Commodore 1541 - Page 19

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Short pin 3 of PS to ground. This will cause the motor to turn.
e The read amplifiers may oscillate when
not in use, but this is a normal
occurrence •
Using the oscilloscope set-up described above and Figure 8-22,
verify the input from the head is per Figure 8-21. The DC bias at
pins 1 and 14 of UH7 is approximately 6 vol ts. Refer to Figure
8-23 for proper oscilloscope display at pins 7 and 8 of UH7. With
oscilloscope connected to output of the first video amplifier,
check mechanical alignment as follows:
e Use a factory recorded disk or a disk
which was formatted on a VIC-1541 that is
known to be properly aligned.
Set oscilloscope to 100 mS/Div. Observe overall envelope of video
and compare it with following general guidelines:
Flat, with constant amplitude-correct mechanical alignment.
Random changes in overall amplitude-incorrect tension or
pressure on head.
Sinewave, 2 div/cycle-drive hub off center.
Sinewave with harmonics-gross misalignment of hub.
Low in amplitude-dirty head, incorrect track position,
incorrect head pressure, incorrect tension, misalignment of
disk seating plane or axial misalignment of the read/write
head. Also check first video amplifier and the heads for
proper electrical operation.
e No provision is made on the VIC-1541
Drive Unit for head alignment, with the
exception of Head Alignment. If a
problem is clearly revealed as a head
alignment problem, the Drive Unit must be
replaced.
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