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8 . B I A S T R A P A D J U S T M E N T
lnformation
A trap coil is inserted between the oscillator bnd
recording amplifier.
Appropriate
bias current
should be supplied to the recording head, but it
should not affect the Oscillator, and the bias cur-
rent must not "leak" into the amplifier block.
o
Effect of detuned bias trap
a. Beat signal is produced between the bias and
the 19kHz broadcast pilot signal.
b. Dolby works improperly and high level de-
creases.
.
Fig.12 shows the Dolby curves at various levels.
When a signal of - 40dB is recorded with a "leaked
signal" of - 20dB, the Dolby circuit works with the
curve of - 20d8.
During playback, the Dolby unit operates at the
level of -40dB because no "leaking,,bias signal is
applied.
As a result, high frequency range is decreased.
F i g . 1 2 - a
c T - F 9 1 9 1
Adjustment
a. Depress Recording and turn the playback
volume level to the maximum.
b. Connect an AC voltmeter and oscilloscope to
the test point (No.95 or 96 and Ground)
Adjust the bias trap coil (L-I02, L-202) so that
the bias waveform becomes minimum.
k-:l #33.
N3 33 Fig 21-E, F on page
c. In every case with the bias selector set at STD
or Chrome and with the recording level control
set at mux or min, the bias leakage should be
within -45dBv.
If not, adjust L-101 and L-201.
L-ch L-101
R-ch L-201
o.s.c.
Fis. t s
I
I
I
i
0 d B
- 1 0 d B
-20d8
S i g n a l
- 3 0 d 8
-40dB
. ' - -
- - -
W h e n r e c o r d i n q
\ {
a.-
When PlaYback
F i s . 1 2 - b
3 5
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