Audio Pro B2-70 Manuel utilisateur

SERVICE MANUAL

FOR

SUBWOOFER B2-70

Audio Pro AB

LIST OF CONTENTS

1. TECHNICAL DESCRIPTION

1.1 Power Supply

1.2 Automatic ON/OFF

1.3 Input stage

1.4 Filter

1.5 ACE-Bass circuits

1.6 Power amplifier

2. TROUBLE SHOOTING TIPS

2.1 B2.70 Does not work

2.2 Fuses F501, F502 blow

2.3 Automatic ON/OFF faulty

2.4 B2-70 oscillates

3. ADJUSTMENTS

3.1 Bias current in Power Amplifier

3.2 Negative output resistance

4. SPARE PARTS

5. ENCLOSURES

5.1 Circuit diagram 55 21295-01

5.2 Component layout

1. TECHNICAL DESCRIPTION

The theoretical background for the ACE-Bass principle used in

Subwoofer B2-70 is described in AES preprint No. 1381.

The functions described below are separated by dotted lines on

the Circuit Diagram.

1.1 Power supply

The primary of the transformer is always connected to the power

cord via a voltage selector and a power fuse ( 4.0 A slow at 117

V, 2.0 A slow at 220-250 V ). The idle current in this transformer

is very low.

The center tapped secondary winding supplies the automatic ON/OFF

via two diodes D201, D202 and a double rectifier D101 via a switch

K101. This switch is operated by a relay in the automatic ON/OFF

circuits.

Rectifier D101 supplies +-40V to the Power Amplifier, and also

stabilized +-15V via voltage regulators, V101, V103.

It is very important that the -15V tracks the +15V accurately

when the power is switched on or off. Mistracking can cause

transients or oscillations in the electronic circuits.

To maintain tracking of the +15V and -15V when power is

disconnected, the +40V must decay faster then the -40V. For this

reason the Power Amplifier is built to always maintain a small

positive offset voltage ( 10-100mV ) on its output terminals under

operating conditions.

If the B2-70 is tested without any load on the Power Amplifier, or

with fuses F501, F502 removed, the balance between +15V and -15V

may be upset during on/off switching. In such cases the operatio-

nal amplifier may start oscillating when power is disconnected, so

that automatic ON is actuated. This is normal under these condi-

tions, and the electronics will not be damaged.

1.2 Automatic ON/OFF

This circuit is powered via diodes D201, D202 and capacitor C201.

The power transformer and the Automatic ON/OFF circuit are

powered at all times when B2-70 is plugged into a live outlet.

The other circuits in B2-70 are powered only when relay K101 is

actuated.

The audio signal on left channel is sensed by operational

amplifier Z201 (2,3,1) which acts as an amplifier with gain 2000

at 150Hz. The amplified voltage (point H) is limited to +7.0V and

-0.7V by zener diode D208.

Operational amplifier Z201 (6,5,7) and transistor V201 work as

a schmitt trigger with ON level 7.5V and OFF level 5.6V. The input

(pin 5) senses the voltage from Z201 (2,3,1) after rectification

in D209 and capacitor C206.

Transistor V201 drives relay K101 and red LED indicating "ON".

When push switch S201 is in position "AUTO", relay K101 is

deenergized about 5 minutes after the audio signal on the left

channel input disappears. The time delay is determined by C206

and R210. ( During testing R210 may be shunted by 100 kohm so

delay time is reduced ).

When push switch S201 is in position "ON" the sensing circuits

are bypassed, and relay K101 is energized as soon as B2-70 is

plugged into a live outlet.

Temperature sensor TS3-85 senses the temperature of the power

amplifier heatsink. If this temperature becomes too high the

resistance in the TS3-85 drops to a low value, and relay K101

is deenergized. ( 85°)

1.3 Input stage

The audio signals on DIN plug J301 pin 1 (left channel) (A) and

pin 4 (right channel) pass through variable high pass filters

formed by C303, R315, R317 and C304, R316, R317 and buffer

amplifiers V303, V301 and V304, V302 to DIN plug J301 pins 3

and 5. From pins 3 and 5, they can be returned to an external

Power Amplifier driving the satellite speakers. The buffer

amplifiers provide low output impedance, so cable capacitances

will not effect the treble response.

The audio signals on DIN plug J301 pins 1 and 4, are summed by

resistors R318, R319 into a first active low pass filter with

kneepoint 138Hz and 10dB gain at 100Hz (when channels are in

parallel).

The signal from the first low pass filter is passed via a volume

control R322 to a second amplifier stage Z301 (10,9,8) with 26dB

gain at 100Hz.

1.4 Filter

The signal from Z301 pin 7 (test point C) passes a variable

low pass filter including R401, R402, R403, C401, C402, and Z401

(9,10,8), and then on to a fixed high filter with kneepoint

20.8Hz. The output terminal is test point (E).

Remaining components in the filter circuits are part of the

ACE-Bass amplifier, and is described in section 1.5.

1.5 ACE-Bass circuits

The Power amplifier together with Z401 (2,3,1) and Z401

(13,12,14) for the ACE-Bass amplifier, which can be represented by

the diagram below.

R418 and Z401(13,12,14) sense the current through the drivers.

The voltage in test point (J) equals the driver current (1V/1A).

Z401 (2,3,1) works as an active bandpass filter, and also sums

the signals in test points (E) and (J).

Part of the driver current signal is fed back positively through

the Power Amplifier, which gives the Power Amplifier a negative

output resistance Rs. The amount of positive feedback, and

thereby, the value of Rs, can be adjusted by trimpot Rrs.

The parallel circuit Lp, Rp, Cp in the ACE-Bass amplifier is

accomplished through negative feedback of the driver current

signal via the bandpass filter, Z401(2,3,1).

1.6 Power Amplifier

The input signal to the Power Amplifier is DC decoupled by

capacitor C414, and is amplified linearly to the output (test

point G).

The output signal is fed back negatively to the input via R511,

C503.

When the output signal is large, negative feedback is also

obtained via a diode - resistor network. The amplitude of the

output signal is compared to the supply voltages (+-40 V) in this

network, and about 2 volts before clipping, the output signal is

rounded off by the diode network so clipping will be soft.

The differential input stage (V503, V506) is supplied via a

current generator (V504). Transistors V501, V502, V505 form

a current mirror, so the output stages from V507, V509 are

symmetrically driven.

The output stage is of triple darlington type with transistors

V512-V517. AC gain is 1. V508 senses the temperature of the

output transistors and assures constant bias current.

V510 and V511 filter the supply voltages to the input stages,

removing hum and transients.

NOTE: If there is a"click" in B2-70 when it is switched on,

the cause could be a short circuit in one of the tran-

sistors V510, V511.

2. TROUBLE SHOOTING TIPS

Use J07 or J08 as common point for measurements of DC or AC

voltages.

2.1 B2-70 Does Not Work

Set push button switch in ON position.

2.1-1 Check that LEDs AUTO and ON are lit.

2.1-2 Check power fuse and fuses F501, F502.

2.1-3 Check that DC voltages are OK.

+40 V : 40+-2 V

-40 V : 40+-2 V

+15 V : 15+-2 V

-15 V : 15+-2 V

2.1-4 To localize the fault, connect a tone generator to DIN plug

J303, pins 1 and 4 in parallel. Set generator to 5mV,

100Hz sine wave. Turn "sensitivity" (R322) and "crossover

frequency - subwoofer" (R402) to max. clockwise position. You

should now measure as follows:

-------------------------------------------------------------

Test Point AC Volts DC Volts

-------------------------------------------------------------

A approx. 5mV

B approx. 13mV

C approx. 275mV

D approx. 255mV

E approx. 255mV

F approx. 95mV

G approx. 1650mV

H approx. 3.9V approx. +7.3V

I approx. +21V

J approx. 340mV

-------------------------------------------------------------

2.2 Fuses F501, F502 Blow

2.2-1 Disconnect drivers, and repeat test per 2.1-4.

Signal at test point (G) now should be around 1V.

2.2-2 Measure resistance of voice coils in drivers.

Should be 6.6 ohm each, 3.3 ohm in parallel.

2.3 Automatic ON/OFF

This can also be checked with the same setup as in 2.1-4.

2.3-1 Set push button switch in position "AUTO".

2.3-2 Signal at test point (H) should be clipped and measure 4V

peak to peak. (Oscilloscope) Signal at test point (I)

should be about 21 V DC.

2.3-3 Shunt R210 by 100k ohm shorten off - delay. Check that ON

is actuated when signal from generator exceeds 0.5mV at 500Hz.

2.4 B2-70 Oscillates

B2-70 Oscillates between ON and OFF with no audio signal on

input.

2.4-1 Check DC offset at test point (G). Should be between

+10mV and +100mV.

NOTE: If DC offset is outside these limits, fault can be in

C414 or in the Power Amplifier (caused by a fault), C414

will probably be damaged, and it should always be replaced. If

V516 or V517 has blown always check C414.

3. ADJUSTMENTS

After certain repairs have been made in B2-70, the following

adjustments must be performed.

3.1 Bias Adjustments in Power Amplifier

After repairs have been made in the Power Amplifier, the

bias current must be readjusted.

Measure the voltage between emitters on V516 - V517

(no signal on input terminals of B2-70). Adjust trimpot

(R525) till voltage reads 10-15mV. (This corresponds to

a bias current of 50mA).

Make final adjustment after B2-70 has been ON for a few

minutes, so the bias current has reached steady state

value.

NOTE: When replacing power transistors, temperature sensor

on V508, heatsink compound (and isolating shims) must

be used.

3.2 Negative Output Resistance (Rs) in ACE-BASS Amplifier

Whenever a driver element or any of resistors R413 - R418 in the

current sensing circuit has been replaced, the negative output

resistance must be reset to equal the positive resistance of the

parallel voice coils in the drivers.

3.2-1 Set jumper below trimpots in position "TRIM", and

disconnect one speaker cable from the circuit board.

3.2-2 Connect a signal generator to the input (J301, pin 1 or

pin 4), and an AC voltmeter across the amplifier output.

3.2-3 Set signal generator to 20.3+-0.5Hz, and adjust level until

voltmeter at amplifier output reads -16dBU (123mV).

3.2-4 Reconnect the speaker cable to the circuit board (so

drivers are connected to amplifier output), and adjust

trimpot Rrs till voltmeter reads OdBU (775mV).

NOTE1:Voice coils of drivers must be at room temperature

when Rrs is set.

NOTE2:Voltages in 3.2-3 and 3.2-4 are correct.

Negative output impedance of ACE-Bass amplifier makes

output voltage larger when amplifier load increases.

3.2-5 Reset jumper in "RUN" position.

NOTE: Do not touch trimpot Ra (R410). This adjustment

requires calibrated microphone equipment and can only be

made at the factory. Ra only effects the sound pressure

level at 20Hz. Change of one driver will cause maximum

0.5dB change in this level (typically less than 0.20dB),

so no correction is required in the field.

3.2-6 Check for correct setting of Rrs as follows:

- Push the cone of forward facing driver inwards,

and release it suddenly.

- Compare the movement of the cone with the curve

below, and fine-adjust Rrs if required.

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