Korg EX-800 Service Manual

7. CIRCUIT DESCRIPTIONS
MAIN CIRCUIT DESCRIPTIONS
The EX-800 is intended to be an expander unit for connec
tion to synthesizers, sequencers, or other equipment that
has a MIDI output. It is essentially the POLY-800 without a
keybord. However, there are some differences in the MIDI
functions, as detailed in the MIDI IMPLEMENTATION
notes.
The memory backup system rs also different. The EX-800
employs an internal lithium battery to protect data in its
memory. The life of this battery is about five years and it
should be replaced after that time has elapsed.
For check and adjustment of the EX-800 please refer to the
POLY-800 service manual.
Below are simple descriptions of each circuit block.
Refer to circuit diagram for number.
1) Tape interface input circuit:
Consists of amplifier and comparator. When command is
executed, data on this line is input to the CPU accumu
lator's 7th bit.
2) CPU:
A CMOS 8-bit microprocessor IG24 (80C85) featuring low
power consumption. Virtually all POLY-800 functions are
handled by this CPU.
3) Reset circuit:
IC40 (PST518) is a 3-pin IC used for reset. It generates an
initial reset voltage of about 4.2V.
4) Sequencer tempo clock oscillator circuit
The tempo circuit includes IC28 (TC40H074) and 1/2 of
IC36(whichis1/2ofaTL072).
The tempo control is connected to CN2 pin 1 providing
10Hz±20% at the knob's 0 position and 100Hz±20% at the
10 position for CPU interrupts. If this circuit fails, then
there will be no sound from the sequencer section.
5) Interrupt oscillator circuit:
This oscillator cycle is used for the EG, MG, LED displays,
and S/H time division processing. Oscillator frequency is
2400Hz-N-3600Hz. Interrupt order is by priority. If this
circuit fails, EG operation and LED indication may become
erratic.
6) Address Decoder:
TTL circuit decodes addresses for RAM and other ICs.
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7) ROM (8192 words x 8bit PROM)
8) RAM (2048 words x 8bit static RAM)
9) Address latch:
IC latches according to CPU ALE (Address Latch Enable)
terminal output signal since CPU uses address LSB 8bits
together with data bus 8bit input.
10) Peripheral I/O:
PA, PB, and PC ports are all used for output. The internal
timer is used for the interface IC26 (63B50) reference clock.
The CPU 3MHz clock frequency is divided by 6 to obtain
500kHz. RAM is used for the program working area.
11) LED display drive circuit:
IC30 (BA618) and IC31 (M54513) form a 6 x 8 matrix for
time sharing indication by the panel's 7-segment LED
display.
12) 8-bit D/A converter:
Uses CMOS noninverting buffer IC32(HD 14050or "4050"),
and BR9 (RKM10L253F or "BR9") a 10-pin (R=25kohm)
R-2R ladder resistor in D/A converter with output of 0V ^
4V.
13) External DC power supply ripple filter:
Diode D2 is used to protect the circuit in case of reverse AC
adapter polarity.
14) LED display power supply:
Circuit is designed so that LEDs become dim when battery
voltage drops below rated level, (about 6V)
15) +5V power supply:
This circuit design is employed because it maintains normal
operation up until just before the batteries drop below
rated voltage of Volts (about 6V)
16) -5V power supply:
A type of DC-DC converter.
18) A/D converter comparator.
19) Master oscillator:
Varactor VC1 and coil KL-003 are used in the oscillator
circuit. This generates a frequency of about 2MHz at the
tune knob's center position. This is divided down (to about
1MHz) to supply the TG. (CL1, CL2)
Bend and vibrato control voltages are D/A converted by
IC35 (3404) and applied to the oscillator.
20) EG S/H circuit:
EG values calculated by the CPU are output by time sharing
and input to the TG.
LED diodes for each voice are there to smooth the stepped
transition.
21) Keyboard panel switch input circuit:
A 9 x 8 matrix is formed by DTC5, IC34 (TC40H138), and
IC33 (TC40H240). This handles keyboard and panel switch
outputs as well as output from the comparator in circuit
diagram (18).
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Korg EX-800 Specifications

General

Brand	Korg
Model	EX-800
Category	Synthesizer
Language	English