Bose 1800-V Manual page 16

2.8 Sequence Send/Receive
The amplifier can be powered up while the power switch is in the OFF position by applying a DC control
voltage of +7V to +15V to the Sequence RCV terminal. Q3 on the Input Board will turn on and carry the
control voltage through to the SND terminal, which is connected to the next amplifier in the sequential
chain. Q1 also turns on which turns on Q2, providing enough current to pass through the LED portion of
optocoupler U1 on the Power Supply Board to illuminate it and turn on the diac. This provides a gate
voltage to fire triac Q1, which powers up the primary circuit. Once the secondary voltages are up, the
+15VDC supply keeps the Receive circuit operating.
2.9 Commutators
Under idle or small-signal conditions the low-rail voltage is applied to the collectors of the output
transistors through D13 and D19 on the Power Supply Board. The output of the amplifier is connected to
the Power Supply Board via J1-10/J2-10. The signal is half-wave rectified by D7 and D14, sending the
positive half of the signal to comparator U2A-1 and the negative half to comparator U2B-7. When the
signal level exceeds the threshold of the comparator, Q4 (positive) or Q10 (negative) turns on. Current
can now flow from ground through Q8 which acts as a current source for Q6. Q6 or Q11 turn on providing
gate drive to the power FET Q9 (positive) or Q14 (negative). When the FETs turn on, the high-rail voltage
is connected to the collectors of the output transistors. D13 and D19 become reversed biased and switch
off the low-rail voltage from the circuit.
Zener diodes D11 and D18 provide gate protection to the FETs. Q7 and Q12 speed up the turn off time of
the FETs.
This two-stage approach minimizes the voltage across each of the output devices which also minimizes
the power dissipation required. Without this approach, the output transistors would be required to support
the entire power supply voltage under small-signal conditions and the "unused" portion of the power
supply voltage would be turned into heat.
2.10 Output Relays
The CH1 and CH2 output relays located on the Output Board, are energized independently of each other.
In CH1, immediately after the power switch is turned on +6V DC is applied to terminal 2 of D105 (READY
LED) on the Display Board via the voltage divider formed by R112/R113. Terminal 1 of D105 is connected
to pin 6 of K100 via J6-5 and J3-8 on the I/O Board. A small amount of current is drawn through R102
and the relay coil, which is enough to illuminate the red LED portion of D105 but not enough to activate
the relay.
In the meantime, C13 begins to charge through R26 on the I/O Board which delays turning on Q2 and Q3
by a few seconds. When Q3 turns on, VLF+ is applied to pin 6 of K100 which activates the relay. VLF+ is
also applied to terminal 1 of D105 which reverse biases the red LED and D107, and forward biases the
green LED, drawing current from ground through R112.
2.11 Display Circuit
In addition to the READY LEDs just discussed, the Display Board contains five Signal LEDs and one
CLIP/PROTECT LED per channel.
The clipping indicators are driven by transistors Q100 (CH1), and Q200 (CH2) located on the Display
Board. The signal for the clipping indicators initially comes from U100A-1 and U100B-7 on the Amplifier
Board. This is the same signal that operates the anti-clipping opto-isolator on the I/O Board. D30 on the I/
O Board half-wave rectifies the positive-going portion of the signal and drives comparator U9B which is a
switch. C9 and R62 establish the time constant of the clipping indicator. D23 rectifies the negative-going
portion and also drives comparator U98. When clipping occurs, U9B-7 changes from positive to negative,
which forward biases D100 on the Display Board and turns on Q100. Q100 supplies current for clipping
LED D104, causing it to illuminate.
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