Sharp RT-31

ELECTRICAL ADJUSTMENT
= BIAS CURRENT AND BIAS OSCILLATION FREQUEN-
CY ADJUSTMENT OF RECORD AMPLIFIER
(See Figure 5-l) * SW7 is not used for model RT-31.
I. Connect VTVM between the test point TPIOI (or
TP201) and ground.
2. Connect the vertical input terminal of oscilloscope to
the VTVM and its horizontal input terminal to CR
oscillator.
3. Apply the signal 105 kHz from the CR oscillator to the
unit, and push the tape selector switch SW4.
4. Place the unit in record mode and adjust bias oscillation
coil L303 so that the bias oscillation frequency is
105 kHz, while watching Lissajous’ figure on the oscillo-
scope.
5. Push tape selector switch SW6.
6. Adjust semi-variable resistor VR301 (ro VR302) so that
VTVM indicates 70 mV.
7. Make sure the indicated value of VTVM varies according
to each position of tape selector switch as tabulated
below.
n ERASE VOLTAGE CHECK (See Figure 5-2)
I. Connect VTVM between test point TP301 and ground.
2. Connect oscilloscope to the output terminal of VTVM,
3. Push the tape selector switch SW6.
4. Place the unit in record mode, then see that VTVM is
value 210 mV.
n RECORD/PLAYBACK HEAD AZIMUTH ADJUSTMENT
(See Figure 5-3)
1. Connect VTVM between the test point TP103 (or
TP203) and ground.
2. Push the tape selector switch SW4 and set the Dolby
NR switch SW3 at off position.
3. Put a test tape (TEAC, MTT-114, IO kHz, 250 pWb/mm,
-10 dB recorded) into the unit and play it.
4. Adjust the head azimuth adjusting screw so that the
indicated value of VTVM reaches the maximum and a
difference in phase becomes as small as possible.
g PLAYBACK FREQUENCY CHARACTERISTIC CHECK
(See Figure 5-4)
1. Connect VTVM to the line output socket.
2. Push the tape selector switch SW4 and set the Dolby
NR switch SW3 at off position.
3. Put a test tape (TEAC, MTT-217G, 12.5 kHz/l kHz/
40 Hz, 250 pWb/mm, -10 dB recorded) into the unit
and play it.
4. See that VTVM is showing the deviation of two different
input signals (40 kHz and 12.5 kHz) against the 1 kHz
input, as tabulated below.
Posit ion
Switch Indicated value
I
normal
Push SW4
37 mV
Fe - Cr
CrOs
Push SW7
Push SW5
41 mV
52 mV
Figure 5-I
Figure 5-2
Figure 5-3
Test Tape
With 40 Hz input
Deviation against 1 kHz input
shown by VTVM
OdBto6dB
I
With 12.5 kHz input ( -3 dB to 4 dB
I
-
Figure 5-4
-5-