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clipj and pop the rotary pot shafts into position. The re-
maining shafts will fall into position. Before snapping the
board into its locked position on the standoffs, check that
the LEDs are aligned with their respective holes. Seat the
board into its locked position. Assemble the washers and
nuts to the phone jacks on the Input Connectors board.
Make sure that the insulating washer is behind the metal
washer on the HEADPHONES jack. Check that the POWER,
TEMP WARNING, and SHUTDOWN LEDs are in their proper
locations. Replace all knobs.
Equalizer Board (A4): This board is located at the center
of the foil side of the Output Controls board (A9). Removal
requires releasing the nylon standoff locking lugs holding
the board in place and pulling the board straight out. Dis-
connect the wiring harness connectors from J401 and J402.
Regulated Power Supply Board (AS): Remove this board
by disconnecting connectors J7, J8, and J9, and removing
six screws along the edges of the board.
Power Amplifier Boards (A6): The two Power Amplifier
boards (Model 700) are identical except for mounting
brackets. Remove either board by first loosening the PC
board-fan assembly. This is accomplished by removing
four screws securing the boards to the bottom panel.
Select the power amplifier to be removed (channel B
amplifier is toward the front of the console and channel A
is toward the back).
Remove the electromagnetic shield (steel) and electro-
static shield (foil) by disconnecting the grounding lug and
disengaging the shields from the nylon standoffs. Discon-
nect connectors J13 and J14 (for channel B; J10 and J11
for channel A). Remove three screws from the edge of the
Power Supply board and, using a small Phillips screw-
driver, two screws from the fan.
Output Connectors Board (A7): Remove all PATCH
BLOCKTM jack retaining nuts and washers, remove board
connectors J5 and J6 and unsolder three leads (yellow,
green and black) to the MIC OUTPUT jack (PI). The board
lifts straight out of the rear of the cover.
Output Controls Board (A9): Remove the knobs from the
right side of the front panel. Remove the four screws with
lockwashers and insulating washers, and disengage the
five nylon standoffs securing the board to the front panel.
Remove connectors J3, J4, J22, J23, J24 and J25.
LED Board ( A l l ) : This board (two boards in the Model 700)
can be removed after the Output Controls board (A9) is
removed. The LED board is located directly below the
equalizer sliders, and can be removed by lifting upward.
DC OUTPUT BALANCE ADJUSTMENT
In order to eliminate dc offset at the output, a dc balance
control (R601, blue) has been provided for each Power
Amplifier (A6). This control is accessible from the left-
hand side of the console. The dc balance should be
checked and adjusted as necessary whenever any Power
Amplifier circuit components are replaced. The dc output
balance adjustment should be made before making the dc
bias current adjustment.
The dc balance adjustment is made with all volume con-
trols down and no speaker load. Connect a dc voltmeter
(preferably with a 0.1 Vdc full-scale range) across a
SPEAKER OUTPUTS jack (J16-J17 for channel A, J18-J19
for channel B).
Use a small screwdriver with an insulated shaft to
make this adjustment; hazardous voltages are pres-
ent in the circuitry near R601.
Carefully adjust the dc balance control for 0 Vdc -C20 mV
at the output. NOTE: The normal adjustment range of R601
+
is
1V. If the dc offset exceeds this value, other problems
exist in the circuit.
DC BIAS CURRENT ADJUSTMENT
Each Power Amplifier (A6) contains a dc bias current
adjustment control (R617, orange) to set the dc idling
current through output transistors Q612-Q615. This adjust-
ment minimizes crossover distortion without drawing ex-
cessive idling power. The bias current should be checked
whenever any Power Amplifier circuit components are re-
placed. The adjustment should be made after the dc output
balance adjustment. Both dc bias current controls are
accessible from the left-hand side of the console.
The dc bias current adjustment is made with all volume
controls down and no speaker load. Remove the Power
Amplifier board shield and connect a dc voltmeter with a
floating ground across resistor R632 (0.3 ohms, 7W). See
Figures 28-31.
Use a small screwdriver with an insulated shaft to
make this adjustment; hazardous voltages are pres-
ent in the circuitry near R617.
Starting with the dc bias current control at full counter-
clockwise, adjust it for 6 f 2 mV across R632. The same
voltage should be present across resistors R633, R638 and
R639.
DB PEAK INDICATORSIFEEDBACK FINDERTM
CALIBRATION
The following calibration procedures should be per-
formed when parts in the DB PEAK LED strings have been
replaced. The potentiometers involved on board A9 are the
A DB (R916), B DB (R919), FULL WAVE CAL A (R1044),
FULL WAVE CAL B (R1045), and FBF (R972). The balance
and dB peak calibrations that follow are given for output
channel B; the identical steps for channel A use poten-
tiometers R916 and R1044. All procedures can be per-
formed with the cover open and the Output Controls board
in place (see Figure 1). Note that the first two procedures
apply to the Model 706 and to channel B of the Model 700.
Full Wave Balance
1. Set the console controls to typical settings (orange
index marks), the INPUT ATTENuators to 0, and the
MASTER controls to 5.
2. Feed a 1 kHz, 150 mV signal from a low-distortion
sine-wave signal generator to a channel input HI IMP
jack. With an audio voltmeter connected (without a
speaker load) to a B SPEAKER OUTPUT jack, adjust
the B MASTER or signal generator level control for a
10V output.
3. Connect an oscilloscope across test point TP B (right
center of A9) and ground (see Figures 18 and 20).
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