At 3:35 AM +0100 1/6/05, Listar wrote:
>Date: Wed, 5 Jan 2005 14:11:02 -0800 (PST)
>From: Tim Hughes <timhughes@xxxxxxxx>
>Subject: [OM] Re: Vivitar 550FD M/P/O discussion
>
>This topic seems to always get recycled, so I don't like to see it
>get recycled with misinformation.
When we come to conclusion, we should add a little paper to the OM
hardware page. A FAQ item if there ever was one.
>Joe wrote contesting the high voltage capability of the OM4 trigger cct:
I do want to hear from the repair guys -- are we interpreting the
inspection criteria correctly? Can you really put 500 volts on the
flash synch contact? Do they actually test this?
> >>The OM-4Ti circuit diagram does show the SCR (Q110), and the
>inspection criteria does list 500 volt insulation resistance. But I
>haven't figured out how transistor Q101 figures in, and why it
>doesn't let the 500 volts into the big IC (IC102). I doubt IC102 can
>handle any such voltage.<<
>
>Joe , Oly uses a rather small triangle to indicate chassis ground.
>The emitter of Q101 you assume is possibly tied to trigger high
>voltage, is grounded to chassis. The OM4T service manual shows one
>line less connected to the FP switch cct +pc socket ,than shown on
>OM4, making this more confusing, but this does not change the
>connection to ground of Q101 emitter. The OM4 **main** schematic
>diagram is clearer, and page E-48 of the OM4 service manual makes it
>much clearer, especially the lower assembly diagram. (unfortunately
>some other diagrams in Mark's scan of OM4 are largely illegible)
>Q101 discharges C103 through thyristor gate to chassis, to trigger
>it. It needs to be set up to do this because the 3V battery is
>otherwise the wrong polarity to trigger scr gate without the +ve
>level shift provided by capacitor. The OM4T has a slightly improved
>C103 charge cct for the capacitor using an added diode.
On E-48, it is clearer. If the triangle is ground and the barred
circle is -3 volts, the circuit makes sense, and the 500 volts cannot
get past the SCR and neighboring components.
>I always wondered about the FP switch. I assume this is how it is
>meant to work: Because Oly wanted to allow external flashes to be
>used via the pc-sync socket, but wanted compatibility with on camera
>T series flashes, they added the FP switch timed so it opens just
>after the trigger is applied to scr. That way the scr has already
>shorted out the Tseries flash trigger and so no high voltage can be
>applied to the T32 trigger pin by the external flash at time when FP
>switch closes. When only an external flash is connected via
>pc-connector, the trigger pulse is long enough to still trigger the
>scr+external flash. Very Obscure Warning : This means if you use a
>high voltage flash and connect it via a hotshoe to pc-sync adater
>you could apply a high voltage via the Tseries/F connector to an Oly
>system flash. (The hot shoe sync terminal goes directly to the
>Tseries 5pin connector, not via FP switch as it does for pc
>connector.) If it works as I assume, it also means by shorting the
>hotshoe sync pin to ground the external pc-socket should trigger an
>external flash even when the camera is set to red manual 1/60 with
>no battery It would be interesting to see if it also works like
>that on an OM3, as this would allow flash with dead batteries or in
>very cold conditions whiich is not possible normally.
All very plausible, but I have never used the FP mode. Is this for
FP-type flash bulbs?
>An interesting bit of flash trivia for the OM4 is that on manual
>speeds, it will trigger strobes at any shutter speeds (with poor
>results >1/60). It makes a bit more sense on OMT with possibility of
>super fp, but even there, since the super fp needs the extra
>dedicated pins, it is strange...
>
>As noted before the only single digit OM I have not checked for high
>voltage capability is OM3 but my guess is its design is similar to
>OM4, esp as it requires a battery to fire. We need to get a
>schematic to verify.
>As far as 2-digit OMs, the OM10 seems ok, as is the OMPC/40, but I
>have not checked others.
Another poster reported the the OM-3 has mechanical contacts only for
flash synch, which would probably follow the OM-2 pattern.
It sounds like a quick test would be to set the camera to B, and use
a low-voltage ohmmeter to measure the resistance of the flash-synch
circuit. If it's only mechanical contacts, the ohmmeter will report
a short circuit. If SCR, it will report an open circuit, for two
reasons: First, the low voltage (less than one diode drop) isn't
enough to turn any semiconductors on. Second, the SCR will have
fired and recovered by the time we manage to measure the resistance.
Joe
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