Fostex A-8 - Page 78

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 (15 ips) and
700mV P-P (7-1/2 ips) , 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
if the voltage is 1.1V P-P.
If
it is off spec, check frequency (lOOKHz, ±0.5KHz) of the bias/erase master
oscillator before rotating the center core of U108 to adjust bias leakage.
6.4.5 Erase current adjustment
In adjusting the erase current, put the track to be adjusted in the record
mode.
To adjust TRACK 1, for example, hook the hot side of the oscilloscope probe
to TP-103 located near relay K-101 and the ground clip of the probe to GND pin in
front of the REP EQ pot, R313. Set the core of L-103 so that voltage at TP-103 is
1.7V P-P (15 ips) and 1.5V P-P (7-1/2 ips).
82