Bose 1600-VI, Service Manual

Page: 14 / 60

14
THEORY OF OPERATION
D2 and D3 on the Regulator Board provide overvoltage protection to U4. U4B-14 is a full-wave
rectifier that outputs positive pulses to comparator U4C. The reference voltage is set at 0.7Vdc
by R10 at pin 3. Where pin 2 crosses the threshold, the output of U4C-1 goes open and C6
begins to charge through R21 and U4A-8.
U4A is a voltage-to-current converter (Howland current pump), whose output current is
determined by the voltage at the junction of R12 and R13. As the voltage increases, the
charging current to C6 increases.
The voltage on C6 is connected to comparator U4D-6. The reference voltage is set at 0.7Vdc
by R22 at pin 5. When pin 6 crosses the threshold, the output of U4D-7 goes open and base
current is supplied to Q1 through R14. Q1 turns on, and current flows through the LED portion
of U3, illuminating it and turning on the diac. This applies voltage to the gate of triac Q1 on the
Power Supply Board which fires it and allows it to conduct current through the primary side of
the power transformer.
When the output of U4B-14 drops below the threshold voltage of U4C-3, U4C-1 goes low and
C6 discharges rapidly through it. When the voltage on C6 drops below the threshold voltage of
U4D-5, U4D-7 goes low and Q1 and U3 turn off, removing the gate voltage to the triac
momentarily interrupting the current through the primary side of the transformer.
The triac is switched on and off every half-cycle of the 60Hz AC line. Thus, the triac switches
the AC line current off at a rate twice the line frequency, at the instant the line current crosses
the zero axis. The triac will then remain off for a number of degrees of the sinusoid, before
switching on again. The phase angle at which the triac switches on is the “firing angle” of the
triac. This produces enough voltage to the primary of the power transformer to allow the
secondary regulator stage to begin to operate.
2.6 Power Supply Regulation (1800
VI
only)
The firing angle of the triac controls the voltage on the primary of the transformer, and is
determined by the conduction of the optocoupler U3 on the Regulator Board.
As the conduction of the optocoupler increases, so does the conduction angle of the triac. The
photodiac conduction of the optocoupler is controlled by the current through the LED portion of
the optocoupler, the amount of current through the LED is equal to the amount of current
through transistor Q1. When the LED in U3 is fully ON, the triac conducts earliest in the AC
cycle: the power supply is operating at maximum output. The LED current is supplied by
voltage regulator U2 on the Regulator PCB. U5A differential amplifier senses the secondary
supply voltages through R37 and R38. The output voltage at U5A-1 increases at the rate
determined by R30 and C8 (slow start-up). The idle secondary voltages are set by R36 on the
Regulator Board .
2.7 Load Regulation (1800
VI
only)
When the amplifier is driven at high power into a load, the high DC supplies (rail voltages) will
begin to “sag”. Differential amplifier U5A-1 senses this and increases the LED current to
optocoupler U1. This action increases the phototransistor conduction, which increases the
output current of U4-8, increasing the charging rate on C6. This ultimately increases the triac
conduction which increases the primary voltage, which increases the secondary voltages, thus
providing steady, regulated DC supplies for the amplifier stage.