Marantz MAR775, MAR770, DR-6000 service manual Circuit description of P816

Circuit description of P816

Input circuit

The input circuit consists of a lightning protection circuit and an EMI filter.

The lightning protection comprises R819.

The EMI filter is formed by C820, L820, C825 and C826. It prevents inflow of noise into the mains.

Primary rectifier/smoothing

The AC input is rectified by rectifier bridge D802 and smoothed into C821. The voltage over C821 is approximately 300V.

It can vary from 100V to 390V.

Start up circuit and Vcc supply

This circuit is formed by R823, R834, C829, D828, R829, R811, C833 and C811.

When the power plug is connected to the mains voltage, the stabilised voltage over D829 (24V ) will charge C833 via R829.When the voltage reaches 14.5V across C811, the control circuit of Q810 is turned on and the regulation starts. During regulation, Vcc of IC7110 will be supplied by the rectified voltage from winding 7-9 via L832, D832 and C833.

Control circuit

The control circuit exists of Q810, C802, C804, C807, C809, C810, R802, R803, R804, R807, R808, R809 and R810. C802 and R810 define the frequency of the oscillator.

Power switch circuit

This circuit comprises MOSFET Q825, Rsense R826, R827 and R828, R825, C827, L825, R812 and R813. R825 is a pull-down resistor to remove static charges from the gate of the MOSFET.

Regulation circuit

The regulation circuit comprises opto-coupler Q801which isolates the error signal from the control IC on the primary side and a reference component D881. The TL431(D881) can be represented by two components:

a very stable and accurate reference diode a high gain amplifier

K

R

2.5V

A

CL 96532076_036.eps

290799

Figure 8-17 ‘TL 431’

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 flows through R806, producing an error voltage, connected to voltage feedback pin 14 of Q810.

Demagnetisation

The auxiliary winding (7-9) voltage is used to detect magnetic saturation of the transformer core and connected via R801 to pin 8 of Q810. During the demagnetisation phase, the output will be disabled.

Overvoltage protection circuit

This circuit consist of D814, C814, R814, R815 and R816. 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 at the auxiliary winding 7-9.

When an overvoltage longer than 2.0 (s is detected, the output is disabled until VCC is removed and then re-applied. 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 5 rectifier/smoothing circuits on the secondary side. Each voltage depends on the number of windings of the transformer.

The -8V supply is regulated by voltage regulator Q871.

On/off circuit

24