Akai M8

Wo
ADJUSTMENT
OF
AMPLIFIER
1, ADJUSTMENT OF RECORDING BIAS
FREQUENCY
Recording/erasing bias frequency has been set at 60 KC
plus or minus 5 KC before shipment. It is decided by
inductance of the oscillator coil (part 3 OSC 60) and its
resonant capacitor (part % C-22). To measure recording
bias frequency, insert a 10 or 100 ohm resistor in series
to the record/play head and connect vertical input
terminals cf an oscilloscope as shown in the Fig. 13.
Another testing instrament to be prepared is an audic
frequency oscillator and its output should be connected
to horizontal input terminals of the oscilloscope. Vary
frequency being generated by the audio frequency osci-
lator around 60 KC, and set the oscillator at the point
where a lissajous figure appears on the oscilloscope. The
recording bias frequency now corresponds to the reading
of the oscillator. Check oscillator coil, C-22, C-27 and
bias head if recording bies frequency measured is not
within 60 KC plus or minus 5 KC.
2. ADJUSTMENT OF RECORDING BIAS
VOLTAGE
A proper recording bias voltage differs on each head
from 130 to 230 volts A.'c., and it is stammed on the
mounting bracket of tape head assembly. To measure
recording bias voltage, connect a V. T. V. M. (Vacuum
T243 Volt Mete:) to the record/play head as shown in
the Fig. 14. Check C-21, C-27 and bias head if record-
ing bias voltage does not meet to the above-mentioned
specification after adjusting C-27, a 100 pf variable
capacitor.
3. ADJUSTMENT OF ERASING BIAS VOLTAGE
A proper erasing bias voltage is decided automatically
as recording bias voltage is set. The erasing bias volta-
ge, however, should always be more than 170 volts A.C.
To measure erasing bias voltage, connect a V.T. V. M. to
the erase head as shown in the Fig. 15. Check C-21
‘(1000 pt mica capacitor) and erase head if erasing bias
voltage measured is lower than 170 volts A.C.
4. ADJUSTMENT OF RECORDING LEVEL
A basic recording level, usually referred to as “O VU",
a set at the poirt where current of 20 wA+10% flows to
the recording head at 1000 cycles. The VU meter indi-
cates 0 VU +2 VU when a signal of 55 db +5 db at the
microphone jack or —15 db +5 db at the line input jack
a given with the volume control set at its maximum.
Adjustment of recording level can be accomplished by
varying sensitivity of VU meter, however, it is not neces-
sary to make in a field of service as all VU meters have
correctly been calibrated at factory before shipment.
See Fig. 16 for details.
NOTE: Make sure to stop oscillation by disconnecting
the oscillator coil $ CSC 60 at the points marked
"RP" and "G" prior to measurement of recording
level.
Cal OOP
— Osc~ tt
OSCILLOSCOPE AUDIO FREQUENCY
OSSRLATCR
Fig. 14
7 - A
nrof Ce
Fig. 15
TO OSC-bU
4 RPH
Gs,
" 1000PF
' RBH
JC, }
@3)
4
VT
Fig. 16
:
Voltage(
V.
TLV.
ML)
Current
(iad)
=
--—
wa
x10