Bose 500PM Servicehandleiding - Pagina 5

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1. Specifications

THEORY OF OPERATION

Model 500PM Loudspeaker SmartProtect Crossover PCB

Note: Refer to Figure 7. Model 500PM Loudspeaker SmartProtect Crossover Assembly

Schematic Diagram, for the following explanation.

The SmartProtect PCB is an electronic switch, in series with the speaker, used to protect

the drivers from high current and power levels. It has a fast turn-on time of about 100

milliseconds. At turn-on, current flows through R1, C1, D1, D2, C2, Q1, C3 and R18.

Capacitors C2 and C3 form a virtual power supply which is used to power the

SmartProtect circuit to provide the bias to FET's Q8 and Q9, limited by the Zener diode

D6 to 10 V. Diodes D1 and D2 form a voltage doubler. Once C2 is charged, Q1 will

conduct and provide a charge path for C3. Once C3 is charged, if the input audio voltage

becomes lower than 4V, Q1 will turn off, and the charge on C3 provides a voltage used to

operate the circuit for up to 5 hours. PTC1 provides the current path if the speaker must

play for a long time at very low volume when input AC voltage to the speaker is less then

1.5V.

Resistors R16 and R17 form the current sensor. It allows the circuit to sense the speaker

current and protect it from the following types of the abuse:

1. DC offset - when too high a DC offset level is detected one, of the two transistors Q6

or Q7 will be turned on through resistor R19, thus tripping the monostable multivibrator

Q2, Q3.

2. Fast DC/AC transients - works the same way for both transient polarities, threshold

depends on the ratio of resistors R19 and R20.

3. Thermal overload - When the AC current is too high for a prolonged time, transistor Q4

turns on through resistor R13, tripping the monostable multivibrator Q2, Q3. The time

constant of current integration depends on C6, R13 while the time constant of "memory" is

defined by R6 and C6. AC voltage divider R15, R14, and C7 defines the frequency

dependent threshold of current protection.

The protection portion of the circuit works as follows: When monostable multivibrator Q2,

Q3 is tripped, by one of the above conditions, transistors Q2 and Q3 turn on, reducing

the gate voltage of Q8 and Q9 to zero, and shutting them off. At this moment the speaker

load is disconnected from the amplifier. During the off cycle, capacitor C4 is charging

through resistor R3, and when the charging is completed, Q2 and Q3 will shut off again,

turning Q8 and Q9 back on, and allowing the speaker to operate again. If the overload

condition persists, the monostable multivibrator will trip the circuit again, and shut off the

speaker again. Duty cycle depends on the current through the speaker.

Troubleshooting

1. If PTC1 is open, the unit will not play at low volume levels (Vin<1.5 V) when the

system is first turned on, but as the volume is increased, the SmartProtect circuit will turn

on, and the speaker will operate. It will then operate for approximately 5 hours, until the

charge on C3 is too low to keep the circuit running, and then the speaker will shut off

again.

2. If either R18, R1, D2 is open, or D1 is shorted there will be no charge of the virtual

power supply formed by C2 and C3, and the circuit will never turn on. The symptom will

be that the speaker will play at low volume levels, but as the volume is turned up, the

speaker will stop operating. This may also happen if for any reason Q7, Q8 or any other

component in the circuit fails, and internal safety "watchdog" C5 turns the Q3 on

preventing the Q8 and Q9 from being overheated.

Shading Network

The shading network consists of one 5.6uF capacitor, one RXE040 PTC, and one 0.13mH

inductor across the two front array drivers, one 33uF capacitor, one RXE040 PTC and one

0.5mH inductor across the driver nearest the port and a trap filter consisting of a 3.9Ω

resistor, a 33uF capacitor, and a 0.8mH inductor. The inductors are iron core.