BBK AV250 Service Manual

AV250T Service Manual

Contents

chapter One Brief Introduction to Product

General description

Chapter Two Operating Principle

Section One Overall structure Section Two Volume board Section Three Signal processing board Section Four CPU board Section Five Panel control and display circuit Section Six Power amplifier board Section Seven Power board Section Eight Video board

Chapter Three Servicing Process

1. No output for Karaoke

2. Power supply not connected

3. No on screen display (OSD)

4. No output Schematic & PCB wiring diagram Spare parts list

Chapter One Brief Introduction to Product

General Description

AV110T is a 5.1CH audio power amplifier for home theatre usage, with the

main features and functions shown as follows:

1) Two-way audio signal input: VCD, DVD;

2) With 5.1CH input terminal, capable of connection with AC-3, DTS decode

output signal;

3) One way subwoofer signal line output, capable of connecting with active

subwoofer speaker;

4) Automatically signal search when power on;

5) 6CH digital volume control and separate channel level adjustment;

6) Three sound field modes: Hi-Fi, Standard and Cyber Logic;

7) Six preset EQ modes, multi-band electronic equalization adjustment snd

scene surround function;

8) Save/Call of user-set parameter;

9) Digital Karaoke, with microphone volume, echo and delay adjustment;

10) Auto mute function;

11) Dynamic display screen; multiple spectrum display modes; English

intelligent operation interface;

12) Bass enhancer function;

13) Digital tuning function;

14) Video switch function;

15) Headphone output function;

16) Full remote control operation.

Chapter Two Operating Principle

Section One Overall structure

The whole unit of AV250T is mainly composed of the following parts:

2.1.1 Volume board: input selection, Cyber Logic and bass enhancer control.

2.1.2 Signal processing board: Karaoke signal processing and front stage amplifying of 5.1Ch signal.

2.1.3 CPU board: whole machine control unit, breadth sampling circuit and auto searching circuit.

2.1.4 Control panel: VFD display, remote control and keyboard scanning and fulfill man-machine conversation function.

2.1.5 Power amplifier board: perform power amplifying for 5.1CH signal or other analog signals.

2.1.6 Power board: supply the working voltage required by each unit circuit, and over-current/over-voltage protection unit.

2.1.7 Digital tuner: receive radio signal and then send to power amplifier to perform signal-amplifying processing.

2.1.8 Video input and output board: fulfill video input/output and video switch function.

Section Two Volume board

AV250T has 4 kinds of input modes in all: FM/AM, VCD, DVD and 5.1CH.

By sampling from L/R channel of AV250T, after Cyber Logic function, C, SL

and SW channel signals are achieved. In this circuit, electronic analog switch is adopted to fulfill the switch in all states, and the signal flow chart is as

FM/AM

VCD

5.l input

L, R

Channel

5.1 input

C, SR, S

channel

CD4052

Input

selection

CD4053

Electronic

switch

N106

M62446

Electronic

volume

adjustment

N108B

N103A

N103B

5.1 input

SW CH

SW1

CD4053

Electronic

switch

Send to L/R

N100

CPU

follows:

2.2.1 Input selection and sound field processing mode

The input selection of AV250T is achieved via electronic switches CD4052

and CD4053, and the truth tables are as follows:

CD4052 Truth table

Tuner VCD DVD 5.1

CD4053 Truth table

XBYC

X1 1

5.1CH input mode: Now A/B/5.1CH control pins of M62446 are in high level. L/R channel signals of 5.1 input terminal is outputted from pin 3/13 of N101 and sent to IC N106 for volume and tone adjustment; meanwhile, C/SR/SL signals of 5.1CH input terminal are outputted from pin 14/15/4 of N102 and then respectively sent to IC N106 for independent volume adjustment. And SW signal of 5.1CH is outputted via pin 4 of N103 and then sent to M62446 after being amplified by N107A.

Three analog input modes: AV250T totally has three analog input modes:

digital tuning demodulation signal, VCD and DVD, which are controlled via A/B signal respectively. AV250T totally has three sound field modes: Standard, Cyber Logic and Hi-Fi.

1. Standard: Under the control of overall CPU, when bass enhancer is off, L/R

channel and subwoofer output are available; when bass enhancer is on, only L/R channel output is available.

2. Hi-Fi: Under the control of overall CPU, only L/R channel output is available

to M62446.

3. Cyber Logic: Pin 9/10 of electronic switch N101 (CD4052) select a group of

analog L/R channel input signals according to the truth table. L/R signals are outputted from pin 13/3 via the internal electronic switch of N101, and divided into two ways. One way is respectively sent into pin 13/15 of M62446 for electronic volume and tone adjustment. The other way produces SW/S-SR/S-SL and S-C signals via buffer, adder-subtractor and low-pass filter. S-C/S-SR/S-SL signals are sent to pin 12/2/5 of N102. N102 select Cyber Logic signal input (see CD4053 truth table) from Cyber Logic and 5.1Ch signals, outputs C/SR/SL signals from pin 14/15/4 and sends into pin 11/8/9 of M62446 for volume adjustment. Still another way of SW signal directly sends to pin 6 of M62446 after being outputted from N107A.

5.1CH signal sent into M62446 is outputted from pin 31-36 after volume and tone adjustment, and then outputted to signal board by XS20 power

distributor. The relation between sound sources in input circuit and sound processing modes is as the following figure.

Press INPU

button to select

circularly

Two

analog

input

modes

Hi-Fi

Standard

Cyber

Only L/R channel output.

Sound field and EQ setup

invalid

Only L/R/SW channel

output. Concert hall

sound field and EQ setup

available

6CH output. Theatre

sound field and EQ setup

available

5.1 input

mode

6CH output. Theatre

sound field and EQ

setup available

2.2.2 Control circuit

Pin 23/26/27 of CPU (N100) output data, PVST and clock signals send to pin 39/40/41 of M62446, control pin 1/2/3/4 of M62446 output control level, so as to select input signal and spectrum sampling signal. It is worth noting that PVST signal is a latch control signal. When data and clock of CPU are sent to M62446, an identification signal will be added, indicating that this signal can only be used by M62446 while other IC of I2C bus cannot use current data and clock signal.

2.2.3 Frequency spectrum sampling circuit

Only S-C/S-SR/S-SL/SW signals are sampled during frequency spectrum sampling in AV250T and added to pin 14 of N103 via a 150K sampling resistor. Another S-C cyber logic signal is added to pin 1 of N103, called S-C＇. 5.1CH and LR-T of M62446 select sampling signals. When cyber logic is selected, the control signal of 5.1CH is of low level while pin 9/11 of N103 is of low level. According to the truth table, it is known that the outputs are X0/Z0. Sampling signal is grounded while LR-T is of H level. Select Y1, S-C＇signal is outputted from pin 15 of N103 to N108B, adding to OK-R signal for the amplification of frequency spectrum signal, and then sent to frequency point gating and auto search circuits via XS20.

2.2.4 Tuning function

The tuning function directly controls tuner and receives audio frequency signal mainly via CPU and then outputs after amplification via power amplifier. The clock and data line of tuner are shared with LM62446 and the other two control lines are connected to CPU directly. L, R signal processed by the tuner can be sent to N101 IC CD4052 directly to input the selected channel.

Section Three Signal processing board

The signal processing board superposes, mixes and amplifies 5.1CH signal sent from the volume board, voice signal from the voice board and Karaoke signal.

2.3.1 Karaoke circuit of AV250T

This circuit processes human voice through power amplifier and reproduces it via speaker. It includes human voice beautification circuit, wide sound field processing circuit, Karaoke echo and delay adjusting circuit. IC and its functions for Karaoke: S.N. of IC Name of IC Functions of IC

N201 PT2308 Transmittal, pre-amplification for Karaoke signal N200 PT2315 Karaoke volume control, including tone control N205 CD4053 Electronic switch N209 PT2399 Karaoke echo processing N207 CD4051 Karaoke delay adjustment N208 CD4051 Karaoke echo adjustment

N204 4558 Phase inverter

PT2315 functional pin

S.N. Name of pin Description 1 REF Reference voltage (1/2VDD) 2 VDD Power supply 3 AGND Analog 4 TREB L 5 TREB R 6 RIN R channel input 7 LOUD-R Loudness control pin of R channel 9 LOUD-L Loudness control pin of LR channel 11 LIN L channel input 12 BIN L 13 BOUT L 14 BIN R 15 BOUT R 16 RFOUT R channel output 17 LFOUT L channel output 18 DGND Digital 19 DATA Control data of sequence transmission (DATA) 20 LCK Clock input of sequence transmission

8，10 NC Not connected

2.3.2 Flow chart of Karaoke signal When the microphone is inserted, MIC signal is sent via MIC to the transmittal circuit combined by N201A for amplification. Amplified MIC signal gives CPU a MIC identification signal after N202A amplification, followed by VD201 rectification and filtering control triode V200. CPU sends PKM signal, which is of low level, causing cutoff of triode V103/V105 and enabling output of MIC signals; another way reaches pin 6 and 11 of PT2315 after C219/C222 coupling, outputs from pin 16/17 after internal volume and tone control, mixed into one way and sent to N202B and then reversely send to N203B for amplification. Signals amplified by N203B are divided into two ways: one way is directly outputted; and the other way is outputted from pin 14 after being coupled by R222/C247 to PT2399 for internal delayed echo adjustment, reversed by N204B and outputted by mixing with karaoke signal. While OK-R is outputted from pin 14 after being gated by N205 and superposed to L/R channel. In this circuit, the bass boost network made up of triode V201 connected to the negative terminal of N202B is primarily for bass boost of 75HZ low frequency signal. During delay adjustment for PT2399, first control signal is given to CPU, which controls N207 after being expanded via N211 IC CD4094 and connects with pin 6 of PT2399 by selecting different resistance values for purpose of delay

Treble control pin of L/R channel

Bass control input/output pin of L channel

Bass control input/output pin of R channel

adjustment. Echo control is to change the resistance value at the connection point to R229, so as to change the superposition on through connect signal for echo control. The wide sound field processing control signal of SOK’s karaoke is in wide sound field mode when it is high level, and now the signal of OK-R is the OK signal inverted by N204A. A sense signal of OK-SW on the MIC plug detection together with the network made up of V200. When MIC is not plugged, it is low level; when plugged, it is of high level. Karaoke auto mute function is also available. When P-KT fails to detect signal for a continuous period, CPU will send a P-KM signal to mute karaoke and avoid MIC receiving noise, which may affect on sound effect.

Signal flow chart of scene mode

AV250T has a special function that switching between 5 scene modes is available without karaoke, which is fulfilled through part of karaoke. When pin 9/10 of N205 are of high level, sampled L/R/C signals are outputted via pin 3 through N205 gating. After amplification by N203, one way is connected directly; and another way is sent to internal of PT2399 for echo delay adjustment (controlled by IC CD4049), with the adjusted effect superposed to L/R/C channel to form different scene modes. In this circuit, MIC shall not be inserted and is only available in 5.1CH mode. N203A is for the purpose of reversal. In addition, this device is added headphone output function. PHSW is low level and each channel has output when headphone is not inserted. But when headphone is inserted, PHSW will be high level for the mechanical settings thus LRM and SCM signals change into high level at the same time to realize muting in each channel, so the signal is only outputted from headphone, that is, there is not signal output with each channel when connecting with headphone output.

2.3.3 Bass enhancer circuit

P-BURST is the switch signal of bass enhancer. When it is of high level and added to the base electrode of V102, V102 will be switched into conduction. When the collector electrode outputs low level, V107 will be cutoff; when the collector electrode is of low level, V100 will also be cutoff. SW signal is normally outputted to external terminal. Meanwhile, the high level signal of P-BURST is added to the emitter electrode of V108. V108 is positively biased and switched into conduction. The collector electrode adds high level to the base electrode of V101. V101 is positively biased and switched into conduction, and ground SW signal, not superposing it to L/R signal. In reverse, when P-BURST is of high level, V100 will be switched into conduction and SWM cannot be outputted from external terminal. Meanwhile, V101 is cut off and SW signal is superposed to L/R channel signal. The bass enhancer of AV250T can be divided into three steps. This principle is to change the volume of bass enhancer by changing the SW output volume of

M62446.

2.3.4 Mixing and amplification circuit of 5.1 signal and karaoke signal

When L/R channel signal of 5.1 signal is superposed with SW signal and amplified by N101B/N100B, it is sent to the reverse phase of N101A/N100A. Meanwhile, OK-R/OK-L signals are also respectively added to the reverse phases of N101A/N100A. After mixing and amplification by N100A/N101A, they are respectively outputted from pin 1 of N100A/N101A to power amplification circuit for power amplification. Meanwhile, the C-1 signal sent by volume board is added to the reverse phase of pin 6 of N102B and added to the reverse phase of N102A after amplification. Now C1-1 signal after electronic echo processing is also added to the reverse phase of N102A and sent to power amplification circuit after mixing and amplification. SR-1/SL-1 of another volume board is also added to the reverse phase of N103B and N104B for amplification and then sent to N103A and N104A for further amplification, and later sent to power amplification circuit. One way of 5.1 signal after being mixed and amplified is sent to power amplification circuit passing through XS9, and the other way forms DIST (distortion error detecting signal) signal passing through R111-R113/R142/R145/VD100-VD104, which will be added to CPU for automatic gain adjustment, so as to control volume output.

Section Four CPU board

2.4.1 CPU control unit

N100, the overall CPU, is the overall control center, inputting all kinds of control instructions to controlled circuits to achieve all kinds of control functions. It adopts +5V supply with pin 40 as its supply pin. Pin 18 and pin 19 connect externally with 12M crystal oscillator to provide working clock frequency for itself. Pin 9 is its reset pin. When starting, +5V charges C106 via R100. The voltage of two ends of capacitance cannot be mutated, thus B-pole of triode V100 is low level, that is, V100 conduction gives a high-level reset signal to CPU. When capacitance C106 finishes charge, V100 stops and then reset finishes. The form of this reset circuit is to reset high level and keep low level. When the machine is working, the static information of Power-on logo on screen and Chinese characters are stored in CPU internal static memory. N101, a status memory, can record the current working status of machine when powering off and show the status when next time power on, avoiding users to re-adjust. The sound field mode set by users is also stored in it and can be called when necessary.

2.4.2 Detect input signal and automatically search circuit

DISPLAY signal from volume board is sent to N103A to amplify and limit level, and then sent to inverse end of voltage comparator N103B after capacitance coupling. It inputs from pin 7 of N103B and then is sent to pin 16 of CPU via

VD103, V101, R109 and R107. When N103B inputs a high level, VD103 is in

reverse cut-off status, B-pole of switch tube V101 is high level and is in conducting status, then gets an about +5V high level (signal input) to CPU after VD101’s stabilization and stop searching. When the output end of N103B outputs a low level, VD103 is in conducting status, B-pole of switching tube V101 is low level and is in cut-off status, and then CPU detects the low level (no signal input). Its working principles are as follows: ① After starting up, under CPU internal program’s control, a data signal is outputted via pin 23 to M62446, and then M62446 scans each input port of N101, N102 and N103 by emitting high and low levels. When the input ports have no signal input, it automatically becomes standby status. When any of ports has signal input, channel paths of input N101, N102 and N103 has A/C signal which is amplified and limited level by N108B and N103A of CPU board, then compares with pin 5 of N103B and gets plus-minus level close to supply power. The co-phase voltage of N103B is about 0.1V. After the direct current voltage is over 0.1V, the output end of N103B outputs low level is close to negative-power voltage, VD103 positive-bias conducts, switch tube V101 (S9014) stops, emitter outputs a low level to pin 16 of CPU which by controlling IC M62446 makes search level lock on the port through which signal inputs, to enter normal play. ② When pressing “SEARCH” button on the remote controller, it is converted from optical signal to electric signal by the remote control receiver of panel. Pin 14 of CPU emits a high level to conduct V102 and search according to the same previous process.

2.4.3 Spectrum Analysis Circuit (see the following figure)

Spectrum analysis circuit is divided into three parts:

DISPLAY

N104

CD4051

uto

spectru

m and

gain

adjustme

N105C

}

CPU

7-segment

band-pass

35HZ

134HZ

300HZ

1KHZ

2.2KHZ

6.3KHZ

16KHZ

N108

CD4051

Frequency

point

gating

}

N102A

V105

Pin 12 of CPU

CPU

+5V

V103

V104

Pin 28 of CPU

/D conversion

Automatic spectrum gain adjustment circuit: To avoid two situations that spectrum display amplitude is too low when input signal is too weak or spectrum display is in full screen when input signal is too strong, AV250T sets automatic spectrum gain adjustment circuit, using a single-track one-from-eight electronic analog switch, its true value diagram is as follows:

CD4051Truth table

X0 X1 X2 X3

0 0 0

X4 X5 X6 X7

Its main working principle is to change the value of inverse ground resistance of transmittal N104 to change the transmittal gain multiple. Let’s see the detailed work of the whole circuit. We’ve referred that spectrum analysis signal source (DISPLAY) is sent to the co-phase input end of transmittal N105C to amplify. Its amplification factor is determined by the value of the resistance connecting with the electronic switch of its inverse end N104. When the main volume is large, CPU will automatically increase the value of ground resistance and decrease the amplification factor; when the main volume is small, CPU will automatically decrease the value of ground resistance and increase the amplification factor. Amplitude sampling circuit: signal amplified by N105C is sent via C115 coupling to seven band-pass filters composed of transmittals. By setting its capacity of feedback capacitance, its frequency-band range can be determined. The frequency value of superscript of the output points is the central frequency-point of the frequency band. The output end of each band-pass filter is connected with a half-wave rectifier circuit. The amplified A/C signal is rectified to direct current. The circuit is mainly to achieve frequency-point sample. It can display the amplitude of all frequency-points of the whole sound signal via direct-current voltage. If the low frequency of sound signal is stronger, the current voltage of output end of 35HZ and 100HZ band-pass filter is higher. When high frequency is stronger, the current voltage of 10K and 16K band-pass filter is higher. The output ends of the seven band-pass filter are connected with the seven input ends of electronic switch N108 (CD4051). These electronic switches will quickly circularly-switch among frequency points (referring to previous true value diagram). Pin 3 output end of N108 will output a string voltage value representing frequency point signal amplitude (see next diagram).

35HZ

100HZ

300HZ

1KHZ

3KHZ

10KHZ

16KHZ

Note: the voltage

amplitude in diagram

is uncertain.

35HZ

Interval of switch

Frequency point cycle gating period

35H

100HZ

Discharge of high level

Interval of switch

Time for charge Time for charge

35H

100HZ

A/D conversion and output circuit display (two situations):

1. When no signal input, pin 28 of CPU sends a high level to B-pole of V104. The positive end of N102B is low voltage, the inverse end of N102B gets partial voltage of R189 and R172, making N102B output a low level, that is, triode V105 stops and C-pole of V105 will give a high level to pin 12 of CPU to let CPU not conduct AD conversion (pin 6/7/8 of CPU are inactive and keep high level).

2. When the machine has detected the signal (the inverse end of N102B has a current voltage representing 35HZ signal amplitude), pin 28 of CPU is converted into low level and +5V voltage charges for C137 via V103. When reaching the voltage value of inverse end, the comparator converts and N102B outputs high level. Once CPU receives low-level signal, it stops 35HZ level gating and converts into next frequency point 100HZ. During conversion, pin 28 of CPU outputs an instant high level to conduct V104, leak the voltage

capacity of C137 and make the co-phase end of N102B restart to charge

100HZ from 0-level. When the charge of 100HZ finishes, the charge and discharge of next frequency point begin, and such process occurs circularly under the control of CPU. The charge time form 0-level to the occurrence of output conversion represents the signal amplitude of current frequency point—the larger the amplitude, the longer the time and the amplitude displaying in screen is higher; the smaller the amplitude, the shorter the time and the amplitude displaying in screen is lower. Digital pulse outputted from N102B output end is added by V105’s inverse to pin 12 of CPU which handle it and output to panel to display dynamic frequency in screen. The display of original frequency points is sequential. However, the above circular process is extremely quick, thus, what we see in screen is the progress of the whole spectrum displaying synchronously.

Section Five Panel control and display circuit

The panel control and display circuit of AV110 adopts the special IC 101 (PT6311), whose external buttons of pin 10/11/12/13 scan buttons matrix. After receiving the control command of users to the machine, processing is done inside and then outputted by two ways: one way is sent to display screen to display the working status; and the other way is transmitted to CPU through pin 5/6/8/9 to ask for performing and finishing the corresponding control function. N102 is remote control receiver, which transforms the received infrared remote control signal into electronic signal and then send it to pin 13 of CPU so as to complete remote control function.

Section Six Power amplifier board

1. L/R channel power amplifying circuit: L/R channel power amplifying

circuit of AV250T (RU) is composed of discrete components, with the composed frame diagram shown as follows. (Take L channel as an example)

V101

V115

Mute

C negative

feedback

R121, R108, C105

L-in

Differential

amplifying stage

V102, V103

Mirror image constant source

V104, V107, VD102, VD103

Voltage

amplifying

stage

V105

Composite powe

amplifying V132,

V112

Temperature

compensation stage

V106

Composite powe

amplifying V133, V113

Relay

Speaker

L channel signal is sent to pole B of differential amplifying stage V102 after being coupled by R101, R103 and C101; V102 and V103 compose single ended input/output differential amplifying circuit. Sound signal outputs from pole C of V102 to pole B of voltage amplifying stage V105, and then outputs to composite power amplifying stage after voltage amplifying. V104, V107, VD102 and VD103 compose mirror image constant source circuit. VD102, VD103 provide V104, V107 with constant base electrode current. Emitter resistor of V104 determines working current of differential amplifying stage; and emitter resistor of V107 determines working current of voltage amplifying stage. V132 and V112 compose multiple unit tube amplifying to supply strong current amplifying capacity for final stage of power amplifier, which composes waveform positive half cycle amplifying. V133 and V113 compose waveform negative half cycle amplifying and the circuit structure is the same with the above. The function of temperature compensation tube V106 in circuit has two: it is the base bias of geminate transistors firstly and its working state determines the static working current of composite power amplifying stage, that is, through adjusting the gating degree of V106, we may set the static working point of composite power amplifying stage. The usual means is to change the base electrode resistor of V107; it may also automatically adjust the working state of composite power amplifying stage after the temperature increases, with the adjustment process shown as follows:

Total current of output stage = working current + leakage current

When temperature increases, the increase of leakage current leads to drift of static working point, and meanwhile leakage current of V106 increases. The decrease of Uce makes the bias current of output stage decrease, the working state change and the working current of back stage decrease to achieve the function of temperature compensation.

Introduce voltage negative feedback into power amplifier circuit of AV250T, composed of R121, R109, C105, which is capable of stabilizing static working point of differential stage. AV250T adopts direct output means and R111, C116 connected to output terminal compose Joubert Network, capable of avoiding high frequency self-excitation caused by the AC inductive reactance of speakers voice coil.

Mote control: when pressing MUTE button on remote controller, a mute signal, achieved through signal photoelectric conversion performed by remote control receiver, is sent to CPU, and then a high level mute order is outputted from pin 35, 36 of CPU to make V115, V101 and V116 on, left/right channel signal has short circuit to ground to fulfill mute control function.

C, SR, SL power amplifying circuit: compared with the former models, the three channels of AV250T adopt special power amplifiers LM1876 and IC LM1875. As for LM1876, it has 15 pins in all, in which pin 2, 15, 4 are its positive/negative power pin; and pin 7, 8, 12, 13 are its in-phase or reverse input terminal. The rated output power of each channel of this power IC may be up to 20W, with auto mute function when power on; as for 1875, it has 5 pins in

all. It is a good performance power amplifying IC, with very simple application

circuit and 15W power output in rated state. Pin 5 and 3 are positive and negative power supply pin.

Section Seven Power board

Power board supplies working voltage required by each unit circuit and over-current/over-voltage protection unit. Protection unit circuit is emphasized here.

The protection means of L, R, and C channel is fulfilled through disconnecting relay Y100 when starting up so as to disconnect the output. SR and SL channel achieves protection function through mute. AV250T is with power-on protection, central point over-voltage-over-current protection and standby protection.

1. Power-on delay attracting protection circuit:

The working of circuit is unstable when power on and the generated percussive current does great harm to speaker and power amplifying circuit, so delay attracting protection circuit is set. This circuit is divided into two steps:

1) C, L, R. The working process is: a 22V voltage is achieved after AC

outputted by transformer being rectified and filtered by VD113, C110, and then charges C115 through R108 to make VD111 struck through reversely and make V105 and V104 in-phase on to finally make Y100 attract to reach delay effect.

2) Surround left and right channels perform anti-percussion protection when

power on: after machine system resets, pin 33 of CPU outputs a high level to pin 9, 14 of LM1876 through R164 to make LM1876 output mute. After delay start-up of machine succeeds, pin 33 of CPU converses to low level immediately and outputs normally through SL/SR channel.

2. Central point over-voltage protection:

An over-voltage sampling resistor is connected to the output terminal of each channel. L channel is R116, R channel is R117, C is R118, SR and SL channels are R119 and R120 respectively. Only when one channel’s central point voltage is above +3.5V or below -3.5V, V101 or V102 is on to make their C pole voltage decrease, then V103 on, relay disconnected and protection circuit start up finally.

Over-voltage

sampling resisto

is connected to

each channel

channel

is R116

When above

+3.5V

When below

+3.5V

V101 on

V102 on

C pole

voltage

decrease

V103

Relay

disconnect

and protection

start up

Over-current and short circuit protection:

An over-current sampling triode is connected to the output load resistor of L/R channel. Sampling tube of L channel is V114 and load resistance is R126 and R127. The other three channels’ power amplifying IC is with over-current protection inside. Only L channel appears over current, the pressure drop produced on R126 and R127 will increase quickly. Once the pressure drop on R129 is above 0.7V, V114 will be on, and V103 is also on. Finally relay disconnects and protection circuit starts up.

Over-current

sampling

triode is

connected to

each channel

channel

is V114

Pressure drop

on R126,

R127

increases

when ove

current

V114

is on

C pole

voltage

decrease

V103

is on

Relay

disconnects

and protection

starts up

The same, when R channel has over current, voltage on R159 will be above

0.7V to make V129 on, and then V103 on. Finally relay disconnects to reach the purpose of protecting speaker.

4. Standby protection:

When standby time is up to 10 minutes and standby by force is needed, PRC signal outputted from pin 34 of CPU is high level, which makes V100 saturated and on through VD108, R101 to make V103 on. Finally relay disconnects to reach the result of standby and energy saving.

Section Eight Video board

Video board fulfills the functions of input, output and switch of the video, with the schematic diagram shown as the following figure:

This switch function is realized through an electronic switch IC CD4051. When high and low level of control signal A and B are changing, IC CD4051 switches between VCD and DVD, and video signal is outputted by pin 3 VOUT.

Chapter Three Servicing Process

1. No output for Karaoke

Firstly confirm whethe

microphone is good.

Whether the pre-amplification

circuit and checking circuit o

microphone work normally.

Whether the microphone

volume adjustment works

normally.

Whether high/low frequency

boost circuit works normally.

Whether mute circuit o

microphone works normally.

Yes

Note: For MIC size is disordered on the market,

that MIC is too long or too short will both lead to

no output of microphone.

Note: here please check whether the

microphone checking signal P-KT is able to

overturn when signal output is available.

Note: check whether the periphery circuit o

volume boost IC2315 or IC M62429 is normal,

and whether the voltage and data line has

abnormalities.

Note: check whether the triode in the

microphone mute circuit has damage, and

whether the mute control signal line P-KM is

normal.

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