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Model 280B
Theory of Operation
SECTION 6
THEORY OF OPERATION
6.1
ELECTRONICS ASSEMBLY
The electronics assembly consists of the heads on the
tape deck, the motherboard PWA, the regulator,
bias oscillator and amplifier PWA, the record/play
back amplifier PWA, and the control panel assembly.
The regulator, bias oscillator and amplifier PWA sup
plies regulated voltages and the 160 kHz bias signal
to the channels 1 and 2 record/playback amplifiers.
One record/playback amplifier PWA is required for
each channel. A separate bias amplifier PWA is re
quired for each pair of channels above two. An ad
ditional power supply provides operating voltages to
the tape transport. Figure 6-1 shows the main stages
in the system for a single channel.
6.2 RECORD MODE
When the transport is in the Start mode and the
RECORD pushbutton is pressed, a Record Enable
and a Record Light signal are made available at the
Function Selector switch. When the RECORD
READY position is selected by the Function Selec
tor switch, the Record circuit is completed and the
following occurs: The RECORD light on the record/
playback control panel lights; record relay K2 ener
gizes, and the bias and erase voltages are progres
sively switched to the appropriate heads.
6.3 PLAY MODE
When the transport is in the Play mode, the Func
tion Selector switch is normally in the SAFE posi
tion and the Monitor switch is in the PLAYBACK
position. The signal from the playback head is con
trolled by the PLAYBACK LEVEL control.
6.4 SYNC MODE
When the transport START pushbutton has been
pressed and the Function Selector switch is in the
SYNC position, the sync relay on the motherboard
is
energized to pennit switching of the record head
for the selected channel(s) to the playback moni
toring mode while recording on other channels. This
feature allows the operator to synchronize recording
with previously recorded material on the selected
channel(s).
6.5 SQUELCH CIRCUIT
The squelch PWA suppresses transients from the
audio circuits.
6.6 TAPE TRANSPORT ASSEMBLY
The tape transport assembly consists of two torque
motors, a two-speed ac capstan drive motor, con
stant tension components, and a power supply. (See
Figure 6-2).
6.7 CONSTANT TENSION
The tension sensor PWA, mounted on the bottom of
the dancer arm assembly, consists of an opto coupler
and an optical grating which control the amount of
light which reaches the transistor. The optical grating
is mechanically connected to the tension dancer arm.
As
the tension dancer arm moves in response to
changes in tape tension, the tension sensor PWA
transmits signals to the constant tension control
unit in the transport power supply which initiates
changes in the hold back voltage applied to the
supply reel.
6.8 TRANSPORT LOGIC
The tape transport control and power circuits are
contained in the transport control chassis. This as
sembly contains the switches, relays, and logic
which control the tape transport in all modes. All
ac high-voltage wiring is run within the insulated
wiring, connectors, or covered relays to provide
protection to personnel. Table 6-1 shows the state
of the motor relays, takeup and supply brake sole
noids, pinch roller solenoid, ATL solenoid and
logic for all modes of operation.
6-1
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