JVC CD-1740 - Page 8

3. Recording bias current
Readjust following the produre below if the head is
replaced or the adjustment seems incorrect.
First adjust for chrome tape, then for normal tape.
Insert a 1009 resistor into the ground side wiring of the
head as shown in Fig. 13.
Remember that the ground side during recording becomes
live during play back. Be sure to insert the resistor to the
recording ground side of the wiring after disconnecting the
wiring from the circuit board.
Connect the AC VTVM across the resistor.
Adjust the bias control semi-fixed variable resistors VR501,
VR502 so that the currect becomes correct for both the L
and R channels setting of the CHROME bias position.
Adjust the bias control semi-fixed variable resistor VR503
setting of NORMAL bias position. R/P HEAD
The standard currents are
NORMAL 400zA (40mV/1002. )
CHROME 600uA (60mV/1002. )
1000 Nite
4. Recording signal current Fig. 13
If the recording signal currents are not adjusted correctly,
the recorded signal does not indicate the reference level on
VU meter.
More over the Dolby recording/playback characteristics
may be so degraded that reproduction of the original sound
is impossible.
The most suitable current intersity depends on the tape
used. Fuji FM-C90 regular tape and Fuji FC-60 chrome
tape were used to adjust the currents in the factory.
Follow the procedure below for readjustment.
Level meter deflection, bias current and playback
sensitivity must be adjusted in advance.
Adjustment for normal tape (e.g. FM-90)
1) Set the equalizer and bias switches to NORMAL.
2) Set the deck to the record mode. Input 1kHz signals
from MIC or LINE IN jacks (with a level of -50dBs
approx. for the MIC input, or -10dBs approx. for the
LINE IN input) and adjust the recording volume
controls so that the level of Line out terminals are
—5.5dBs.
3) After setting the signal level in this way, record the
signal on blank tape.
Playing back the tape just recorded, adjust VR104
(L-ch) and VR204 (R-ch) so that the level of LINE OUT
terminals are -5.5dBs. Repeat the recording and
Playback adjustments if necessary.
5. Dolby NR circuit
(See Dolby NR circuit board = page 14)
1) Set the Dolby NR circuit in the record mode, and the
FET gate short to ground.
2) Adjust two semi-fixed variable resistors VR301,401 so
that the voltage of it’s center terminal become
maximum.
3) Apply 5kHz sine wave signal to input termina! of Dolby
circuit. Adjust the INPUT level so that the level of REC
MONI terminal is 17.5mV (—33dBs).
4) Check the level of NR OUT terminal, when NR SW
terminal shorts to ground.
CD-1740/A/B/E/U
No. 4133
5) When NR SW terminal opens, adjust two semi-fixed
variable resistor VR303,402 so that the level of NR OUT
terminal is +10dB +0.25dB rise at 4) step.
6) When FET gate opens, adjust two semi-fixed variable
resistors VR301,401 so that NR OUT terminal level is
-2dB +0.25dB rise at 5) step.
7) Replace fet gate short and check that level at REC OUT
terminal returns exactly to that observed in step 6)
above.
8) Adjust the INPUT level so that REC MONI terminal
level is -2.5dBs.
Check the NR OUT terminal level is not changed (0.5dB
is limit) when NR SW terminal open and short to
ground.
9) Set the playback mode.
Adjust the INPUT level so that REC MONI terminal
level is -2.5dBs. Check Line out terminal level is not
changed (0.5dB is limit) when the NR SW terminal open
and short to ground.
Mechanism adjustment
1. Take-up torque
Standard torque: 40~ 60 gr-cm
If not as specified,
1) Replace the take-up lever arm (Fig. 14 @).
2) Replace the take-up lever arm pressure spring.
3) Clean or replace the capstan belt (Fig. 14 Q).
4) Clean the right-side reel rubber outer surface, or replace
the reel rubber.
2. Fast forward torque
Standard torque: 70 gr-cm or more
If not as specified,
1) Clean the right-side reel rubber outer surface, or replace
the reel disc ass’y (Fig. 15 @)).
2) Clean the fast forward idler, or replace the fast forward
idler ass’y @).
3) Clean the flywhee! outer surface and belt.
SPEED ADJ
Fig. 14
3. Rewinding torque
Standard torque: 70 gr-cm or more
If not as specified,
1) Clean the left-side reel @) outer surface, or replace the
reel disc ass’y ©.
2) Clean the rewind idler ©).
3) Clean the flywheel! outer surface and belt.
4. Tape speed
Standard speed: 3000Hz +2% (at playing a 3000Hz test
tape)
If not as specified,
1) Insert your driver in hole with the motor up side and
adjust a semi-VR for speed control in the motor case.
2) Replace the motor.