Hi,
> Yesterday I received an OMPC that I'm evaluating. Without
> film and with good 357's it works fine in Program and Manual
> modes. In Auto, however, occasionally the mirror locks up.
> The mirror comes down by turning the dial to "test" (just
> like an OM-2N). The glitches occur when speeds less
> that 1/30 are indicated and they occur intermittently and
> unpredictably. Occasionally at slower speeds the shutter
> will close OK, but it will seem to have taken longer than
> the indicated speed in the display.
Yes, a well (?) known problem of the OM-40... Unfortunately mine also
suffers from this problem. The worst I've experienced with it was 3 weeks
ago when I tried to shoot the beautiful glass-in-lead windows in the Notre
Dame (Paris), and the bloody thing kept locking up every time (some 5
consecutive shots!), lately on a few other occasions it has also been bad
(nightshots), and when taking similar shots with the 4Ti it didn't happen.
> We've had a lot of rain lately and the humidity is very
> high. But it's not logical that the same conditions that
> make the one camera get better make the other one get worse.
I would be highly surprised if the weather has anything to do with it at
all. I've had the mirror lock up in all sorts of conditions, and also in the
above cases there was no excessive humidity, so I think it's pretty safe to
rule that option out...
> Any speculations greatly appreciated!
Since I haven't seen any of the internal schematics (John, do you have some
for me?) I'm venturing out on a limb; therefore the following is indeed
nothing more than speculation.
To me it seems that the timer is flip-flop controlled; suppose the flip-flop
doesn't get reset properly or goes into a race state, it is very well
possible that the output signal used to keep the shutter locked doesn't get
reset and hence the mirror stays up. This could also be consistent with the
releasing when the camera is put into test mode, provided that the
flip-flop's reset line then receives a "hard reset" (i.e. the line is pulled
down (or up, depending on the logics)), and hence the flip-flop gets reset,
causing the mirror to be released.
Now why wouldn't Olympus have spotted this during development? Well, this
could be caused by them not exhaustively checking all conditions, or by them
using a multiplexed reset line, which in some conditions doesn't get (re)set
timely and hence messes up the (re)set line...
Just my 2 cents on what might be happening, it can of course also be
something else, but based on what I've seen it very much seems to be buggy
reset behaviour (this can also be caused by mismatchings in the timing of
the internal components, hence "missing" the right moment to release the
mirror)...
Either way, I'd have to check the schematics to be able to see if the above
theory is plausible, but even then typically one would need to know what
kind of microprogram is used to control the mirror lock-up...
Cheers!
Olaf
PS: Don't consider the body a total loss, mine has served me very well, it's
just that occasionaly the thing messes up. If you have a second camera with
a different internal control for the mirror lock-up, just use that camera
for these long exposures (I'm quite certain the OM-4(T(i)) is a lot more
stable, and so far it hasn't locked up on me yet)...
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