Philips 900 Series - Limitation of Remedies

CIRCUIT DESCRIPTION
ADJUSTMENT TABLE
(1) ON FLEXIBLE CABLE OF DCC HEAD, THE ADVISED WRITE CURRENT HAS BEEN INDICATED, VCLAMP » NOTED WRITE CURRENT x Rtot(t0 Ohen}
USE f-COUNTER
ADJUST BOTH SIDES TO
fz S50 Hr +h 025%
&CS 83 522
PHILIPS SOUND & VISION, BG AUDIO - Service Training
2.4 Write current adjustment.
In the next an explanation of the write current
adjustment as given in the adjustment table
of the service manual has been given. The
write current has to be adjusted a.o. in case
the head support assembly has been
replaced.
A. First of all take care that tape speed is
within the prescribed tolerances
(3150 Hz + 0.25%).
B. Short circuit the Automatic gain control
(AGC) on the read/write board:
Interconnect measuring points 7 and 10.
C. Insert the DCC 9.6 kHz level calibration
tape (4822 397 30264). With this tape the
reference level of the play back mode can
be noted. (VppRDMUX)
D. Play back the Calibration level Tape and
measure the average peak-peak level of
the RDMUX signal on measuring point 3
on RW board. Trigger oscilloscope on
the RDSYNC signal, measuring point 2.
E. In case the RDMUX signal is > 3 Vpp the
signal is clipping. Than add a
potentiometer of 47kQ between
measuring point 8 and ground (measuring
point 10) and adjust with that
potentiometer until the RDMUX signal
does not clip any more.
F. Insert now a random DCC blanc cassette.
G. Make a recording of a random audio
signal.
H. Measure the peak-peak value of the write
current between measuring point 9 and
ground (measuring point 10). Note this
Vclamp value. (Neglect the 4 negative
pulses)
This value of Vclamp is defined by:
Veclamp = Iwrite x Rtot.
(Rtot = Resistor value of shunt 3881,
3882, 3883, 3884 = 102)
CIRCUIT DESCRIPTION
On the flexible cable of the DCC-head the
advised write current in mA has been
indicated.
Vclamp should be:
Noted current x Rtot.
Example: Noted current reads: 120
Vclamp is:
120mMA_x 10Q = 1.2Vpp
Adjust Vclamp to that calculated value
under item H by resistor 3888.
In example to 1.2Vpp.
Make a recording with this current on a
virgin piece of tape.
Play back the own recording and
measure the average peak-peak level of
the RDMUX signal on measuring point 3.
Trigger oscilloscope again on the
RDSYNC signal, measuring point 2.
Calculate the desired write current via
next formula:
Dt = TT TO terme x1 TOMA
Example: VppROMUX 9.6 kHz reads: 2.4Vpp
VppRDMUX own recording: 2.3Vpp
Results in desired write current of
114.78 mA, rounded to 115 mA.
M. Put the set in record mode again and
adjust the write current with potentiometer
3888 to that calculated value. Measure
between measuring point 9 and ground
GNDD (measuring point 10)
In the example: Velamp= Iwrite x Rtot:
Velamp = 115 mA x 10 = 1.15 Vpp
Adjust with potentiometer 3888 to this
value.
. Do not forget to remove short circuit
between measuring point 7 and 10 again,
because otherwise the AGC will not
operate. In case potentiometer 47kQ2 has
been added, this additional potentiometer
should be removed too.
FHILIPS SOUND & VISION, BG AUDIO - Service Training
PCS 83 523