Akai GXC-750

6. RECORDING BIAS VOLTAGE AND
(1)
(2)
ERASE VOLTAGE CHECK
(Refer to Fig. 24)
Set the GXC-75D to recording mode.
Connect a V.T.V.M. between the coil terminals
of the recording/playback head as shown in
Fig. 24 and read indication. Recording bias
voltage should be about 3V A.C. with low
noise tape and about 4V A.C. with chromium
dioxide tape.
(3) Connect a V.T.V.M. between the coil terminals
of the erase head and read indication. Erase
voltage should be 27V A.C. with low noise
tape and 35V A.C. with chromium dioxide
tape.
7. DOLBY N.R. SYSTEM CIRCUIT
ADJUSTMENT (Refer to Figs. 25 and 26)
(1)
19 kHz FILTER ADJUSTMENT
(Refer to Figs. 21 and 25)
a) Connect the various measuring instruments as
shown in Fig. 21.
b) To prevent damage by the FM stereo pilot
signal (19 kHz) use a Frequency Counter and
set the oscillation frequency of the Audio
Frequency Oscillator to exactly 19 kHz and
supply this signal to the line input.
c) Set Recording Level Controls VR1 S50 kA
(left channel) and VR2 50 kA(right channel)
as well as Volume Controls VR3 10 kA(left
channel) and VR4 10 kA(Right channel) to
maximum (10 position).
d) Set machine to recording mode and adjust 19
kHz filter coils L2 (eft channel) and L2b
(right channel) of Dolby N.R. P.C. Board
(CP-5003) to obtain the lowest possible output
level.
(2) RECORDING CIRCUIT ADJUSTMENT
(Refer to Figs. 21 and 25)
a) Connect the various measuring instruments as
shown in Fig. 21.
b) Turn Dolby N.R. P.C. Board (CP-5003) gain
adjustment semi-fixed resistors VR2 500B (left
channel) and VR2b 500B (right channel) as
well as FET gate bias adjustment semi-fixed
resistors VR1 5 kB (left channel) and VR1b
5 kB (right channel) as far as they will go in
the direction of the arrows shown in Fig. 25.
c) Turn Dolby N.R. Switch SW20 to OFF position
and, as shown in Fig. 24, ground “TP” test
point (FET Gate).
d) Next, set Recording Level Controls VR1 50 kA
(left channel) and VR2 to kA (right channel)
as well as output adjustment Volume Controls
VR3 10 kA (left channel) and VR4 10 kA
(sight channel) to maximum (10 position).
e) Set the GXC-75D to recording mode and supply
a 5 kHz signal which has been checked with
a frequency counter to the line input from the
audio Frequency Oscillator and adjust the
Attenuator to obtain a +2 dB line output level
indication.
f) At this time, adjust the Attenuator to obtain
a +2 dB line output level indication.
g) At this condition, further adjust the Attenuator
so that the line output level at point © in
Fig. 25 is -28.5 dB. (This adjustment must
be exactly -28.5 dB) and confirm that the level
at point®) is -28.5+0.5 dB.
h) Next, turn Dolby N.R. Switch SW20 to ON
position. Adjust Dolby N.R. P.C. Board gain
adjustment semi-fixed resistors VR2 SOOB (left
channel) and VR2b 500B (right channel) until
the voltage at point ® in Fig. 25 is increased
by 10 dB.
i) Disconnect GND (ground) from FET gate.
Adjust FET gate bias volumes VR1 5 kB
(left channel) and VR1b 5 kB (right channel)
until the voltage at point(B)is reduced by 2 dB.