JVC GRAX770U
Service Manual

2.4 TAPE TRANSPORT ADJUSTMENT In most cases the deck section is in need electrical adjustment, 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 instruments 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 0.7x10-3 � 1.37x10-3 N�m. 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 1.47x10-3 � 2.45x10-3 N�m. If necessary, replace the center pulley unit.

8. When the FM waveform breaks in the level varying process, subtly adjust the height of guide rollers at every breaking point so that the waveform varies as flat as possible. Repeat the above steps 6. and 7. several times to confirm that the waveform is flat as a whole. 9. Playback the SP stairstep signal of alighment tape and adjust the tracking control to maximize the FM waveform, confirm that FM waveform variation is always flat. 10. Record the signal and play it back in both of the SP and EP 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 EP model only.

CH-2 1 field

Back tension 0.7x10-3 - 1.37x10-3 N�m (7-14gf·cm)

Play torque 1.47x10-3 - 2.45x10-3 N·m (15-25gf·cm)
Causer by wrong height of supply guide roller

Flatten waveform.

Caused by wrong height of take-up guide roller

Fig. 2-4-2 FM waveform-1

Fig. 2-4-1 Cassette torque meter 2.4.2 Tape pattern 1. Remove the Cover (JIG) shwon on Fig. 2-6-1 (Page 210). 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 simultaneously 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 � directions) 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.

Correct variation of waveform

Bad variation of waveform

Fig. 2-4-3 FM waveform-2 (U)

2-7