Roughly translated from page Z-98 of the repair manual (the "oops" section),
the problem was called "String in Picture", or as we lovingly referred to
it, "Curtain String Bounce", or "Aw, sh*t, another one!"
Again roughly paraphrased, it was recommended to check all cameras dated
"TND4", or May 84, and earlier, by setting the camera to 1/60th, no lens,
back open, and winding and firing while holding the camera vertically with
the wind lever down (use of a winder is suggested), and watching through the
focal aperture from the back. (We found checking with fluorescent lighting
helpful, due to it's pulsing.) Periodically you may see the curtain string
bounce into the focal aperture from the top or bottom. (Since the image is
inverted, with the sky usually projected to the bottom of the camera, the
artifact appears most often to be cast by the lower string, but may occur at
either side, top or bottom.)
The problem is caused by weak brakes on the first curtain. Instead of
slowing the first curtain roller to a stop then holding the shaft, the first
curtain is allowed to slam to a stop, and sometimes bounce back slightly.
When this happens, the strings go slack and loop into the focal aperture.
At speeds above 1/60th, a significant portion of the focal aperture is
obscured by the second curtain, hence the fault will not be apparent. Only
at slower speeds can the shadow of the looping curtain string cast it's
shadow onto the film.
The solution is to turn the brake eccentric to it's tightest setting, then
bend the brake spring even tighter. (Woe be unto him that lets the brake
spring slip, though! Getting the end of the spring back onto the eccentric
post is not a fun job!)
This may make the wind stiff at the first, which may be improved by
exchanging the brake release arm with a different sized part or not. Given
the choice, a little tug at the start of the wind stroke is better than
string bounce any day.
> Dear OM Doctors,
> I'm having an intermittent fault with my OM4. Lately I have had a few
> returned prints (perhaps 10 in 72) which have a peculiar 'stain' on
> them.
> It is always exactly the same elongated arc shape, a thin line approx
> 5mm wide, that is positioned in exactly the same place on the print - on
> the upper edge of the photo.
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