Dual 704 - Stroboscope Adjustment; Pitch Control Adjustment

*A,
Changeover to
78 rpm nominal
speed Fig. 5
Stroboscope
Instead of 45 rpm
the Dual 704 can be chang-
ed to a
nominal speed of 78 rpm.
To
change the speed bring the fine speed
control (242/R 21) in center
position using
knob
(44). Using control R 20 on the motor
electronics
board (172) adjust for 78 rpm
(refer to
Fig. 6). Check with strobe disk.
Stroboscope
Accurate setting of the platter speeds
33 1 / 3 and 45
rpm can be checked during play
with the aid of the stroboscope.
When the platter
(4) is rotating at exactly
33 1/3 or 45 rpm the
lines of the strobos-
cope appear to stand
still. If the lines
move in the direction of
rotation of the
platter, the platter speed is too high. If
the lines move backwards, the platter is
rotating more slowly than the nominal speed.
Adjustment of platter speeds 33 1/3 and 45
rpm is carried out separately with the
"pitch"
controls (44).
Strobe markings are provided on the outer
edge of the platter for 50 and 60 Hz line
frequencies.
To replace glow lamp (103) remove machine
screws (105) and remove strobe cover (104).
lt can happen that the stroboscope lines
appear to move slightly although the exact
speed setting with stroboscope
stationary
has not been altered. This apparent
contra-
diction is explained by the fact
that the
electronic central drive motor operates
fully independently of line frequency
whilst
the only relatively accurate line frequency
of the PC current supply is used for speed
measurement with the light
stroboscope. The
constantly detectable fluctuations of line
frequency by + 2 % (according to the infor-
mation of the electricity supply companies)
brief frequency fluctuations up to 1 %
arm
possible - only effect the stroboscope in-
dication and can cause the lines to "wander"
although the platter speed is as constant
and absolutely accurate as before.
Pitch Contml
Each of the two standard speeds 33 1/3 and
45 rpm (78 rpm) can be varied by about 10 %.
The variable speed control (242/R 21) loca-
ted in the voltage divider is adjusted by
turning
the
pitch control knob
(44). By this
means the differential amplifier is altered
and the motor speed accordingly.
44
Fig. 6
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7
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