Bose 1600VI Service Manual page 15

The -15Vdc supplied to U5A-4 is backed up with a voltage divider off the -106Vdc supply (R26/
R27). Without this, if the -15Vdc supply should fail for some reason, the output of U5A-1 would
go high, drawing maximum current through the LED in U1 and latching the triac into full con-
duction. To prevent this, D8 will become forward biased and supply negative DC to U5A-4,
keeping it operating normally. Note that if the +15Vdc supply should fail, the output of U5A-1
would go negative, turning off the triac.
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 Module
Board will turn on and carry the control voltage through to the SND terminal, which is con-
nected 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 ampli-
fier 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 (posi-
tive) or Q14 (negative). When the FETs turn on, the high-rail voltage is connected to the collec-
tors 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 Display Circuit
In addition to the READY LEDs discussed in section 2.3 (Output Relays), 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 U9B.
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.
THEORY OF OPERATION
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