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Model 285B
Theory of Operation
SECTION 4
THEORY OF OPERATION
4.1
GENERAL
4.2
The 285B theory of operation
for
the tape transport is identical
to the 280B.
The electronics assembly is discussed in this section.
4.3
ELECTRONICS ASSEMBLY
4.4
The electronics assembly consists of the play head on the tape
deck and the two-channel audio amplifier printed wiring assembly.
The two-channel audio amplifier, Figures 4-1 and 7-1, contains two
identical amplifier circuits.
The following discussion covers
channell.
4.5
The audio signal from the play head is connected through Jl14
and J130, on the play electronics assembly, to a preamplifier circuit
consisting of Ql, Q3 and associated components.
High or low speed
equalization is selected by the
CAPSTAN LOW/HIGH
switch on the trans
port control panel through the transport logic PWA and applied to
analog switch Ul and inverter Q5.
The amplifiers of Ul close the
solid state relay contacts when +2.5 to +5.5
volts
appears at their
inputs and open when 0 to +0.4 volt appears at their inputs.
Equali
zation is adjusted by variable resistors R9
(LO) and R7 (HI).
The
amplitude of the preamplifier output (Q3 collector) is adjusted by R23
and applied to the input of power amplifier U2.
U2 raises the level
of the equalized audio signal to provide the 600-ohm output through
the line output audio transformer (Tl).
4.6
The audio output of line driver U2 is also applied to
VOLUME
control RlOl on the transport control chassis assembly.
The wiper of
the VOLUME control is connected by the audio interconnect harness
4-1
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