BIAS
LEVEL
ADJUST: This adjustment is made
while
you are recording
a
tone on the type of tape
you'll be using for the session.
It
will be different for
each brand of tape.
Set up the signal generator (oscillator). The frequency
is
10 kHz. The level should be
O
VU on the meters of
the 80-8 on INPUT. Start the
machine. record the
signal, and switch to MONITOR on the OUTPUT
SELECT.
Begin the adjustment by making
sure trim pot
#8
R112,
50
kOhms BIAS LEVEL
is
in the fully counter-
clockwise position (off, no bias at all). Now,
as
you
rotate the trim pot
#8
clockwise, the VU meter will
rise to some peak
reading. Continue the clockwise rota-
tion slowly until the reading on the meter drops back
3dB from the peak. If, at peak the meter goes off
scale, adjust the front panel input level
contro1 to keep
the reading on scale.
What
is
important here
is
not the zero. It is the reduc-
tion of the peak by precisely
3
dB. If you have moved
the input level pot on the front panel of the 80-8 to
keep your reading on scale, the next adjustment will
correct your input reference.
Fig.
27
Bias
Level Adjustment Squencr
I
BIAS
(Pot rotated
clockwise)
1.307
Fig.
28
Bias Limits
Chart
If there
is
insufficient CW rotation of trim pot
#8
to
achieve
a
peak, dealer service of the Bias amplifieri
oscillator system will be required. Many voltages in the
circuit must be adjusted accurately and this type of
problem is not considered to be "Daily Maintenance".
Bring it in.
When doing bias adjustments,
al1
8
channels should be
recording at once, even though you are adjusting only
one at
a
time.
With the oscillator running at 1
kHz,
switch back to
INPUT. Adjust the front panel input knob to
2
o'clock
position and adjust trim pot
#6
R107 INPUT MONIT
LEVEL for
O
VU indication on meters.
BIAS TRAP ADJUST:
Now
is
the time to do the bias trap in the record
circuit: the card
extender
is
used. With no input signal,
test point
is
located on the card. Positive side of
VTVM
is
connected to test point, negative side to
ground.
Tune inductor L3 for minimum.
Fig.
29
16
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