Akai GX-370D Service Manual

I. SPECIFICATIONS

An asterisk next to a figure indicates the minimum guaranteed performance.
TRACK SYSTEM		4 track 2-channel stereo/monaural system
REEL CAPACITY		Up to 7" reel
TAPE SPEED		7-1/2 and 3-3/4 ips ±0.5%
WOW AND FLUTTER		Less than 0.07% (*0.1,%) RMS at 7-1/2 ips
		Less than 0.1% (*0.15%) RMS at 3-3/4 ips
FREQUENCY RESPONSE		30 to 26,000 Hz (*30 to 24,000 Hz) ±3 dB at 7-1/2 ips (AKAI S.R.T. Tape)
		30 to 24 000 Hz (*30 to 22,000 Hz) ±3 dB at 7-1/2 ips (Regular Tape)
		30 to 22:000 Hz (*30 to 19,000 Hz) ±3 dB at 3-3/4 ips (AKAI S.R.T. Tape)
		30 to 19,000 Hz (*30 to 18,000 Hz) ±3 dB at 3-3/4 ips (Regular Tape)
SIGNAL TO NOISE RATIO		Better than 50 dB (*48 dB)
DISTORTION		Less than 1.5% (*2%) at 7-1/2 ips 1,000 Hz "0" VU recording
		Less than 3% at 3-3/4 ips 1,000 Hz "0" VU recording
CROSSTALK		Better than 70 dB (Monaural)
		Better than 45 dB (Stereo)
ERASE RATIO		Better than 70 dB
BIAS FREQUENCY		103 kHz ±5 kHz
BIAS LEAK		Less than –20 VU
HIGH FREQUENCY DEVIATION		Within 3 dB, using a 8,000 Hz 3-3/4 ips recorded tape at 7-1/2 ips
(between left and right channel)
INPUTS\myttrMic input	0.7 mV Impedance: 10 kΩ
\myttrLine input	70 mV Impedance: 150 kΩ
\myttrDin input	7 mV (low) and 70 mV (high)
OUTPUTS\myttrLine output	1.228V (4±1.5dB) Irnpedance: 100Ω, using a 250Hz "0" VU recorded tape
\myttrDin output	0.4V
RECORDING CAPACITY		60 min. stereo recording, using a 1,200 ft. tape at 7-1/2 ips
FAST FORWARD & REWIND TIME		68/83 sec., using a 1,200 ft. tape at 60/50 Hz
MOTORS\myttrCapstan Motor	2-speed servo control outer rotor motor
		Type: SCM2-24
		Revolutions: 520 r.p.m. at 7-1/2 ips
		260 r.p.m. at 3-3/4 ips
Reel Motor		Two 6-pole eddy current outer rotor motors
		Type: 24X0-II
		Revolutions: 930 r.p.m. at 50 Hz
		1120 r.p.m. at 60 Hz
HEADS
Combination Recording & Erase Head		Type: RE4-1
		Gap: 4µ±15%, 0.2x2 mm
		Impedance: 1,800Ω at l00 kHz
		210Ω ±15% at 100 kHz
Playback Head		Type: P4-200
		Gap: 1.75µ ±15%
		Impedance: 3 ±1 kΩ at 1 kHz
TRANSISTORS		2 . . . 2SA564(R) 4 . . . 2SC454(C)
		8 . . . 2SC458LG(B)(C) 16 . . . 2SC711(D)(E)
		4 . . . 2SC871(F) 17 . . . 2SC945(Q)(R)(S)
		6 . . . 2SC968(3) 2 . . . 2SC971(2)(3)(red)
		1 . . . 2SC1013 2 . . . 2SD234(Y)
		3 . . . TSC9000-1(B)(C)
DIODES		27 . . . 1N34A 17 . . . 10D1
		3 . . . 10D4 2 . . . IODC-l(black)
		1 . . . lODC-l(red)
IC		2 . . . LD-3141
THERMISTORS		1 . . . 1N359A1 . . . RD9A
VARISTOR		1 . . . TH201
POWER SUPPLY		100 to 240V AC, 50/60 Hz
POWER CONSUMPTION		130 W
INSULATION RESISTANCE		More than 50 MΩ
INSULATION DURABILITY		1,000V AC for more than 1 min. duration
DIMENSIONS		445 (W) x 503 (H) x 252 (D) mm (18.2 x 20.5 x 10.3")
WEIGHT		25.5 kg (56 Ibs.)
NOTE: Specifications subject to change without notice.

II. MEASURING METHOD

1. TAPE SPEED DEVIATION

As shown in Fig. 1, connect a Frequency Counter to the Line Output of Model GX-370D. Playback a 1,000 Hz prerecorded test tape. Take a Frequency Counter reading at the beginning, middle, and end of tape winding during playback. The maximum value of these respective readings will represent tape speed deviation.

2. WOW AND FLUTTER

Method A
As shown in Fig. 2, connect the Line Output of Model GX-370D to the Input of a Wow and Flutter Meter. Playback a 3,000 Hz prerecorded test tape and take a Wow and Flutter Meter reading at the beginning, middle, and end of tape winding. The maximum value of these respective readings will represent the Wow and Flutter.

Method B
Supply a 3,000 Hz sine wave signal from an Audio Frequency Oscillator and make a recording on a blank tape at the beginning, middle, and end of tape Rewind and playback the resultant signal. Measure Wow and Flutter with a Wow and Flutter Meter. (The Wow and Flutter value of Method B will be close to twice that of Method A.)

3. FREQUENCY RESPONSE

For measuring Frequency Response, connect instruments as shown in Fig. 3 and proceed as follows:

1) Supply a 1,000 Hz sine wave signal to the Line Input of Model GX-370D from an Audio Frequency Oscillator through an Attenuator.

2) Set recorder to recording mode and turn recording level control volume and line output level control volume to maximum. Adjust attenuator to obtain a +4 dB V.T.V.M. reading.

3) Under conditions described in 2) above, readjust attenuator so that the Line Output is –16 dB, and record 30 to 24,000 Hz spot frequencies.

4) Rewind tape and playback from the beginning. Take V.T.V.M. spot frequency readings and plot values on a graph.

NOTE: When measuring Frequency Response, new tape should be used.

4. SIGNAL TO NOISE RATIO

As shown in Fig. 4, connect a High Sensitivity V.T.V.M. to the Line output of Model GX-370D. Playback a 250 Hz "0" VU prerecorded test tape and measure the output. Then remove the tape and measure the noise level under the same condition. Convert each of the measured values into decibels.

5. TOTAL HARMONIC DISTORTION

Connect the measuring instruments as shown in Fig. 5 and record a 1,000 Hz sine wave signal at "0" VU. Playback the resultant signal and measure the overall distortion factor. Measure the noise level of the tape recorder without the tape. Connect the Audio Frequency Oscillator directly to the distortion meter for measurement of the distortion factor of the oscillator. The required distortion factor can be obtained from the results of the above measurement by the following formula:

d0 = d – d1 – d2

where,  d0 – Required distortion factor
            d   – Overall distortion factor
            d1 – Noise level
            d2 – Distortion factor of the oscillator

NOTE: When measuring the distortion factor, new tape should be used.

6. CROSS TALK (Cross talk between the tracks)

As shown in Fig. 6, first record a 1,000 Hz sine wave signal on Track No. 3 at +3 VU level. Next, record under a non-input condition. Then, playback the tape on Tracks No. 3 and 1 (reversed condition of tape) through the B.P.F. (band pass filter sensitivity... 1:1) and obtain a ratio between the two from the following formula:
.

7. ERASE RATIO

As shown in Fig. 4, connect a High Sensitivity V.T.V.M. to the Line Output of Model GX-370D. Playback a virgin tape and take a V.T.V.M. reading of the output level. Next, record a 1,000 Hz sine wave signal at +3 dB, then playback this recorded signal and take a V.T.V.M. reading of the output level. Next, using this prerecorded tape, record under a non-input condition and take a reading of the noise output level of the erased signal and obtain a ratio between the two from the following formula:

III. DISMANTLING OF UNIT

In case of trouble etc. necessitating disassembly, please disassemble in the order shown in the photographs. Reassemble in reverse order.

IV. MECHANISM ADJUSTMENTS

1. TAPE SPEED ADJUSTMENT
(SERVO MOTOR CIRCUIT ADJUSTMENT)

1) Connect a Frequency Counter to the Line Output.
2) Set the Tape Speed Selector to 3-3/4 ips and playback a 1,000 Hz prerecorded tape.
3) Adjust the core of Coil Ll (V12031SC-01) shown in Fig. 8 to obtain a Frequency Counter indication of 500 Hz ± 1%.
4) When the 3-3/4 ips tape speed adjustment is completed, set the Tape Speed Selector to 7-1/2 ips and adjust Servo P.C. Board (KB-1011) semi-fixed resistor VR-202 (2k B) shown in Fig. 9 to obtain a Frequency Counter indication of 1,000 Hz +1/–0.5%.

NOTE: When making tape speed adjustment, it is necessary to make the low speed (3-3/4 ips) adjustment first.

2. DIRECT FUNCTION TIME CONSTANT
ADJUSTMENT (See Fig. 9)

1) FWD <--> REV Time Constant
Adjust System Control P.C. Board (KH-1009) semi-fixed resistor VR-1 (30k B) so that the time constant to and from FWD and REV mode is about 3 seconds.
2) F-FWD or RWD to FWD or REV Time Constant Adjust System Control P.C. Board (KH-1009) semi-fixed resistor VR-2 (10k B) so that the time constant from F-FWD or RWD to FWD or REV mode is about 1.5 seconds.

NOTE: In making the adjustments outlined in Items (1) and (2) above, when the machine is switched from the various modes to FWD or REV, it is important that proper capstan motor revolutions be attained by the time the pinch wheel contacts the capstan.

3. BRAKE TENSION ADJUSTMENT

Use a 60 mm diameter tape wound on a 5" reel and measure the brake tension with a tension gauge. (See Fig. 10) Ideal tape tension is 350 grams.

Brake tension adjustment can be made as follows:
1) Adjust position of suspended springs (d) (d').
2) Loosen screws, (a) (b) as well as (a') (b') and adjust the vertical (upper/lower) position of spring suspension metal (c) (c').
3) Loosen screws (e) (f) as well as (e') (f') and adjust the horizontal (left/right) position of brake band suspension metal (g) (g').
Adjust as described above until proper brake tension is attained. (Refer to Fig. 1 1)

NOTE: In making brake tension adjustment, when the machine is set to other than stop mode, confirm that the brake band definitely does not touch the cloth tape on the brake drum.

4. REEL HEIGHT ADJUSTMENT

Loosen the reel table height adjustment screws shown in Fig. 12 and adjust by moving the reel table in the direction of arrow and positioning so that the tape winds in the center of the reel.

5. PINCH WHEEL PRESSURE
MEASUREMENT AND ADJUSTMENT

Use a tension gauge and measure the pinch wheel pressure as shown in Fig. 13. Read the value on the tension gauge as soon as the pinch wheel separates from the tape and tape travel stops. Ideal pinch wheel pressure is 1.5 kg. Pinch wheel pressure can be increased or decreased by adjusting screws (a)(b) shown in Fig. 13.

V. HEAD ADJUSTMENTS

1. HEAD HEIGHT ADJUSTMENT
(See Figs. 14 & 15)

1) Playback Head

a) For FWD playback mode head height adjustment, during FWD playback, turn head height control screw (d) to left and right until the upper edge of the tape as it passes the head is aligned with the upper edge of channel 1 head core.

b) For REV playback mode head height adjustment, during REV playback, turn head height control screw (e) to left and right until the lower edge of the tape as it passes the head is aligned with the lower edge of channel 1 head core.

2) Recording and Erase Heads

a) For FWD recording and erase head height adjustment, at FWD playback mode, adjust head height control screws (f) (g)and (h) by turning to left and right until the upper edge of the tape as it passes the head is aligned with the upper edge of channel 1 recording head core.

b) For REV recording and erase head height adjustment, at REV playback mode, adjust head height control screws (j) (k) and (l) by turning to left and right until the lower edge of the tape is aligned with the lower edge of channel 1 recording head core.

3) When making the various head height adjustments, confirm that the tape and head core surface is at a right angle with the head chassis during tape travel.

2. HEAD AZIMUTH ALIGNMENT
ADJUSTMENT (See Figs. 14 & 15)

1) Playback Head

a) Connect a High Sensitivity V.T.V.M. to the line output and playback a test tape (Ampex Alignment Tape, 8,000 Hz at 3-3/4 ips) at 7-1/2 ips.

b) At FWD playback mode, turn adjustment screws (a) and (b) to left and right until the line output level of both channels is maximum.

c) When Item (b) adjustment is completed, loosen screws (n) and (o) and move the head gap side of the playback head to the left and right. When the tension increases on the supply reel side and the line output level of both channels do not fluctuate, fix screws (n) and (o) to maintain this condition.

d) At REV playback mode, make the same adjustment as outlined above to attain maximum line output of both channels.

2) Recording and Erase Heads

a) Connect an Audio Frequency Oscillator to the line input and connect a High Sensitivity V.T.V.M. to the line output and load a blank tape.

b) Set the Monitor Switch to TAPE position an record a 16,000 Hz signal at -10 dB recording level.

c) At FWD recording mode, adjust Azimuth Alignment screw (f) so that the line output level of both channels is maximum and does not fluctuate.

d) After completing Item (c) adjustment, loosen screw (i) and move the head gap side of the recording head to left and right. When the tension increases on the supply reel side and the line output level of both channels do not fluctuate, fix screw (i) to maintain this condition.

e) At REV recording mode, make the same adjustment as outlined above by adjusting screws (k) and (m) to attain maximum line output of both channels without fluctuation.

3. To obtain the best results make adjustments outlined in Paragraphs 1 and 2 above two or three times. Also new blank tape should be used.

VI. AMPLIFIER ADJUSTMENTS

1. PLAYBACK LEVEL ADJUSTMENT
(See Fig. 16)

1) Connect a High Sensitivity V.T.V.M. to the line output.

2) Set Tape Speed Selector to 7-1/2 ips.

3) Depress both LEFT and RIGHT Track Selector Switches.

4) Set Monitor Switch to TAPE position and Line Output Level Controls to maximum.

5) Playback a 250 Hz, 7-1/2 ips prerecorded tape.

6) With P.B. Amp. P.C. Board (KH-5014) semi-fixed resistors VR-1 and VR-1b (20k B), set the line output level of both channels to 4 ±1.5 dB.

2. VU METER SENSITIVITY ADJUSTMENT
(See Fig. 16)

Adjust P.B. Amp. P.C. Board (KH-5014) semi-fixed resistors VR-2 and VR-2b (5k B) to obtain a VU meter indication of "0" VU on both channels.

MY NOTES – in the absence of a prerecorded test tape, proceed as follows – record 400 Hz sinewave and monitor the playback (select TAPE position). Adjust record input level for 70 – 100 mV r.m.s. at TR2 collector, P.B. amp. (= top of VR1 [20kB] on P.B. Board). Then do the set-ups on this page.

3. RECORDING LEVEL ADJUSTMENT
(See Fig. 16)

1) Connect an Audio Frequency Oscillator to the line input and connect a High Sensitivity V.T.V.M. to the line output.

2) Set Tape Speed Selector to 7-1/2 ips.

3) Depress both LEFT and RIGHT Track Selector Switches.

4) Load an AKAI I00L (Fuji S-100) blank tape. Set the Monitor Switch to TAPE position and the Line Output Level Controls to maximum.

5) Set recorder to FWD recording mode and supply a 1,000 Hz sine wave signal to the line input from the Audio Frequency Oscillator. Adjust line recording level volume controls VR-la and VR-1b (20k B) to obtain a line output level of 4 dB (0 VU) on both channels.

6) Set Monitor Switch to SOURCE position and adjust Monitor Switch P.C. Board (KH-5012) semi-fixed resistors VR-1 and VR-2 (5k B) to obtain a 4 dB line output level o both channels.

7) When the FWD recording level adjustment is completed, set the recorder to REV recording mode and set the Monitor Switch to TAPE position.

8) Adjust Rec. Amp P.C. Board (KH-5013) semi-fixed resistors VR-1 and VR-1b (10k B) to obtain a line output level of 4 dB on both channels.

4. RECORDING BIAS FREQUENCY
MEASURING METHOD AND ADJUSTMENT

Method I

1) Install a 1 to 2.2Ω resistor in series with the recording head and connect these terminals (a)(b) to the vertical input of an oscilloscope. (See Fig. 19)

2) Supply a sine wave signal to the horizontal input of the Oscilloscope from an Audio Frequency Oscillator. Set recorder to the REC mode.

3) Vary the frequency of the Audio Frequency Oscillator until the oscilloscope waveform displays a circular or linear pattern.

4) If the audio frequency oscillator indication is 103 ±5 kHz, the recording bias frequency is correct.

Method II

1) Connect a Frequency Counter to points (a) and (b) as shown in Fig. 19. Set recorder to the REC mode, and take a frequency counter reading at this time.

2) If the Frequency Counter indication is 103 ±5 kHz, the recording bias frequency is correct.

Adjustment
The recording bias frequency can be adjusted by changing the value of OSC. circuit condenser C-4 (4500P/500).

5. RECORDING BIAS VOLTAGE ADJUSTMENT (Frequency Response Adjustment)

1) Refer to section regarding Frequency Response Measuring Method (Fig. 3 of this manual)

2) Adjust OSC. P.C. Board semi-fixed capacitors C-5 to C-8 (70P) so that a 10 kHz signal output level is within +1/-0 dB in relation to 1,000 Hz. (See Figs. 16 & 17)

3) The bias voltage after the frequency response adjustment has been made is about 5V AC.

NOTE: The frequency response will vary depending upon the tape being used.

6. ERASE VOLTAGE

There is no way to adjust the erase voltage, but correct value is about 23V AC.

7. COMPUTE-0-MATIC RECORDING LEVEL SENSITIVITY ADJUSTMENT

1) Connect an Audio Frequency Oscillator to the left microphone input and connect a High Sensitivity V.T.V.M. to the left line output.

2) Supply a 1,000 Hz sine wave from the audio frequency oscillator.

3) Set the Monitor Switch to SOURCE position and depress both the LEFT and RIGHT Track Selector Switches.

4) Adjust COM-DET (Compute-0-Matic Detector) P.C. Board (RD-A514) semi-fixed resistor VR-201 (100k B) to obtain a 4 dB V.T.V.M. indication when the Compute-0-Matic Button is depressed.

Continue Transport Mechanism . . .