Epson PX-8 Technical Manual page 154

REV.-A
3.1.5.3 Power Supply
Because IC 2 requires a negative voltage supply, pin 5 is grounded and pin 4 is connected to the
VBSW voltage.
Input - Monostable Circuit
Motor speed control is accomplished by comparing the time constant determined by VR1, R5, and
C7 with the frequency fed back from the capstan motor tachogenerator. The relationship bet-
ween the time constant and the rpm of the capstan motor is as follows:
NP= 1.17RxC7
1
2400· 1 0 = 1.1 7 Rx. 0.01
24 x 10
3
.
Rx = 1.17 x 0.01 ..... ApproxImately 205 kQ
where N: rpm of capstan motor - 2,400 rpm
3-8
P: Number of tachogenerator poles - 10
Rx: Total resistance of VR1 and R5 (kohms)
C7: 0.01 uF (pF)
The specified speed should be attained by ad-
justing VR 1 nearly at the center. The opti-
mum frequency output from the tachogenera-
tor, 400 Hz, is deviced as follows:
2,400 rpm
60 x 10 (poles) = 400 Hz
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