13
10. PERIPHERAL DEVICE ACCESS.14
11. ROM PACK ACCESS . 15
12. LED DRIVE CIRCUIT .16
13. RESET & POWER ON/OFF DETECTION CIRCUITS.17
14. POWER SUPPLY CIRCUIT .. .18
15. TROUBLESHOOTING ..19
PARTS LIST
SK-8.20
SK-8A .27

1. SCHEMATIC DIAGRAM
1-1. Main PCB M3123-MA1M
A I B
To LEDs
I ''
DC !N HEAD PHONE
TC4066BP13/A 1
VA
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Power
a.
Amp. 2 J
v 2 2
J V
C22I
138
10V47M |
Apo [50V2.2A
IK 10KB
_, MAIN VR
1 -"— " 5j—
t Mixing Amp.
5l)V1M
1 '* Tuft
2 )
1 100K
~68K
r
iwtlHIwj
y
TC40S6BP !3/4I
Go in Con t r o 11 er
12SC1740LNSR V0O_
>CV 15K 15K 15K

1 -2. Console PCB M3123-CN1M
A|B_j_C[0|E_j_F|G|H
A B C D E F G H

J_j_K|L|M|N_j_0|P
-2 -
i
2
j 3
L
5
6
7
8
9
10
I
! ii
i

I
i
wr

MAIM

P 2. PCB VIEW & MAJOR CHECKPOINTS

3. MAJOR WAVEFORMS
Notes: Photographs of waveforms have been taken using a memory scope.
Attenuation Radio of probes 10:1
(?) RESET signal
When AC adaptor is inserted
0.2V/div., 0.1 sec/div.
(?) Signal Ml
At Power ON
0.2V/div , Ims/div
( 3 ) CPU clock
Percussion Generator Clock
0.2V/div., 0.5ms
©
©
Signal SH
Melody Circuit Output
Tone: Pipe Organ
Key: C2
50mV/div., Ims/div.
-6-

LED
Driver
Key
&
Switches
4. BLOCK DIAGRAM

5. KEY & SWITCH MATRIX
KI1
KI2
KI3
KI4
KI5
KI6 KI7
KI8
KOO
FI
F#1
G1
G#1
KOI Piano
Vibra¬
phone
Guitar Pipe Org.
A1
A#1
B1
C2
K02
Trumpet
Violin Flute Clarinet
C#2 D2
D#2
E2
K03
Sample
1
Sample
2
Sample
3
Sample
4
F2
F#2
G2
G#2
K04
Memory
Stop
Memory
Start
Edit
A2
A#2 B2
C3
K05
Sample
Normal
Sample
Long
C#3 D3
D#3 E3
K06
Rock 1
Rock 2
Disco 1
Disco 2
F3
F#3
G3 G#3
l<07
1G Beat March
Bossa Nova
Samba
A3
A #3
B3 C4
l<08
Slow
Rock
Waltz
Sample
Pad 1
Sample
Pad 2
Sample
Pad 3
Sample
Pad 4
l<09
Play
Record Demo
Power
Off
KOIO
Tempo
▼
Tempo
A
Tune
T
Tune
A
KO11
Accomp.
Obbligato Melody
- 8 -
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6. CPU (MSM6283-05GS)
O Generates Pitch and Envelope signals for Melody, Chord, Bass and Obbligato sounds,
o Controls Keys, Switches and peripheral devices.
The following table is the pin functions of the CPU.
Pin No. Terminal Name In/Out
Function
1
BGND
5V source for internal ADC (Analog to Digital Converter)
2
VADC
Ground (OV) source for internal ADC
3, 4 No function
5~11
100-106 In/Out
Data bus (DO — D6)
12
AO Out
Address bus (AO)
13 107
In/Out Data bus (D7)
14
A1 Out
Address bus (A1)
15 CE
Out
Chip enable signal for memory devices. LOW active.
16 —18 A2, A10, A3
Out Address bus (A2, A3, A10)
19 No function
20-29 A4—9, All-14
Out
Address bus (A4 — A9, A11 — A14)
30 A15
Out
Output enable signal for memory devices. LOW active.
31
WE
Out
Write enable signal for RAM. LOW active.
32-38 No function
39 CE1
Out
Write enable signal for Percussion generator. LOW active.
40 GND 5V source
41
CE2
Out
ROM pack's chip select signal. LOW active.
42
No function
43
VDD1
Ground (OV) source for internal digital circuit
44 VDD2
Ground (OV) source for internal analog circuit
45
No function
46, 47
CPI, CP2 Out ROM pack's clock pulse
48 OP
Out
ROM pack's data/address change signal.
Data bus for ROM pack becomes data or address in accordance-with the
voltage level of this signal.
49-52
DO - D3
Out
Data bus for Percussion generator.
53-56
D4- D7 Out
Data bus for Percussion generator and data/address bus for
ROM pack.
57, 58
OSI, OSO
In/Out
7,277MHz clock pulse input 0 and output
59
Ml
In
Power ON detection signal input. HIGH active.
Receiving a pulse at Power ON, CPU starts functioning.
60
RESET
In
Reset signal input. HIGH active.
The terminal receives a pulse which resets CPU internal circuit when
new batteries or AC adaptor is set.
61
No function
62
K014
Out
Accompaniments' volume control signal.
63 K013
Out
ROM and RAM chip select signal.
- 9 -

Pin No.
64
Terminal Name
ln / Out
Out
64 K012
Out
65~76
KOII-KOO
Out
77-84
KI1 ~ KI8
1 n
85
POO
Out
86-88 POI ~ P03 Out
89
Out
90 EG
Out
91 EF
Out
' 92
EE
Out
93
SH
Out
94
SG
Out
95 SF
Out
96 SE
Out
97
98 AGND
99 VDAC
100
Vin
In
Timing
Function
APO (Auto Power Off) signal output.
Terminal drops to LOW level to shut the voltages off when the unit is
not operated for approximately 00 minutes.
Key common and LED drive signals outputs.
Key input signals
Reset signal for Percussion generator.
The terminal holds LOW level at power ON for a while during which
the internal circuits of the Percussion generator are initialized.
LED common signals output.
Melody envelope signal output
Melody or obbligato envelope signal output
Melody or chord envelope signal output
Melody or bass envelope signal output
Melody pitch signal output
Melody or obbligato pitch signal
Melody or chord pitch signal
Melody or bass pitch signal
No function
+5V source for DAC (Digital to Analog Converter)
Ground (0V) source for DAC
Sampling sound input signal from MIC terminal
n n n n
h n n n
n n n n
n n n n
■ i
L_
4t
i - 1
1 1
i_i-
1 1 "
6t
i
Address
)( Address
\
7t_
t
i-r
n_r
Data (into RAM)
n
n
Write Cycle (+ pulse refresh)
Floating Cycle (+ pulse refresh)
i - 1/fPG («7,802MHz)
art of sampling CPU for ROM and RAM

7. PERCUSSION GENERATOR (MSM6294-02)
Containing a internal ROM (128 Kbit) for percussion data, and internal 8 bit DAC (Digital to
Analog Converter), the LSI provides percussion sounds.
Percussion
Out
Pin Function of Percussion Generator
Pin No.
Signal
In/Out
Function
1
RESET
In
Reset signal input. LOW active.
The terminal holds LOW level at power switch on for initializing the internal
circuit
2 CLK
In
Clock pulse (256 kHz) input
3
No function
4 WR In
Write signal input. LOW active.
The LSI reads the percussion data from the CPU when the terminal is
LOW level.
5
cs
In
Chip select signal input.Fixed at LOW level.
6 ~ 13 D0~ D7
In
Data bus (DO — D7)
14
DVSS
Ground (OV) source for internal digital circuit
15, 16 No function
17
AVSS
Ground (OV) source for internal DAC
18 MIX-0
Out
Analog percussion signal output
19 AVDD
+5V source for internal DAC
20-27 No function
28 DVDD +5V source for internal digital circuit

MELODY CIRCUIT
VD -
p 'f O
VDo-,
15K
15K
_ 111 _ 1
n
L5
rev
Js 1
ID
CD
O
li
<XO -7-.MA—O I
- AAA-1—W—(I—C
68 K 18|
\ 2.5 V
Mixer
47 K 11
Amp.
2.5V 220K
47K
Gain Controller
Envelope ^11 M e '°?V or
5 ^—,- 4 ' Obbligato
| t 3 Pitch SE
-II-
14 Envelope EE
(
—
I ^
r Pitch SF
-it-—-——-i
Envelope EF U^ 0 '
--iJ Ch0rd
Envelope
CPU outputs four each of pitch and envelope signals.
Normally all the signals are for melody sounds however, when a ROM Pack music is played, each
pitch and envelope signals become obbligato, bass, chord, or melody sounds.
CA91A 02 contains four mixing amplifiers and merges each pitch and envelope signals.
Gain Controller balances the sound volume of melody and accompaniments for ROM Pack musics.
Signal K014 normally stays HIGH level so that all the switches turn on. When a ROM Pack
music is played, K014 falls to LOW level turning the switches off and all the accompaniment
signals are reduced as they pass through the resistors.

9. SAMPLING CIRCUIT
VC
Fig. 1 Sample Sound Input Circuit
Start
Fig. 2 Vin Input Trigger Level
As shown in Fig. 1, the circuit provides sampling signals to the Vin terminal of the CPU. Resistors
lOOKohm and (100 + 4.7) Kohm bias the sampling signal on 2.863V.
When the sampling sound level exceeds ± 0.315V as shown in Fig. 2, the CPU starts to transmit
the sampling sound data to the RAMs during the following period.
While sampling, the CPU transmits the sampling sound data to the sampling RAMs directly.
While sampling, CPU does not output key common signals (KOO ~ K09) so that the keyboard
becomes inoperative.
Sampled sounds are digitized in CPU and transmitted to the RAM.