13
THEORY OF OPERATION
2.3 Output Relays
Note: Refer to the Output PCB schematic for the following.
Relay K100 is used to connect the output signal to the Speaker Output Binding Posts through
J1. The CH1 and CH2 (K100 and K200) output relays are energized independantly of each
other. In CH1, immediately after the power switch is turned on, +6 Vdc is applied to terminal 2
of D105 (ready LED) on the display board via the voltage divider formed by R112/R113. Termi-
nal 1 of D105 is connected to pin 6 of K100 via J3-8 and J6-5 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.
2.4 Magnetic Field Power Supply
When the power switch is OFF (S1 across E1 and E2) and the linecord is connected to an AC
voltage, D1 illuminates (Standby LED). When the power switch is turned ON (S1 across E2
and E3), the LED goes off and gate voltage is applied to triac Q1, which turns it on and ener-
gizes the transformer primary winding.
The secondary winding of the power transformer has two taps that supply the two pairs of DC
supply voltages, ±90 Vdc and ±45 Vdc for the 1600VI and ±106 Vdc and ±53 Vdc for the
1800VI, each having its own bridge rectifier and filter capacitors. The ±15 Vdc is tapped from
the ±53 Vdc for the 1800VI and the ±45 Vdc for the 1600VI through R6 and R8, and regulated
by Q2, Q3 and zener diodes D3 and D6. The ±15 Vdc supply powers the op-amps and small-
signal transistors.
Note: Refer to the Regulator Board schematic diagram for the following.
The Regulator Board is not used on the 1600VI.
In the 1800VI, the triac Q1 drives the primary of the magnetic field power transformer by
operating as a phase controlled switch; its gate signal depends on the signal supplied to
opto-isolator U3 located on the regulator board. U4B provides steering for the photodiac in U3,
allowing the triac to fire on both alternations of the power line. U2 on the Regulator Board
provides AC to DC conversion, with the AC line voltage providing the input to the converter
through limit resistors R3 and R4, and the 12.5 Vdc output determined by feedback resistor
R6. This voltage provides the positive supply for U4, DC reference for comparators U4C and
U4D, and the current through opto-isolator U1 which sets the voltage for the voltage-to-current
converter U4A. Note that U4 (MC3405) is a dual op-amp and dual-voltage comparator in a
singe package (U4A/U4B are op-amps and U4C/U4D are open collector comparators).
2.5 Start-up Sequence (1800VI only)
When the power switch is OFF (S1 across E1 and E2) and the linecord is connected to an AC
voltage, D1 illuminates (STANDBY LED). When the power switch is turned ON (S1 across E2
and E3), the LED goes off and AC HI is supplied to the Regulator Board.
(2 pages)
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