Sony TC-640 Service Manual page 22

Procedure
1. Thread a blank tape (SONY SLH tape) along the
tape path.
20. Erase Ratio
Measurement
Test Setup:
blank tape
3. Set the MIC VOL and LINE VOL controls to the
specified position on page 12.
MIC jack eee . Deliver a 1 kHz signal of —50 4B (2.45 mV) to the
MIC jack.
. Place the unit in the stereo-record mode.
. Rewind a half of recorded tape.
LINE OUT jack : .
. Disconnect the audio oscillator,
. Terminate the MIC jack with a 600 {2 resistor.
Specification:
more than
65 dB (with SONY SLH tape)
9. Erase the tape in the record mode with no input
signal.
. Rewind again to the beginning of the recorded
portion.
. Play back the tape.
. Make sure that the difference between the two
portions is more than 65 dB on the VT VM.
21. Erase Head . Unsolder the erase head lead wires.
Dummy Coil
Test Setup:
= — §X rae Head
/
. Connect the frequency counter across the lead wires.
Adjustment
. Place the unit in the stereo-record mode.
. Make sure that the counter indicates 120 kHz
+10%.
. Place the L-CH (R-CH) only in the record mode.
. Adjust L187 (L188) to obtain the same reading as
obtained in the step 4.
299%