TM 11-5835-241-13&P
PLAYBACK CIRCUITS.
Refer to block diagram, figure 4-1, and schematic diagram,
a.
In playback operation, magnetic fields caused by the tape
figure 4-2.
These
moving past the play head induce audio signals in the head.
signals are coupled through contacts of RECORD switch S6 (in the play
position) and C18 to the base of playback preamplifier Q5, Q6. The
Dc degenerative feed-
amplifier uses two direct coupled amplifiers.
back from the output to the input is provided by R28 to stabilize the
amplifier and improve linearity.
Additional dc degeneration is pro-
vided by C21 and R24 (for Q5) and R29 and C23 (for Q6). R30, R31,
and C22 form an equalization network to provide high frequency boost.
Signals from the collector of Q6 are coupled through C22 and either
R30 or R31 to R25 in the emitter of Q5.
The signals are in phase and
increase the gain at high frequencies, when the reactance of C22 de-
creases.
R30 is selected for tape speeds of 3.75 inches per second
and R31 for 7.5 inches per second by contacts of S9.
The feedback is
adjusted using R30 and R31 to obtain "flat" response.
b.
The preamplifier output is coupled to the power amplifier
input stage Q7 through C24 and PLAY LEVEL control R3B.
The audio
signals are amplified in Q7 and applied to the bases of driver stages
Q8 and Q10.
The signal path to Q10 base includes CR3, CR4, R39, and
CRS, which provides little attenuation to the audio signals but which
establish the dc bias on driver Q10.
c. Driver Q8 and Q10 form a push-pull emitter follower pair
which drives output amplifiers Q9 and Q11. Q9 and Q11 form a compli-
mentary emitter follower pair.
The output is taken from the common
emitter circuit through C29 and applied through contacts of EXTENSION
SPEAKER jack J3 and SPEAKER switch S7 to loudspeaker LS1. With S7
in the off position, the speaker is disconnected and the amplifier
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