Yamaha DRX-2 Service Manual Download

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DRX-2

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CIRCUIT DESCRIPTIONS

1. Display Board

1.1 Microcontroller

The core element of the Display Control unit is the
m i c r o c o n t r o l l e r T M P 8 7 C H 7 4 A F [ 7 1 1 0 ] . T h e
TMP87CH74AF is an 8 bit microcontroller fitted with 32kB
ROM and 1kB RAM. It requires 5V supply and is
responsible for the following functions:

– Interface to Central Controller-P

– Evaluation of the keyboard matrix

– Decoding the remote control commands from the

infrared receiver

– Activation and control of the local display

– Heater voltage generation

The 8 MHz resonator (Pos. 1111) generates the system
clock.

The reset is generated by the CC-P via “POR_DC”-signal
where the transistor [7106] is used as a level-shifter from
3V3 to 5V.

1.2 Interface to the Central Control P

The communication to the main microcontroller (CC) on
the P-Sub- PCB is done via I2C-Interface, where the
TMP87CH74AF acts in slave-mode.

An additional wire (“INT”-line) is used to signal the Central
controller that data are ready, e.g. when a key has been
pressed.

1.3 Evaluation of the keyboard matrix

There are 12 different keys on the display board. A resistor
network is used to generate a specific direct voltage value,
depending on the pressed key. Via the resistors 3107 and
3102 on the analog/digital (A/D) ports (7103 pin 37 and
38) the evaluation is done.

1.4 IR receiver and signal evaluation

The IR receiver [7107] contains a selectively controlled
amplifier as well as a photo-diode. The photo-diode
changes the received infra red transmission (approx.
940nm) to electrical pulses, which are then amplified and
demodulated.

On the output of the IR receiver [7107], a pulse sequence

with TTL-level, which corresponds to the envelope curve
of the received IR remote control command, can be
measured. This pulse sequence is fed into the controller
or further processing via port TC1 [7103, pin20].

1.5 Vacuum Fluorescence Display

The VFD “BJ900GNK” [POS 7100] is fully controlled by
the microcontroller. The C also includes the driving
stages. Only two additional drivers [POS 7101 and 7102]

are necessary for the grids 8 and 9 because of their large
size.

1.6 VFD Heater Voltage Generator

The circuit around POS [7106, 7108 and 7109] is used to

generate a proper AC-Voltage for the filament of the VFD.
For this the microcontroller generates an appropriate
rectangular signal with 50%duty-cycle and a frequency of
30 kHz at pin 19.

Pos. [5104] and [2113] are acting as a resonance-circuit.
Via Zener-Diode (POS[6100]) and resistors [3119, 3122
and 3123] the two heater-pins of the VFD (“FIL1” and
“FIL2”) are clamped so that the grids and segments can be
fully switched off.

1.7 REC-LED

The REC-LED-ring is made with 3 red LED, controlled via
pin 3 (only for flashing) and pin 12 for on/off switching, of
the microcontroller. The POS [7105] is used as a driver for
the led.

1.8 EPG-LED

The EPG led is a white led and controlled from the pin 14
from the microcontroller. The POS [7110] is used as a
driver for the led.

1.9 TRAY-LED

There are 6 LEDs (chip) necessary to illuminate the tray,
these 6 leds are located on a little sub-pcb connected over
a 4 pin connector POS [1911] from the DC-print. The leds
are controlled from pin 11 of the microcontroller.

2 Microcontroller Sub Board (UP SUB Board)

2.1 General

This small PCB is directly soldered in on top of the
Analogue-Board.

It is used with no diversity in all three different basic
versions (Europe, NAFTA and APAC-Pal). Only the
software being loaded into the external Flash-memory is
not the same.

2.2 Microcontroller

The main part of the Sub-PCB is the central controller
(CC) P [7804] TMP91CW12AF, which is a 16-bit CPU with
128kBROM and 4kB RAM.

It works with a 3V3 supply and a system clock of
24,576MHz [1801].

The 3V3-supply is made out of the “5VSTBY” by the circuit
around [7816].

After connecting the set to the mains (power-up) the IC
[7806] generates a reset pulse. This signal (“IPOR”) is
directly fed to first priority interrupt input (pin 63) for power