Tape Transport Adjustment; Back Tension; Tape Pattern - JVC GR-FXM270A Service Manual

2.4

TAPE TRANSPORT ADJUSTMENT

In most cases the deck section is in need electrical adjust-
ment, it results from replacement of worm mechanical parts
or video heads. In the event of malfunction with electrical
circuits, troubleshouting with the aid of proper test instru-
ments most be done first, and then commence necessary
repair, replacement and adjustment, etc.

2.4.1 Back tension

1. Set a cassette torque meter onto the deck and measure
the back tension in standard REC mode to confirm that
the back tension is 7 – 14 gf
2. If not, replace the tension band.
When the value widely fluctuates in the measurement,
replace the supply reel disk.
3. With the cassette torque meter, confirm that the play
torque is 15 – 25 gf cm.
•
If necessary, replace the center pully unit.
Back tension
-3 -3
0.7x10 -1.37x10 N cm
•
(7-14gf·cm)
Fig. 2-4-1 Cassette torque meter

2.4.2 Tape pattern

1. Remove the Cover (JIG) shwon on Fig. 2-6-1 (Page 2-10).
2. Connect the jig connector cable to CN25 on the MAIN
board as shwon on Fig. 2-6-1 (Page 2-10).
3. Observe signal at V. TP FM with external trigger from V.
FF on the jig connector cable.
4. Playback the SP stairstep signal of the alignment tape
and maximize the FM waveform by the tracking button.
5. Set the tracking control to the center position by simul-
taneously pressing the tracking (-) and (+) buttons and
maximize the FM waveform by the tracking button.
6. If the observed FM waveform is not flat, adjust the height
of the supply of take-up guide roller with the roller driver.
Note: To prevent the tape from damage, turn the guide
rollers slowly.
7. By operating the tracking button (both in + and – direc-
tions) in the manual tracking mode, vary the output level
of the FM waveform from maximum to minimum and
vice versa to confirm that the waveform varies nearly in
a flat shape.
2-7
cm.
•
Play torque
-3 -3
1.47x10 -2.45x10 N·m
(15-25gf·cm)
8. When the FM waveform breaks in the level varying proc-
ess, subtly adjust the height of guide rollers at every
breaking point so that the waveform varies as flat as pos-
sible.
Repeat the above steps 6. and 7. several times to con-
firm that the waveform is flat as a whole.
9. Playback the LP stairstep signal of alighment tape and
adjust the tracking control to maximize the FM wave-
form, confirm that FM waveform variation is always
flat.
10. Record the signal and play it back in both of the SP and
LP modes, and confirm that the FM waveform is flat in
both modes.
Note: Among the above-mentioned adjustment steps,
the items of No. 9 and No. 10 are needed for the
LP model only.
CH-2
Causer by wrong height
Caused by wrong height
of supply guide roller
of take-up guide roller
Fig. 2-4-2 FM waveform-1
Correct variation of waveform
Bad variation of waveform
Fig. 2-4-3 FM waveform-2
1 field
Flatten waveform.