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I inch. From the table, the number of bars required on the
stroboscope is theoretically 20-1 x J = 15-1.
In practice you would use 15 bars and the projector would
, 16 x 15 run at — =-=— = — = 15-9 f.p.s.
When you fit the stroboscope, take great care not to damage the working face of the capstan which comes in contact with the tape. If this does not run perfectly true, the recording will wow on reproduction. If the end of the capstan is accessible, stick the disc to it. You may have to extend the capstan shaft a little. As a temporary measure, you can do this with a match stick held in place with modelling clay. The matchstick will be amply strong enough to support the disc at the top. A more permanent arrangement may require drilling and tapping the end of the shaft to take a screw thread. If you are not equipped to do this yourself, you can get it done at a local garage.
On some recorders, the capstan shaft seats on a steel ball at each end. You cannot then extend the shaft in this way. But you can still apply stroboscopic bars to the flywheel, provided you can arrange for it to be visible and illuminated by the projector. This may entail fitting a door or window in the side of the cabinet through which to observe the flywheel. In other cases it may be sufficient to remove the back of the cabinet for this purpose.
You can illuminate the stroboscope in one of three ways. In a small room you may get sufficient light reflected from the screen. More generally you will have to make special provision for lighting the stroboscope. You can project through a thin piece of glass or Perspex inclined so as to tip a fraction of the light on to the tape recorder. Alternatively use a small mirror just in front of the projector lens. This intercepts the extreme upper edge of the beam to deflect light to the stroboscope. Because the mirror is very close to the lens, it will not produce a sharp shadow on the screen.
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