Audio Pro B2-70 User manual

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.

4. SPARE PARTS
Spare parts for B2-70 can be obtained from distributors and
service centers.
When ordering spare parts, please refer to component number per
diagram 55 21295-01.