Broadcast Electronics 3000 Series - Theory of Operation; Playback;Logic Board Circuit Description; Configuration; Program Circuitry

SECTION IV
THEORY OF OPERATION
4-1. PLAYBACK/LOGIC BOARD CIRCUIT DESCRIPTION
4-2. CONFIGURATION
The Playback/Logic board contains the playback amplifiers for the
program tracks, the output audio muting, the cue track amplifier, the 1 kHz
stop cue sensor, and the stop/start logic. When the optional Q Trips are
installed, the Q1 150 Hz and Q11 8 kHz sensors are incorporated on this
board. Additionally, the 3kHz stop cue sensor required for units equipped
with the
fast forward option is mounted on this board.
In all, there are eight possible configurations of the Playback/
Logic board as shown in the assembly diagram in Section VI. The exact con-
figuration used in a particular machine is indicated in Table 3-1. This varies
depending upon whether a unit is monophonic or stereophonic, equipped with
Q
Trips, or equipped for single speed, manual or automatic fast forward.
The schematic diagram in Section VI shows the most complex arrangement re-
quired for a stereophonic machine with all options.
4-3.
PROGRAM CIRCUITRY
The program tracks of the playback head are coupled through Cll
(C17) to the input of the preamplifier IC-1A (IC-1B). This dual integrated
circuit amplifier provides gain and equalization as determined by the net-
work Rl (R3), R2 (R4), RIO (Rll), R34 (R33), Cl (C4). The variable high
and low frequency trimmers permit adjusting the equalization to match in-
dividual heads to NAB or other standards (IEC. CCIR, etc.). Internal de
voltage is supplied to both halves of IC-1 through pin 14 from the decou-
pling network
R26, C13. Bias de is provided from the voltage divider R20,
R25,
C12 through R19, R24.
The output of the preamplifier is coupled through C26 (C25) to
the
FET program muting switch Q5 (Q6). This N channel device is controlled
by the logic so that a ground on either CR7 or CR8 shuts off the audio
signal. This is provided whenever the machine is in the stop mode or when
units equipped for automatic fast forward are so operating. Bias (12 VDC)
for the FET is provided from the voltage divider R35, R98, C24 through R36,
R37, (R38), R39, (R48). Filtering is provided by C23 which shunts to ground
any ac appearing on the gate.
The output of the FET is coupled through C27 (C28) to the output
level control
R15 (R16) mounted on the playback board. The audio signal is
also available through board pin 14 (13) for the VU meter installed in
record/playback models.
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