Dual 721 - Pitch Control; Adjustment of Nominal Speeds

6
lt can happen that the stroboscope lines
appear to
move slightly although the exact
speed setting with stroboscope lines sta—
tionary has not been altered. This
apparent
contradiction 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 AC 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
am constant
and absolutely accurate am before.
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).
Pitch
Control
Each
of the
two standard
speeds 33
1/3 and
45 rpm can
be varied by
approximately 10 %.
The
variable speed
control (R
33) located
in the
voltage divider is
adjusted
by turn—
ing the pitch
control
knob (44). By
this
means the
operation
amplifier is
altered
and the
motor
speed
accordingly.
Adjusting
the nominal
speed
Using knob (44) bring the variable speed
control (191) into
midposition.
Nominal
speed 33 1/3 rpm using R 22 control
Nominal
speed 45
rpm using
R 21 control
Check with
strobe disk.
Fig. 9 Stroboscop
Fig. 10 Pitch
control
11