TM 11-5835-241-13&P
to properly magnetize the tape with the audio signals.
In erasing,
the ac erase signal continually changes the polarity at a. constant
amplitude.
As a given point on the tape approaches and passes the
erase head, the magnetic field increases and then decreases linearly
This leaves the tape demagnetized.
to zero.
In the recording head,
the bias current (which is about ten times the average audio current)
and audio currents are added.
Since the current in the record head
changes at an audio rate, the residual magnetic pattern on the tape
reflects the audio level.
Record Bias Adj R51 is adjusted to obtain
operation at the "peak bias" mode where a 1 kHZ signal is recorded at
its maximum level.
The bias oscillator is of the push-ull type. Oscillations
b.
at the base of the transistors are amplified and developed across
tuned circuit C36 and T2.
This circuit determines the oscillator
Signals from the collector of each transistor are coupled
via C33 and C34 to the base of the opposite transistor to sustain
oscillations.
C30, C31, R45, and R46 provide dc degeneration to
The bias signal
stabilize the operating point of the oscillator.
is coupled through T2 and C37 to the erase head and to the record
head circuit through Record Bias Adj R51 and C38. L1 and C12 is a
It presents a high
parallel resonant circuit at the bias frequency.
The bias oscillator is operative only when S6 is in the
c.
This supplies B+
RECORD position and PLAY switch S4 is depressed.
to the oscillator through contacts of S6 and S4 and Erase Bias Adj
R50.
Erase Bias Adj R50 varies the oscillator B+ and thus the ampli-
tude of oscillation.
It is adjusted for the correct level of erase
Record Bias Adj R51 is then adjusted for the correct level
current.
of record bias.
4-9
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