Yamaha DRX-1 Service Manual Download Page 73

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DRX-1

Control Circuit
The control circuit exists of T7140, D6141, C2144 and
2145, C2147, R3147 and 3148.
This circuit is fed by supply vVb via R 3141. This
circuit controls the conduction time and the switching
frequency of the power switch circuit. It switches off the
MOSFET as soon as the voltage over Rsense reaches a
certain value. This value depends on the error voltage at
the emittor of T7140, which can be positive or negative (+/
- 0,66V). The voltage fed back by the regulation circuit
defines this error voltage.

Power Switch Circuit
This circuit comprises MOSFET 7125, Rsense formed by
R3133, 3134, 3135, 3136 and 3137, R3131, R3132,
D6146.
Diodes 6130, 6131 and 6132 protect the control circuit in
case of failure of the MOSFET.

Regulation Circuit
The regulation circuit comprises opto-coupler 7200, which
isolates the base voltage of transistor 7140 at the primary
side from a reference component 7251 at the secondary
side. The TL431(7251) can be represented by two
components:
• a very stable and accurate reference diode
• a high gain amplifier

2.5V

Fig. 2

TL431 will conduct from cathode to anode when the
reference is higher than the internal reference voltage of
about 2.5V. If the reference voltage is lower, the cathode
current is almost zero.
The cathode current flows through the LED of the opto-
coupler.
The collector current of the opto-coupler will adjust the
feedback level of the error voltage at the emittor of T7140.

Overload Protection Circuit
This circuit consists of R3145, C2143, a thyristor circuit
formed by T7141 and T7143, R3143 and R3142. When
the output is shortened, the thyristor circuit will start to
conduct and switch off the supply voltage over C2140.
This results in a switching off of the drain current of the
MOSFET 7125 and the output will be disabled. The start
circuit will try to start up the power supply again. If the
circuit is still shortened, the complete start and stop
sequence will repeat. The power supply comes in a hiccup
mode (is ticking).

Overvoltage Protection Circuit
This circuit consists of R3149, D6144, 6143, R3144,
C2142 and T7142.
When the regulation circuit is interrupted due to an error in
the control loop, the regulated output voltage will increase
(overvoltage). This overvoltage is sensed on the primary
winding 7-8.
When an overvoltage is detected, the circuit will start up
the thyristor circuit T7141-7143. The power supply will
come in a hiccup mode as long as the error in the control
loop is present.

Secondary Rectifier/Smoothing Circuit
There are 6 rectifier/smoothing circuits on the secondary
side.
Each voltage depends on the number of windings of the
transformer.
From these circuits a lot of voltages are derived and fed to
3 connectors. The following voltages are present at the
output: Connector 209
Functional use: to Digital board + Dvio board
1.

+3V3(for dig pcb + DVio)

GND(for dig pcb + DVio)

+12V(for dig pcb + DVio)

GND(for dig pcb + DVio)

+5V(for dig pcb + DVio)

10. STBY control(for dig pcb + DVio)
11. GND(for dig pcb + DVio)
12. -5V(for dig pcb + DVio)
The +12V is switched off by the STBY_ctrl signal.
When the +12V is switched off, also the +3V3, +5V and -
5V are switched off. All these voltages are low drop
regulated.
Connector 0205
Functional use: to analogue board + display board + flap
motor ‘STBY’ indicates that the voltage will not be
switched off in the standby situation.
1. +12VSTBY(= +12V Standby, for display heating,

8Vstby)

2. +5VSTBY(= +5V Standby; general use)
3. -5NSTBY(= -5V Standby; neg. voltage for drivers)
4. VGNSTBY(= -32V Standby; for display grids)
5. +33STBY(= +33V Standby; for tuner)
6. FLYB(flyback pulse for power fail + measurement)
7. GNDA(Ground for the analogue board)
Connector 0207
Functional use: to engine
1. +3V3(for engine servo board)
2. +5V(for engine servo board)
3. GND(for engine servo board)
4. +4V6E(for engine analog part)
5. GND(for engine servo board)
6. -5V(for engine servo board)
7. GND(for engine motor currents)
8. +12V(for engine motor currents)