AUDIO PRO B2-50 free download

File name B2-50 schematic.pdf

SERVICE MANUAL FOR SUBWOOFER B2-50 Audio Pro AB LIST OF CONTENTS 1. TECHNICAL DESCRIPTION 1.1 Power Supply 1.2 Automatic ON/OFF 1.3 Input Stage 1.4 Filters 1.5 Ace-Bass Amplifier 1.6 Power Amplifier TROUBLE SHOOTING TIPS 2.1 B2-50 Does Not Work 2.2 Fuses F501, F502 Blow 2.3 Automatic ON/OFF Faulty 2.4 B2-50 Goes On and Off Repeatedly with no signal in. ADJUSTMENTS 3.1 Bias Adjustment in Power Amplifier 3.2 Negative Output Resistance (Rs) in Ace-Bass Amplifier SPARE PARTS ENCLOSURES 5.1 Circuit Diagram 2-125010 5.2 Component Layout 3-125004 2. 3. 4. 5. 1. TECHNICAL DESCRIPTION The theoretical Background for the Ace-Bass principle used in Subwoofer B2-50 is described in AES Preprint #1381. The functions described below are separated by dotted lines on the Circuit Diagram. 1.1 Power Supply The primary of the power transformer is always connected to the power cord via a voltage selector and a power fuse (5.0A slow at 117 V, 2.5A slow at 220-250 V). The idle current in this transformer is very low (approx. 25mA at 117 V supply). The center tapped secondary winding supplies the Automatic ON/OFF via two diodes D201, D202, and a double rectifier D101 via a DPST switch K101. This switch is operated by a relay in the Automatic ON/OFF circuits. Rectifier D101 supplies +- 40 V to the Power Amplifier, and also stabilized +- 15 V via voItage regulators, V101, V103. It is very important that the -15 V tracks the +15 V accurately when power is switched on or off. Mistracking can cause transients or oscillations in the electronic circuits. To maintain tracking of the +15 V and -15 V when power is disconnected, the +40 V must decay faster than the -40 V. 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 B2-50 is tested without any load on the Power Amplifier, or with fuses F501, F502 removed, the balance between +15 V and -15 V may be upset during on/off switching. In such cases the operational amplifiers may start oscillating when power is disconnected, so that Automatic ON is actuated. This is normal under these conditions, 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 B2-50 is plugged in to a live outlet. The other circuits in B2-50 are powered only when relay K101 is actuated. The audio signal on left channel is sensed by operational amplifier Z201 (3, 2, 1) which acts as an amplifier with gain 5,000 at 500Hz. The amplified voltage (point H) is limited to +7.0 V and -0,7 V by zener diode D208. Operational amplifier Z201 (6, 5, 7) and transistor V201 work as a Schmitt trigger with ON level 7 V and OFF level 5 V. The input (pin 5) senses the voltage from Z201 (3, 2, 1) after rectification in D209 and capacitor C206. Transistor V201 drives relay K101 and a red LED indicating "