Broadcast Electronics 3000 Series - Bias Oscillator; Program Track Record and Bias Control Circuitry

4-15. BIAS OSCILLATOR
Transistors Ql1 and Q12 form a push-pull oscillator providing a
100 kHz sine wave voltage. The primary of T3 and C39 form a tuned circuit
resonant at 100 kHz. Positive feedback is provided by C24 and C25. To
ensure smooth cut-off and minimize distortion, negative feedback is supplied
to the nonconducting
transistor by C26. Bias voltages for the transistors
are supplied by R55, R56, R58 and R61. Emitter dropping resistors R54 and
R57 provide a small amount of negative feedback to maintain purity of the
100 kHz output.
Transistor Q13 controls the oscillator. When the base of Q13 is
grounded through CR1 to the record flip-flop, Q13 turns on to supply +24 VDC
to the oscillator. To minimize starting transients, C23 provides a gradual
turn-on.
The oscillator runs whenever the unit is in the record mode.
To provide isolation and minimize cross talk, the secondary of
T3 is divided into two parts. Bias for the program tracks is taken from
transformer terminal 5; bias for the cue track is taken from terminal 7.
4-16. PROGRAM TRACK RECORD AND BIAS CONTROL CIRCUITRY
The rear panel line and microphone input jacks are connected to
separate windings of
T1 (T2). The secondary of the transformer is directly
coupled to the input of the preamplifier IC-1A (IC-1B). Gain in this stage
is switchable 0 dB (LO) for use with line level sources or 35 dB (HI) for
microphone input. The gain is determined by R15 (R17) and R14 (R16) as
selected by Sl. The frequency response of the preamplifier is flat. Bias
is provided from the voltage divider Rl8, Rl9, and C3.
The output of IC-1A (IC-1B) is coupled through C4 (C5) to the
front panel record level control. The output of the level control is re-
turned to the Record Amplifier/Bias Board and to the VU meter amplifier (on
the Record Control and Generator Board Signal is coupled through C11 (C12)
to booster amplifier IC-2A (IC-2B) which supplies an additional 32 dB of
gain as determined
by R24 (R21) and R25 (R26). Bias for IC-2A (IC-2B) is
supplied through R22 (R23).
Following
amplification, the signal is coupled through C13 (C10)
to the record equalizer. The record equalization network is composed of R5
(R4), R30 (R29), R31 (R32), R27 (R28), C15 (C14). The high frequency trimmer
R5 (R4) permits adjusting the equalization for the individual head, type of
tape used,
and the various standards (NAB, IEC, CCIR, etc.).
The equalized signal is coupled through C17 (C16) to the head
driver amplifier Q2 (Q3). This stage provides sufficient current to drive
.the record head.
The amplified signal is coupled through C32 (C33), R39
(R42) to the program record control FET Q4 (Q5). This P channel FET is held
off by voltage supplied to the gate through R47 and R49. When the unit is
in the record mode, the gate grounds to the record flip-flop through CR6.
The FET conducts to allow the program signal to reach record head. At all
other times the signal is shut off.
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