Fostex A-8 - Page 38

As a result of phase check, with a lOKHz signal, the adjustment is finished if
difference in phase is less than 90° between tracks and azimuth adjustment is
at the best point.
4) Check the playback frequency response of each channel by playback of the Frequency
Response section of the Reproduce Alignment Tape. The recorder VU meters can be
used for this check but if a more accurate measurement is necessary, the level
meter is plugged one by one into the recorder rear panel 1 a , 8 OUTPUT jacks and
the levels measured here.
The normal playback frequency response should be within ±3dB for a frequency
range of 50 't 18, OOOHz.
If it is not within spec, adjust REP EQ R313, 10K0 b (odd number tracks) or R323,
IOKQ b (even number tracks).
5) Whenever R313 and R323 are adjusted, the Dolby encode must be recalibrated -
(Item 5.2).
6.4.4 Bias leakage check
Two bias trap modules are provided for each channel. One is in the first stage
of the reproduce amplifier and the other in the output stage of the record amplifier.
1) Reproduce bias trap module (U109 -- odd number channels; U209 -- even number
channels)
To check bias leakage of TRACK 1, the oscilloscope probe is hooked to TP-105 and
the probe ground clip to the nearest GND.
Put TRACK 1 in the reproduce mode, the adjacent TRACK 2 in the record mode and
check bias leakage at TP- 105. If this is less than 250mV P-P, it is normal. (At
checking TRACK 2, put the adjacent tracks 1 or 3 in the record mode.) If the
voltage is high, it is adjusted by rotating the center core of U109 but before
doing this, check the frequency (lOOKHz, ±0.5KHz) of the erase/bias master
oscillator. To check the oscillator frequency, the record/reproduce amplifier
PCB is pulled out from the A-8 and the frequency at connector pin No. 2 is
checked.
If the oscillator frequency is largely off spec, replace the erase/bias master
oscillator module (U13) .
2) Record bias trap module (U108 -- odd number channels; U208 --- even number
channels)
To check bias leakage of TRACK 1, the oscilloscope probe is hooked to TP-104
and the probe ground clip to GND nearest to TP-105.
Put TRACK 1 in the record mode and check bias leakage at TP-104. It is normal
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